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This information is produced and provided by the National Cancer Institute (NCI). The information in this topic may have changed since it was written. For the most current information, contact the National Cancer Institute via the Internet web site at http://cancer.gov or call 1-800-4-CANCER.
This summary discusses primary epithelial breast cancers in women. The breast is rarely affected by other tumors such as lymphomas, sarcomas, or melanomas. Refer to the following PDQ summaries for more information on these cancer types:
Breast cancer also affects men and children and may occur during pregnancy, although it is rare in these populations. Refer to the following PDQ summaries for more information:
Incidence and Mortality
Estimated new cases and deaths from breast cancer (women only) in the United States in 2017:
Breast cancer is the most common noncutaneous cancer in U.S. women, with an estimated 61,000 cases of in situ disease and 246,660 cases of invasive disease in 2016. Thus, fewer than one of six women diagnosed with breast cancer die of the disease. By comparison, it is estimated that about 71,280 American women will die of lung cancer in 2017. Men account for 1% of breast cancer cases and breast cancer deaths (refer to the Special Populations section in the PDQ summary on Breast Cancer Screening for more information).
Widespread adoption of screening increases breast cancer incidence in a given population and changes the characteristics of cancers detected, with increased incidence of lower-risk cancers, premalignant lesions, and ductal carcinoma in situ (DCIS). (Refer to the Ductal Carcinoma In Situ section in the Breast Cancer Diagnosis and Pathology section in the PDQ summary on Breast Cancer Screening for more information.) Population studies from the United States  and the United Kingdom  demonstrate an increase in DCIS and invasive breast cancer incidence since the 1970s, attributable to the widespread adoption of both postmenopausal hormone therapy and screening mammography. In the last decade, women have refrained from using postmenopausal hormones, and breast cancer incidence has declined, but not to the levels seen before the widespread use of screening mammography.
Anatomy of the female breast. The nipple and areola are shown on the outside of the breast. The lymph nodes, lobes, lobules, ducts, and other parts of the inside of the breast are also shown.
Increasing age is the most important risk factor for most cancers. Other risk factors for breast cancer include the following:
Age-specific risk estimates are available to help counsel and design screening strategies for women with a family history of breast cancer.[23,24]
Of all women with breast cancer, 5% to 10% may have a germline mutation of the genes BRCA1 and BRCA2. Specific mutations of BRCA1 and BRCA2 are more common in women of Jewish ancestry. The estimated lifetime risk of developing breast cancer for women with BRCA1 and BRCA2 mutations is 40% to 85%. Carriers with a history of breast cancer have an increased risk of contralateral disease that may be as high as 5% per year. Male BRCA2 mutation carriers also have an increased risk of breast cancer.
Mutations in either the BRCA1 or the BRCA2 gene also confer an increased risk of ovarian cancer [28,29] or other primary cancers.[28,29] Once a BRCA1 or BRCA2 mutation has been identified, other family members can be referred for genetic counseling and testing.[30,31,32,33] (Refer to the PDQ summaries on Genetics of Breast and Gynecologic Cancers; Breast Cancer Prevention; and Breast Cancer Screening for more information.)
(Refer to the PDQ summary on Breast Cancer Prevention for more information about factors that increase the risk of breast cancer.)
Protective factors and interventions to reduce the risk of female breast cancer include the following:
(Refer to the PDQ summary on Breast Cancer Prevention for more information about factors that decrease the risk of breast cancer.)
Clinical trials have established that screening asymptomatic women using mammography, with or without clinical breast examination, decreases breast cancer mortality. (Refer to the PDQ summary on Breast Cancer Screening for more information.)
When breast cancer is suspected, patient management generally includes the following:
The following tests and procedures are used to diagnose breast cancer:
Pathologically, breast cancer can be a multicentric and bilateral disease. Bilateral disease is somewhat more common in patients with infiltrating lobular carcinoma. At 10 years after diagnosis, the risk of a primary breast cancer in the contralateral breast ranges from 3% to 10%, although endocrine therapy decreases that risk.[52,53,54] The development of a contralateral breast cancer is associated with an increased risk of distant recurrence. When BRCA1 /BRCA2 mutation carriers were diagnosed before age 40 years, the risk of a contralateral breast cancer reached nearly 50% in the ensuing 25 years.[56,57]
Patients who have breast cancer will undergo bilateral mammography at the time of diagnosis to rule out synchronous disease. To detect either recurrence in the ipsilateral breast in patients treated with breast-conserving surgery or a second primary cancer in the contralateral breast, patients will continue to have regular breast physical examinations and mammograms.
The role of MRI in screening the contralateral breast and monitoring women treated with breast-conserving therapy continues to evolve. Because an increased detection rate of mammographically occult disease has been demonstrated, the selective use of MRI for additional screening is occurring more frequently despite the absence of randomized, controlled data. Because only 25% of MRI-positive findings represent malignancy, pathologic confirmation before treatment is recommended. Whether this increased detection rate will translate into improved treatment outcome is unknown.[58,59,60]
Prognostic and Predictive Factors
Breast cancer is commonly treated by various combinations of surgery, radiation therapy, chemotherapy, and hormone therapy. Prognosis and selection of therapy may be influenced by the following clinical and pathology features (based on conventional histology and immunohistochemistry):
The use of molecular profiling in breast cancer includes the following:
On the basis of ER, PR, and HER2/neu results, breast cancer is classified as one of the following types:
ER, PR, and HER2 status are important in determining prognosis and in predicting response to endocrine and HER2-directed therapy. The American Society of Clinical Oncology/College of American Pathologists consensus panel has published guidelines to help standardize the performance, interpretation, and reporting of assays used to assess the ER/PR status by immunohistochemistry and HER2 status by immunohistochemistry and in situ hybridization.[66,67]
Gene profile tests include the following:
The following trials describe the prognostic and predictive value of multigene assays:
Patients in this study with a low-risk score were found to have very low rates of recurrence at 5 years with endocrine therapy. Primary endpoint results from this study are awaited.
Results from the RxPONDER (NCT01272037) trial will help to determine if there is a benefit from adjuvant chemotherapy in patients with ER-positive, node-positive early breast cancer treated with endocrine therapy, and a RS below 25.
Many other gene-based assays may guide treatment decisions in patients with early breast cancer (e.g., Predictor Analysis of Microarray 50 [PAM50] Risk of Recurrence [ROR] score, EndoPredict, Breast Cancer Index).
Although certain rare inherited mutations, such as those of BRCA1 and BRCA2, predispose women to develop breast cancer, prognostic data on BRCA1 /BRCA2 mutation carriers who have developed breast cancer are conflicting. These women are at greater risk of developing contralateral breast cancer. (Refer to the Prognosis of BRCA1- and BRCA2-related breast cancer section of the PDQ Genetics of Breast and Gynecologic Cancers summary for more information.)
Hormone replacement therapy
After careful consideration, patients with severe symptoms may be treated with hormone replacement therapy. For more information, refer to the following PDQ summaries:
Other PDQ summaries containing information related to breast cancer include the following:
Table 1 describes the histologic classification of breast cancer based on tumor location. Infiltrating or invasive ductal cancer is the most common breast cancer histologic type and comprises 70% to 80% of all cases.
The following tumor subtypes occur in the breast but are not considered typical breast cancers:
The American Joint Committee on Cancer (AJCC) staging system provides a strategy for grouping patients with respect to prognosis. Therapeutic decisions are formulated in part according to staging categories but primarily according to the following:
Definitions of TNM and AJCC Stage Groupings
The AJCC has designated staging by tumor, node, and metastasis (TNM) classification to define breast cancer. When this system was modified in 2002, some nodal categories that were previously considered stage II were reclassified as stage III. As a result of the stage migration phenomenon, survival by stage for case series classified by the new system will appear superior to those using the old system.
Treatment Option Overview for Early/Localized/Operable Breast Cancer
Standard treatment options for early, localized, or operable breast cancer may include the following:
Postoperative radiation therapy:
Postoperative systemic therapy:
Preoperative systemic therapy:
Stage I, II, IIIA, and operable IIIC breast cancer often require a multimodal approach to treatment. The diagnostic biopsy and surgical procedure that will be used as primary treatment should be performed as two separate procedures:
To guide the selection of adjuvant therapy, many factors including stage, grade, and molecular status of the tumor (e.g., ER, PR, HER2/neu, or triple-negative status) are considered.[1,2,3,4,5]
Selection of a local therapeutic approach depends on the following:
Options for surgical management of the primary tumor include the following:
Surgical staging of the axilla should also be performed.
Survival is equivalent with any of these options, as documented in the European Organization for Research and Treatment of Cancer's trial (EORTC-10801)  and other prospective randomized trials.[9,10,11,12,13,14,15] Also, a retrospective study of 753 patients who were divided into three groups based on hormone receptor status (ER-positive or PR-positive; ER-negative and PR-negative but HER2/neu-positive; and triple-negative) found no differences in disease control within the breast in patients treated with standard breast-conserving surgery; however, there are not yet substantive data to support this finding.
The rate of local recurrence in the breast with conservative treatment is low and varies slightly with the surgical technique used (e.g., lumpectomy, quadrantectomy, segmental mastectomy, and others). Whether completely clear microscopic margins are necessary has been debated.[17,18,19] However, a multidisciplinary consensus panel recently used margin width and ipsilateral breast tumor recurrence from a meta-analysis of 33 studies (n = 28,162 patients) as the primary evidence base for a new consensus regarding margins in stage I and stage II breast cancer patients treated with breast-conserving surgery plus radiation therapy. Results of the meta-analysis include the following:
For patients undergoing partial mastectomy, margins may be positive after primary surgery, often leading to re-excision. A clinical trial of 235 patients with stage 0 to III breast cancer who underwent partial mastectomy, with or without resection of selective margins, randomly assigned patients to have additional cavity shave margins resected (shave group) or not (no-shave group). Patients in the shave group had a significantly lower rate of positive margins than those in the no-shave group (19% vs. 34%, P = .01) and a lower rate of second surgery for clearing margins (10% vs. 21%, P = .02).[Level of evidence: 1iiDiv]
Axillary lymph node management
Axillary node status remains the most important predictor of outcome in breast cancer patients. Evidence is insufficient to recommend that lymph node staging can be omitted in most patients with invasive breast cancer. Several groups have attempted to define a population of women in whom the probability of nodal metastasis is low enough to preclude axillary node biopsy. In these single-institution case series, the prevalence of positive nodes in patients with T1a tumors ranged from 9% to 16%.[22,23] Another series reported the incidence of axillary node relapse in patients with T1a tumors treated without axillary lymph node dissection (ALND) was 2%.[Level of evidence: 3iiiA]
The axillary lymph nodes are staged to aid in determining prognosis and therapy. SLN biopsy is the initial standard axillary staging procedure performed in women with invasive breast cancer. The SLN is defined as any node that receives drainage directly from the primary tumor; therefore, allowing for more than one SLN, which is often the case. Studies have shown that the injection of technetium-labeled sulfur colloid, vital blue dye, or both around the tumor or biopsy cavity, or in the subareolar area, and subsequent drainage of these compounds to the axilla results in the identification of the SLN in 92% to 98% of patients.[25,26] These reports demonstrate a 97.5% to 100% concordance between SLN biopsy and complete ALND.[27,28,29,30]
On the basis of the following body of evidence, SLN biopsy is the standard initial surgical staging procedure of the axilla for women with invasive breast cancer. SLN biopsy alone is associated with less morbidity than axillary lymphadenectomy.
Evidence (SLN biopsy):
On the basis of the following trial results, ALND is unnecessary after a positive SLN biopsy in patients with limited SLN-positive breast cancer treated with breast conservation or mastectomy, radiation, and systemic therapy.
Evidence (ALND after a positive SLN biopsy in patients with limited SLN-positive breast cancer):
For patients who require an ALND, the standard evaluation usually involves only a level I and II dissection, thereby removing a satisfactory number of nodes for evaluation (i.e., at least 6-10), while reducing morbidity from the procedure.
For patients who opt for a total mastectomy, reconstructive surgery may be performed at the time of the mastectomy (i.e., immediate reconstruction) or at some subsequent time (i.e., delayed reconstruction).[36,37,38,39] Breast contour can be restored by the following:
After breast reconstruction, radiation therapy can be delivered to the chest wall and regional nodes in either the adjuvant or local recurrent disease setting. Radiation therapy after reconstruction with a breast prosthesis may affect cosmesis, and the incidence of capsular fibrosis, pain, or the need for implant removal may be increased.
Postoperative Radiation Therapy
Radiation therapy is regularly employed after breast-conserving surgery. Radiation therapy is also indicated for high-risk postmastectomy patients. The main goal of adjuvant radiation therapy is to eradicate residual disease thus reducing local recurrence.
For women who are treated with breast-conserving surgery without radiation therapy, the risk of recurrence in the conserved breast is substantial (>20%) even in confirmed axillary lymph node-negative women. Although all trials assessing the role of radiation therapy in breast-conserving therapy have shown highly statistically significant reductions in local recurrence rate, no single trial has demonstrated a statistically significant reduction in mortality. However, a large meta-analysis demonstrated a significant reduction in risk of recurrence and breast cancer death. Thus, evidence supports the use of whole-breast radiation therapy after breast-conserving surgery.
Evidence (breast-conserving surgery followed by radiation therapy):
A 2011 meta-analysis of 17 clinical trials performed by the Early Breast Cancer Trialists' Collaborative Group (EBCTCG), which included over 10,000 women with early-stage breast cancer, supported whole-breast radiation therapy after breast-conserving surgery.[Level of evidence: 1iiA]
With regard to radiation dosing and schedule, the following has been noted:
Additional studies are needed to determine whether shorter fractionation is appropriate for women with higher nodal disease burden.
Regional nodal irradiation
Regional nodal irradiation is routinely given postmastectomy to patients with involved lymph nodes; however, its role in patients who have breast-conserving surgery and whole-breast irradiation has been less clear. A randomized trial (NCT00005957) of 1,832 women showed that administering regional nodal irradiation after breast-conserving surgery and whole-breast irradiation reduces the risk of recurrence (10-year DFS, 82.0% vs. 77.0%; HR, 0.76; 95% CI, 0.61-0.94; P = .01) but does not affect survival (10-year OS, 82.8% vs. 81.8%; HR, 0.91; 95% CI, 0.72-1.13; P = .38).[Level of evidence: 1iiA]
Similar findings were reported from the EORTC trial (NCT00002851). Women with a centrally or medially located primary tumor with or without axillary node involvement, or an externally located tumor with axillary involvement, were randomly assigned to whole-breast or thoracic-wall irradiation in addition to regional nodal irradiation or not. Breast-conserving surgery was performed for 76.1% of the study population, and the remaining study population underwent mastectomy. No improvement in OS was seen at 10 years among patients who underwent regional nodal irradiation when compared with patients who did not undergo regional nodal radiation (82.3% vs. 80.7%, P = .06). Distant DFS was improved among patients who underwent regional nodal irradiation when compared with patients who did not undergo regional nodal irradiation (78% vs. 75%, P = .02).[Level of evidence: 1iiA]
A meta-analysis that combined the results of the two trials mentioned above found a marginally statistically significant difference in OS (HR, 0.88; 95% CI, 0.78-0.99; P = .034; absolute difference, 1.6% at 5 years).
Postoperative chest wall and regional lymph node adjuvant radiation therapy has traditionally been given to selected patients considered at high risk for locoregional failure after mastectomy. Patients at highest risk for local recurrence have one or more of the following:[54,55,56]
In this high-risk group, radiation therapy can decrease locoregional recurrence, even among those patients who receive adjuvant chemotherapy.
Patients with one to three involved nodes without any of the high-risk factors are at low risk of local recurrence, and the value of routine use of adjuvant radiation therapy in this setting is unclear.
Evidence (postoperative radiation therapy in patients with one to three involved lymph nodes):
Further, an analysis of NSABP trials showed that even in patients with large (>5 cm) primary tumors and negative axillary lymph nodes, the risk of isolated locoregional recurrence was low enough (7.1%) that routine locoregional radiation therapy was not warranted.
Timing of postoperative radiation therapy
The optimal sequence of adjuvant chemotherapy and radiation therapy after breast-conserving surgery has been studied. Based on the following studies, delaying radiation therapy for several months after breast-conserving surgery until the completion of adjuvant chemotherapy does not appear to have a negative impact on overall outcome. Additionally, initiating chemotherapy soon after breast-conserving surgery may be preferable for patients at high risk of distant dissemination.
Evidence (timing of postoperative radiation therapy):
The following results were observed:
These studies showed that delaying radiation therapy for 2 to 7 months after surgery had no effect on the rate of local recurrence. These findings have been confirmed in a meta-analysis.[Level of evidence: 1iiA]
In an unplanned analysis of patients treated on a phase III trial evaluating the benefit of adding trastuzumab in HER2/neu-positive breast cancer patients, there was no associated increase in acute adverse events or frequency of cardiac events in patients who received concurrent adjuvant radiation therapy and trastuzumab. Therefore, delivering radiation therapy concomitantly with trastuzumab appears to be safe and avoids additional delay in radiation therapy treatment initiation.
Late toxic effects of radiation
Late toxic effects of radiation therapy are uncommon, and can be minimized with current radiation delivery techniques and with careful delineation of the target volume. Late effects of radiation include the following:
Modern radiation therapy techniques introduced in the 1990s minimized deep radiation to the underlying myocardium when left-sided chest wall or left-breast radiation was used. Cardiac mortality decreased accordingly.[69,70]
An analysis of the National Cancer Institute's Surveillance, Epidemiology, and End Results Program (SEER) data from 1973 to 1989 reviewing deaths caused by ischemic heart disease in women who received breast or chest wall radiation showed that since 1980, no increased death rate resulting from ischemic heart disease in women who received left chest wall or breast radiation was found.[71,72][Level of evidence: 3iB]
Postoperative Systemic Therapy
Stage and molecular features determine the need for adjuvant systemic therapy and the choice of modalities used. For example, hormone receptor (ER and/or PR)-positive patients will receive hormone therapy. HER2 overexpression is an indication for using adjuvant trastuzumab, usually in combination with chemotherapy. When neither HER2 overexpression nor hormone receptors are present (i.e., triple-negative breast cancer), adjuvant therapy relies on chemotherapeutic regimens, which may be combined with investigational targeted approaches.
An international consensus panel proposed a risk classification system and systemic therapy treatment options. This classification, with some modification, is described below:
The selection of therapy is most appropriately based upon knowledge of an individual's risk of tumor recurrence balanced against the short-term and long-term risks of adjuvant treatment. This approach allows clinicians to help individuals determine if the gains anticipated from treatment are reasonable for their particular situation. The treatment options described below should be modified based upon both patient and tumor characteristics.
Adjuvant chemotherapy 1970s to 2000: Anthracycline-based regimens versus cyclophosphamide, methotrexate, and fluorouracil (CMF)
The EBCTCG meta-analysis analyzed 11 trials that began from 1976 to 1989 in which women were randomly assigned to receive regimens containing anthracyclines (e.g., doxorubicin or epirubicin) or CMF (cyclophosphamide, methotrexate, and fluorouracil). The result of the overview analysis comparing CMF and anthracycline-containing regimens suggested a slight advantage for the anthracycline regimens in both premenopausal and postmenopausal women.
Evidence (anthracycline-based regimens):
Study results suggest that particular tumor characteristics (i.e., node-positive breast cancer with HER2/neu overexpression) may predict anthracycline-responsiveness.
Evidence (anthracycline-based regimen in women with HER2/neu amplification):
Adjuvant chemotherapy 2000s to present: The role of adding taxanes to adjuvant therapy
A number of trials have addressed the benefit of adding a taxane (paclitaxel or docetaxel) to an anthracycline-based adjuvant chemotherapy regimen for women with node-positive breast cancer.
Evidence (adding a taxane to an anthracycline-based regimen):
An Eastern Cooperative Oncology Group-led intergroup trial (E1199 [NCT00004125]) involving 4,950 patients compared, in a factorial design, two schedules (weekly and every 3 weeks) of the two drugs (docetaxel vs. paclitaxel) after standard-dose AC chemotherapy given every 3 weeks.[Level of evidence: 1iiA] Study findings include the following:
Chemotherapy schedule: Dose-density
Historically, adjuvant chemotherapy for breast cancer was given on an every 3-week schedule. Studies sought to determine whether decreasing the duration between chemotherapy cycles could improve clinical outcomes. The overall results of these studies support the use of dose-dense chemotherapy for women with HER2-negative breast cancer.
Evidence (administration of dose-dense chemotherapy in women with HER2-negative breast cancer):
Docetaxel and cyclophosphamide
Docetaxel and cyclophosphamide is an acceptable adjuvant chemotherapy regimen.
Evidence (docetaxel and cyclophosphamide):
Timing of postoperative chemotherapy
The optimal time to initiate adjuvant therapy is uncertain. A retrospective, observational study has reported the following:
Toxic effects of chemotherapy
Adjuvant chemotherapy is associated with several well-characterized toxic effects that vary according to the individual drugs used in each regimen. Common toxic effects include the following:
Less common, but serious, toxic effects include the following:
(Refer to the PDQ summary on Treatment-Related Nausea and Vomiting; for information on mucositis, refer to the PDQ summary on Oral Complications of Chemotherapy and Head/Neck Radiation; for information on symptoms associated with premature menopause, refer to the PDQ summary on Hot Flashes and Night Sweats.)
The use of anthracycline-containing regimens, however-particularly those containing an increased dose of cyclophosphamide-has been associated with a cumulative risk of developing acute leukemia of 0.2% to 1.7% at 5 years.[104,105] This risk increases to more than 4% in patients receiving high cumulative doses of both epirubicin (>720 mg/m2) and cyclophosphamide (>6,300 mg/m2).
Cognitive impairment has been reported to occur after the administration of some chemotherapy regimens. However, data on this topic from prospective, randomized studies are lacking.
The EBCTCG meta-analysis revealed that women who received adjuvant combination chemotherapy did have a 20% (standard deviation = 10) reduction in the annual odds of developing contralateral breast cancer. This small proportional reduction translated into an absolute benefit that was marginally statistically significant, but indicated that chemotherapy did not increase the risk of contralateral disease. In addition, the analysis showed no statistically significant increase in deaths attributed to other cancers or to vascular causes among all women randomly assigned to receive chemotherapy.
HER2/neu-negative breast cancer
For HER2/neu-negative breast cancer, there is no single adjuvant chemotherapy regimen that is considered standard or superior to another. Preferred regimen options vary by institution, geographic region, and clinician.
Some of the most important data on the benefit of adjuvant chemotherapy came from the EBCTCG, which reviews data from global breast cancer trials every 5 years. In the 2011 EBCTCG meta-analysis, adjuvant chemotherapy using an anthracycline-based regimen compared with no treatment revealed significant improvement in the risk of recurrence (RR, 0.73; 95% CI, 0.68-0.79), significant reduction in breast cancer mortality (RR, 0.79; 95% CI, 0.72-0.85), and significant reduction in overall mortality (RR, 0.84; 95% CI, 0.78-0.91), which translated into an absolute survival gain of 5%.
Triple-negative breast cancer (TNBC)
TNBC is defined as the absence of staining for ER, PR, and HER2/neu. TNBC is insensitive to some of the most effective therapies available for breast cancer treatment including HER2-directed therapy such as trastuzumab and endocrine therapies such as tamoxifen or the aromatase inhibitors.
Combination cytotoxic chemotherapy administered in a dose-dense or metronomic schedule remains the standard therapy for early-stage TNBC.
Evidence (neoadjuvant chemotherapy on a dose-dense or metronomic schedule for TNBC):
Platinum agents have emerged as drugs of interest for the treatment of TNBC. However, there is no established role for adding them to the treatment of early-stage TNBC outside of a clinical trial. One trial that treated 28 women with stage II or stage III TNBC with four cycles of neoadjuvant cisplatin resulted in a 22% pCR rate.[Level of evidence: 3iiiDiv] A randomized clinical trial, CALGB-40603 (NCT00861705), evaluated the benefit of carboplatin added to paclitaxel and doxorubicin plus cyclophosphamide chemotherapy in the neoadjuvant setting. The Triple Negative Trial (NCT00532727) is evaluating carboplatin versus docetaxel in the metastatic setting. These trials will help to define the role of platinum agents for the treatment of TNBC.
Poly (ADP-ribose) polymerase (PARP) inhibitor agents
The PARP inhibitors are being evaluated in clinical trials for patients with BRCA mutations and in TNBC. PARPs are a family of enzymes involved in multiple cellular processes, including DNA repair. Because TNBC shares multiple clinicopathologic features with BRCA-mutated breast cancers, which harbor dysfunctional DNA repair mechanisms, it is possible that PARP inhibition, in conjunction with the loss of DNA repair via BRCA-dependent mechanisms, would result in synthetic lethality and augmented cell death.
HER2/neu-positive breast cancer
Treatment options for HER2-positive early breast cancer:
Standard treatment for HER2-positive early breast cancer is 1 year of adjuvant trastuzumab therapy.
Several phase III clinical trials have addressed the role of the anti-HER2/neu antibody, trastuzumab, as adjuvant therapy for patients with HER2-overexpressing cancers. Study results confirm the benefit of 12 months of adjuvant trastuzumab therapy.
Evidence (duration of trastuzumab therapy):
The HERA (BIG-01-01 [NCT00045032]) trial examined whether the administration of trastuzumab was effective as adjuvant treatment for HER2-positive breast cancer if used after completion of the primary treatment. For most patients, primary treatment consisted of an anthracycline-containing chemotherapy regimen given preoperatively or postoperatively, plus or minus locoregional radiation therapy. Trastuzumab was given every 3 weeks starting within 7 weeks of the completion of primary treatment.[Level of evidence: 1iiA] Patients were randomly assigned to one of three study arms:
Of the patients in the comparison of 1 year of trastuzumab versus observation group, the median age was 49 years, about 33% had node-negative disease, and nearly 50% had hormone receptor (ER and PR)-negative disease.[114,115][Level of evidence: 1iiA]
A number of studies have evaluated the use of subcutaneous (SQ) trastuzumab in the neoadjuvant and adjuvant settings.
Cardiac toxic effects with adjuvant trastuzumab
Cardiac events associated with adjuvant trastuzumab have been reported in multiple studies. Key study results include the following:
Lapatinib is a small-molecule tyrosine kinase inhibitor that is capable of dual-receptor inhibition of both epidermal growth factor receptor and HER2. There are no data supporting the use of lapatinib as part of adjuvant treatment of early-stage HER2/neu-positive breast cancer.
Evidence (against the use of lapatinib for HER2 positive early breast cancer):
Hormone receptor-positive breast cancer
Much of the evidence presented in the following sections on therapy for women with hormone receptor-positive disease has been considered in an American Society of Clinical Oncology guideline that describes several options for the management of these patients.
Tamoxifen has been shown to be of benefit to women with hormone receptor-positive breast cancer.
Evidence (tamoxifen for hormone receptor-positive early breast cancer):
The optimal duration of tamoxifen use has been addressed by the EBCTCG meta-analysis and by several large randomized trials.[83,128,129,130,131] Ten years of tamoxifen therapy has been shown to be superior to shorter durations of tamoxifen therapy.
Evidence (duration of tamoxifen therapy):
The results of the ATLAS trial indicated that for women who remained premenopausal after 5 years of adjuvant tamoxifen, continued tamoxifen for 5 more years was beneficial. Women who have become menopausal after 5 years of tamoxifen may also be treated with AI. (Refer to the Aromatase inhibitors section in the Hormone-receptor positive therapy section of this summary for more information.)
Tamoxifen and chemotherapy
Based on the results of an EBCTCG analysis, the use of tamoxifen in women who received adjuvant chemotherapy does not attenuate the benefit of chemotherapy. However, concurrent use of tamoxifen with chemotherapy is less effective than sequential administration.
Ovarian ablation, tamoxifen, and chemotherapy
Evidence suggests ovarian ablation alone is not an effective substitute for other systemic therapies.[133,134,135,136,137] Further, the addition of ovarian ablation to chemotherapy and/or tamoxifen has not been found to significantly improve outcomes.[135,137,138,139,140]
Evidence (tamoxifen plus ovarian suppression):
Aromatase inhibitors (AI)
AI have been compared with tamoxifen in premenopausal women in whom ovarian function was suppressed or ablated. The results of these studies have been conflicting.
Evidence (comparison of an AI with tamoxifen in premenopausal women):
In postmenopausal women, the use of AI in sequence with or as a substitute for tamoxifen has been the subject of multiple studies, the results of which have been summarized in an individual patient-level meta-analysis.
Evidence (AI vs. tamoxifen as initial therapy in postmenopausal women):
Sequential tamoxifen and AI versus 5 years of tamoxifen
Several trials and meta-analyses have examined the effect of switching to anastrozole or exemestane to complete a total of 5 years of therapy after 2 to 3 years of tamoxifen.[150,151,152] The evidence, as described below, indicates that sequential tamoxifen and AI is superior to remaining on tamoxifen for 5 years.
Evidence (sequential tamoxifen and AI vs. 5 years of tamoxifen):
In the meta-analysis, which included 11,798 patients from six trials, the 10-year recurrence rate was reduced from 19% to 17% in the AI-containing groups (RR, 0.82; 95% CI, 0.75-0.91; P = .0001). The overall 10-year mortality was 17.5% in the tamoxifen group and 14.6% in the AI-containing group (RR, 0.82; 95% CI, 0.73-0.91; P = .0002).[Level of evidence: 1A]
Sequential tamoxifen and AI for 5 years versus 5 years of an AI
The evidence indicates that there is no benefit to the sequential use of tamoxifen and an AI for 5 years over 5 years of an AI.
Evidence (sequential use of tamoxifen and an AI vs. 5 years of an AI):
In the meta-analysis, which included 12,779 patients from the trials, the 7-year recurrence rate was slightly reduced from 14.5% to 13.8% in the groups that received 5 years of an AI (RR, 0.90; 95% CI, 0.81-0.99; P = .045). Overall mortality at 7 years was 9.3% in the tamoxifen-followed-by-AI groups and 8.2% in the AI-alone groups (RR, 0.89; 95% CI, 0.78-1.03; P = .11).[Level of evidence: 1A]
One AI versus another for 5 years
Switching to an AI after 5 years of tamoxifen
The evidence, as described below, indicates that switching to an AI after 5 years of tamoxifen is superior to stopping tamoxifen at that time.
Duration of AI therapy
Evidence (additional 5 years of letrozole vs. placebo):
The role of bisphosphonates as part of adjuvant therapy for early-stage breast cancer is unclear.
Evidence (bisphosphonates in the treatment of early breast cancer):
An ongoing phase III trial (NCT01077154) is examining the activity of the bone-modifying agent, denosumab, in stage II and stage III breast cancer.
Preoperative Systemic Therapy
Preoperative chemotherapy, also known as primary or neoadjuvant chemotherapy, has traditionally been administered in patients with locally advanced breast cancer in an attempt to reduce tumor volume and allow for definitive surgery. In addition, preoperative chemotherapy is being used for patients with primary operable stage II or stage III breast cancer. A meta-analysis of multiple randomized clinical trials has demonstrated that preoperative chemotherapy is associated with identical DFS and OS compared with the administration of the same therapy in the adjuvant setting.[Level of evidence: 1iiA] Current consensus opinion for use of preoperative chemotherapy recommends anthracycline- and taxane-based therapy, and prospective trials suggest that preoperative anthracycline- and taxane-based therapy is associated with higher response rates than alternative regimens (e.g., anthracycline alone).[167,168][Level of evidence: 1iiDiv]
A potential advantage of preoperative systemic therapy is the increased likelihood of success with definitive local therapy in those presenting with locally-advanced, unresectable disease. It may also offer benefit to carefully selected patients with primary operable disease by enhancing the likelihood of breast conservation and providing prognostic information where pCR is obtained. In these cases, a patient can be informed that there is a very low risk of recurrence compared with a situation in which a large amount of residual disease remains.
pCR has been utilized as a surrogate endpoint for long-term outcomes, such as DFS, EFS, and OS, in preoperative clinical trials in breast cancer. A pooled analysis (CTNeoBC) of 11 preoperative randomized trials (n = 11,955) determined that pCR, defined as no residual invasive cancer in the breast and axillary nodes with presence or absence of in situ cancer (ypT0/is ypN0 or ypT0 ypN0), provided a better association with improved outcomes compared with eradication of invasive tumor from the breast alone (ypT0/is). pCR could not be validated in this study as a surrogate endpoint for improved EFS and OS.[Level of evidence: 3iiiD] However, on the basis of a strong association of pCR with substantially improved outcomes in individual patients with more aggressive subtypes of breast cancer, the FDA has supported use of pCR as an endpoint in preoperative clinical trials for patients with high-risk, early-stage breast cancer.
Postoperative radiation therapy may also be omitted in a patient with histologically negative axillary nodes after preoperative therapy, irrespective of lymph node status before preoperative therapy, allowing for tailoring of treatment to the individual.
Potential disadvantages with this approach include the inability to determine an accurate pathological stage after preoperative chemotherapy. However, the knowledge of the presence of residual disease may provide more personalized prognostic information, as noted above.
Patient selection, staging, treatment, and follow-up
Multidisciplinary management of patients undergoing preoperative therapy by an experienced team is essential to optimize the following:
The tumor histology, grade, and receptor status are carefully evaluated before preoperative therapy is initiated. Patients whose tumors have a pure lobular histology, low grade, or high hormone-receptor expression and HER2-negative status are less likely to respond to chemotherapy and should be considered for primary surgery, especially when the nodes are clinically negative. Even if adjuvant chemotherapy is administered after surgery in these cases, a third-generation regimen (anthracycline/taxane based) may be avoided.
Before beginning preoperative therapy, the extent of the disease within the breast and regional lymph nodes should be assessed. Staging of systemic disease may include the following:
Baseline breast imaging is performed when breast-conserving therapy is desired to identify the tumor location and exclude multicentric disease. Suspicious abnormalities are usually biopsied before beginning treatment and a marker placed at the center of the breast tumor(s). When possible, suspicious axillary nodes may be biopsied before initiation of systemic treatment.
The optimal timing of sentinel lymph node (SLN) biopsy has not been established in patients receiving preoperative therapy. The following points should be considered:
When considering preoperative therapy, treatment options include the following:
Regular clinical assessment of response to therapy is necessary after beginning preoperative therapy. Repeat radiographic assessment is also required if breast conservation is the surgical goal. Patients with progressive disease during preoperative therapy may either transition to a non-cross-resistant regimen or proceed to surgery, if feasible.[175,176] Although switching to a non-cross-resistant regimen results in a higher pCR rate than continuing the same therapy, there is no clear evidence that other breast cancer outcomes are improved with this approach.
Early trials examined whether anthracycline-based regimens used in the adjuvant setting would prolong DFS and OS when used in the preoperative setting. The evidence supports higher rates of breast-conserving therapy with the use of a preoperative anthracycline chemotherapy regimen than with postoperative use, but no improvement in survival was noted with the preoperative strategy.
Evidence (preoperative anthracycline-based regimen):
In an effort to improve the results observed with AC alone, a taxane was added to the chemotherapy regimen. The following study results support the addition of a taxane to an anthracycline-based chemotherapy regimen for HER2-negative breast tumors.
Evidence (anthracycline/taxane-based chemotherapy regimen):
Promising results have been observed, however, with the addition of carboplatin to anthracycline-taxane combination chemotherapy regimens in patients with triple-negative breast cancer (TNBC). Future definitive studies evaluating survival endpoints and the identification of biomarkers of response or resistance are necessary before the addition of carboplatin to standard preoperative chemotherapy can be considered a new standard of care.
Evidence (adding carboplatin to an anthracycline-taxane based chemotherapy regimen in patients with TNBC):
Importantly, results of studies in the adjuvant and metastatic settings have not demonstrated an OS benefit with the addition of bevacizumab to chemotherapy versus chemotherapy alone. However, the addition of bevacizumab to preoperative chemotherapy has been associated with an increased pCR rate alongside increased toxicity such as hypertension, cardiac toxicity, hand-foot syndrome, and mucositis (e.g., NSABP B-40 [NCT00408408] and GeparQuinto [NCT00567554]).[186,187][Level of evidence: 1iiDiv] However, it is not clear that the modest benefit observed will translate into a longer term survival advantage.
After the success in the adjuvant setting, initial reports from phase II studies indicated improved pCR rates when trastuzumab, a monoclonal antibody that binds the extracellular domain of HER2, was added to preoperative anthracycline- and taxane-based regimens.[Level of evidence: 1iiDiv] This has been confirmed in phase III studies.[189,190]
A phase III (HannaH [NCT00950300]) trial also demonstrated that the pharmacokinetics and efficacy of preoperative SQ trastuzumab is noninferior to the IV formulation. This international, open-label trial (n = 596) randomly assigned women with operable, locally advanced, or inflammatory HER2-positive breast cancer to preoperative chemotherapy (anthracycline/taxane-based), concurrent with either SQ-administered or IV-administered trastuzumab every 3 weeks before surgery. Patients received adjuvant trastuzumab to complete 1 year of therapy.[Level of evidence: 1iiD] The pCR rates between the arms differed by 4.7% (95% CI, 4.0-13.4); 40.7% in the IV-administered group versus 45.4% in the SQ-administered group, demonstrating noninferiority for the SQ formulation. Data regarding the DFS and OS differences between the arms are not yet available.
An ongoing trial, SafeHer (NCT01566721), is evaluating the safety of self-administered versus clinician-administered SQ trastuzumab. SQ trastuzumab is approved for use in Europe in early- and late-stage breast cancer.
Newer HER2-targeted therapies (lapatinib, pertuzumab) have also been investigated. It appears that dual targeting of the HER2 receptor results in an increase in pCR rate; however, no survival advantage has been demonstrated to date with this approach.[194,195]
Pertuzumab is a humanized monoclonal antibody that binds to a distinct epitope on the extracellular domain of the HER2 receptor and inhibits dimerization. Pertuzumab, in combination with trastuzumab with or without chemotherapy, has been evaluated in two preoperative clinical trials in an attempt to improve on the pCR rates observed with trastuzumab and chemotherapy.
On the basis of these studies, the FDA-granted accelerated approval for the use of pertuzumab as part of preoperative treatment for women with early-stage, HER2-positive breast cancer whose tumors are larger than 2 cm or node-positive. The FDA approved no more than three to six cycles of pertuzumab. Thus, a pertuzumab-based regimen as outlined above is a new treatment option for patients with HER2-positive breast cancer who are candidates for preoperative therapy. There is insufficient evidence to recommend concomitant anthracycline/pertuzumab or sequential use of doxorubicin with pertuzumab.
The ongoing APHINITY (NCT01358877) trial, a randomized, phase III, adjuvant study for women with HER2-positive breast cancer, is the confirmatory trial for this accelerated approval. Results are expected in 2016.
Lapatinib is a small-molecule kinase inhibitor that is capable of dual receptor inhibition of both epidermal growth factor receptor and HER2. Study results do not support the use of lapatinib in the preoperative setting.
More definitive efficacy data were provided by the phase III ALLTO (NCT00490139) trial that randomly assigned women to trastuzumab or trastuzumab plus lapatinib in the adjuvant setting. The trial did not meet its primary endpoint of DFS. The doubling in pCR rate observed with the addition of lapatinib to trastuzumab in the NeoALTTO trial did not translate into improved survival outcomes in the ALTTO trial at 4.5 years of median follow-up. This indicates that there is currently no role for the use of lapatinib in the preoperative or adjuvant settings.
Cardiac toxic effects with pertuzumab and lapatinib
A pooled analysis of cardiac safety in 598 cancer patients treated with pertuzumab was performed using data supplied by Roche and Genentech.[Level of evidence: 3iiiD]
A meta-analysis of randomized trials (n = 6) that evaluated the administration of anti-HER2 monotherapy (trastuzumab or lapatinib or pertuzumab) versus dual anti-HER2 therapy (trastuzumab plus lapatinib or trastuzumab plus pertuzumab) was performed.[Level of evidence: 3iiiD]
Preoperative endocrine therapy
Preoperative endocrine therapy may be an option for postmenopausal women with hormone receptor-positive breast cancer when chemotherapy is not a suitable option because of comorbidities or performance status. Although the toxicity profile of preoperative hormonal therapy over the course of 3 to 6 months is favorable, the pCR rates obtained (1%-8%) are far lower than have been reported with chemotherapy in unselected populations.[Level of Evidence: 1iDiv]
Longer duration of preoperative therapy may be required in this patient population. Preoperative tamoxifen was associated with an overall response rate of 33%, with maximum response occurring up to 12 months after therapy in some patients. A randomized study of 4, 8, or 12 months of preoperative letrozole in elderly patients who were not fit for chemotherapy indicated that the longer duration of therapy resulted in the highest pCR rate (17.5% vs. 5% vs. 2.5%, P -value for trend < .04).[Level of Evidence: 1iiDiv]
The AI have also been compared with tamoxifen in the preoperative setting. Overall objective response and breast-conserving therapy rates with 3 to 4 months preoperative therapy were either statistically significantly improved in the AI-treated women  or comparable to tamoxifen-associated outcomes. An American College of Surgeons Oncology Group trial is currently comparing the efficacy of anastrozole, letrozole, or exemestane in the preoperative setting.
The use of preoperative endocrine therapy in premenopausal women with hormone-responsive breast cancer remains investigational.
There is currently no clear role for adjuvant chemotherapy in cases in which pCR is not obtained after receipt of an anthracycline/taxane combination chemotherapy regimen. Clinical trials of novel therapies should be considered in these individuals (after neoadjuvant or preoperative trials).
Radiation therapy is administered after breast conservation in most women who have received preoperative therapy to reduce the risk of locoregional recurrence. Baseline clinical and subsequent pathologic staging should be considered in deciding whether to administer postmastectomy radiation.
Other adjuvant systemic treatments may be administered either postoperatively, during, or after completion of adjuvant radiation, including adjuvant hormonal therapy for patients with hormone receptor-positive disease and adjuvant trastuzumab for those with HER2-positive disease. (Refer to the Hormone receptor-positive breast cancer subsection in the Early/Localized/Operable Breast Cancer section of this summary for more information.)
The frequency of follow-up and the appropriateness of screening tests after the completion of primary treatment for stage I, stage II, or stage III breast cancer remain controversial.
Evidence from randomized trials indicates that periodic follow-up with bone scans, liver sonography, chest x-rays, and blood tests of liver function does not improve survival or quality of life when compared with routine physical examinations.[203,204,205] Even when these tests permit earlier detection of recurrent disease, patient survival is unaffected. On the basis of these data, acceptable follow-up can be limited to the following for asymptomatic patients who complete treatment for stages I to III breast cancer:
Current Clinical Trials
Use our advanced clinical trial search to find NCI-supported cancer clinical trials that are now enrolling patients. The search can be narrowed by location of the trial, type of treatment, name of the drug, and other criteria. General information about clinical trials is also available.
Treatment Option Overview for Locally Advanced or Inflammatory Breast Cancer
Based on available evidence, multimodality therapy delivered with curative intent is the standard of care for patients with locally advanced or inflammatory breast cancer.
The standard treatment options for locally advanced or inflammatory breast cancer may include the following:
Initial surgery is generally limited to biopsy to permit the determination of histology, estrogen receptor (ER) and progesterone receptor levels, and human epidermal growth factor receptor 2 (HER2/neu) overexpression.
The standard chemotherapy regimen for initial treatment is the same as that used in the adjuvant setting (refer to the Postoperative Systemic Therapy section of this summary for more information), although trials done solely in patients with locally advanced disease have not shown a statistically significant advantage to dose-dense chemotherapy.
For patients who respond to preoperative chemotherapy, local therapy may consist of total mastectomy with axillary lymph node dissection followed by postoperative radiation therapy to the chest wall and regional lymphatics. Breast-conserving therapy can be considered for patients with a good partial or complete response to preoperative chemotherapy. Subsequent systemic therapy may consist of further chemotherapy. Hormone therapy is administered to patients with ER-positive or ER-unknown tumors.
Although the evidence described below has not been replicated, it suggests patients with locally advanced or inflammatory breast cancer should be treated with curative intent.
Evidence (multimodality therapy):
Subsequent trials have confirmed that patients with locally advanced and inflammatory breast cancer can experience long-term DFS when treated with initial chemotherapy.
All patients are considered candidates for clinical trials to evaluate the most appropriate manner in which to administer the various components of new multimodality regimens.
Recurrent breast cancer is often responsive to therapy, although treatment is rarely curative at this stage of disease. Patients with locoregional breast cancer recurrence may become long-term survivors with appropriate therapy.
The rates of locoregional recurrence have been reduced over time, and a meta-analysis suggests a recurrence rate of less than 3% in patients treated with breast-conserving surgery and radiation therapy. The rates are somewhat higher (up to 10%) for those treated with mastectomy. Nine percent to 25% of patients with locoregional recurrence will have distant metastases or locally extensive disease at the time of recurrence.[3,4,5]
Before treatment for recurrent breast cancer, restaging to evaluate the extent of disease is indicated. Cytologic or histologic documentation of recurrent disease is obtained whenever possible. When therapy is selected, the estrogen-receptor (ER) status, progesterone-receptor (PR) status, and human epidermal growth factor receptor 2 (HER2/neu) status at the time of recurrence and previous treatment are considered, if known.
ER status may change at the time of recurrence. In a single small study by the Cancer and Leukemia Group B (MDA-MBDT-8081), 36% of hormone receptor-positive tumors were found to be receptor negative in biopsy specimens isolated at the time of recurrence. Patients in this study had no interval treatment. If ER and PR statuses are unknown, then the site(s) of recurrence, disease-free interval, response to previous treatment, and menopausal status are useful in the selection of chemotherapy or hormone therapy.
Treatment options for locoregional recurrent breast cancer include the following:
Patients with locoregional recurrence should be considered for further local treatment (e.g., mastectomy). In one series, the 5-year actuarial rate of relapse for patients treated for invasive recurrence after initial breast conservation and radiation therapy was 52%.
Treatment options also depend on the site of recurrence, as follows:
(Refer to the Metastatic (systemic) disease section of this summary for information about treatment for recurrent metastatic breast cancer.) All patients with recurrent breast cancer are considered candidates for ongoing clinical trials.
Treatment of metastatic disease is palliative in intent. Goals of treatment include prolonging life and improving quality of life. Although median survival has been reported to be 18 to 24 months, some patients experience long-term survival. Among patients treated with systemic chemotherapy at a single institution between 1973 and 1982, 263 patients (16.6%) achieved complete responses. Of those, 49 patients (3.1% of the total group) remained in complete remission for more than 5 years, and 26 patients (1.5%) were still in complete remission at 16 years.[Level of evidence: 3iiDiii]
Treatment options for metastatic breast cancer include the following:
Cytologic or histologic documentation of metastatic disease is obtained whenever possible.
Treatment of metastatic breast cancer will usually involve hormone therapy and/or chemotherapy with or without trastuzumab. All patients with metastatic breast cancer are considered candidates for ongoing clinical trials.
Hormone Receptor-Positive or Hormone Receptor-Unknown Breast Cancer
Tamoxifen and aromatase inhibitor (AI) therapy
Initial hormone therapy
Initial hormone therapy depends, in part, on the patient's menopausal status.
For postmenopausal patients with newly diagnosed metastatic disease and estrogen receptor (ER)-positive tumors, progesterone receptor (PR)-positive tumors, or ER/PR-unknown tumors, hormone therapy is generally used as initial treatment. Hormone therapy is especially indicated if the patient's disease involves only bone and soft tissue and the patient either has not received adjuvant antiestrogen therapy or has been off such therapy for more than 1 year.
While tamoxifen has been used for many years in treating postmenopausal women with newly metastatic disease that is ER-positive, PR-positive, or ER/PR-unknown, several randomized trials suggest equivalent or superior response rates and progression-free survival (PFS) for the AI compared with tamoxifen.[3,4,5][Level of evidence: 1iiDiii]
Evidence (initial hormone therapy in postmenopausal women):
Another initial treatment option for postmenopausal women is AI therapy combined with cyclin-dependent kinase inhibitor therapy (refer to the Cyclin-dependent kinase inhibitor therapy section of this summary for more information).
In premenopausal women, several randomized but underpowered trials have tried to determine whether combined hormone therapy (luteinizing hormone-releasing hormone [LH-RH] agonists plus tamoxifen) is superior to either approach alone. Results have been inconsistent.[9,10,11]
Evidence (initial hormone therapy in premenopausal women):
Second-line hormone therapy
Women whose tumors are ER-positive or ER-unknown, with bone or soft tissue metastases only, and who have been treated with tamoxifen, may be offered second-line hormone therapy. Examples of second-line hormone therapy in postmenopausal women include selective AI, such as anastrozole, letrozole, or exemestane; megestrol acetate; estrogens; androgens;[13,14,15,16,17,18,19,20,21] and fulvestrant, an ER down-regulator.[22,23]
Evidence (second-line hormone therapy):
Mammalian target of rapamycin (mTOR) inhibitor therapy
Endocrine therapy is recommended for patients with metastatic hormone receptor-positive disease. However, patients inevitably develop resistance to endocrine therapy. Preclinical models and clinical studies suggest that mTOR inhibitors might enhance the efficacy of endocrine therapies.
Evidence (mTOR inhibitor therapy):
Cyclin-dependent kinase inhibitor therapy
Cyclin-dependent kinases 4 and 6 (CDK4 and CDK6) have been implicated in the continued proliferation of hormone receptor-positive breast cancer resistant to endocrine therapy. CDK inhibitors have been approved by the U.S. Food and Drug Administration (FDA) in the first-line setting. Palbociclib is an orally available CDK4/6 inhibitor that has been shown in two trials to enhance the efficacy of endocrine therapy.
Evidence (cyclin-dependent kinase inhibitor therapy):