The American Society of Clinical Oncology (ASCO) has updated a clinical practice guideline on the use of adjuvant endocrine therapy in women with stage I-III hormone receptor-positive breast cancer. The guideline was published in the online edition of the Journal of Clinical Oncology (JCO). The guideline was updated by ASCO in 2010, and this revision is based on a review of new data from five studies of tamoxifen maintenance therapy that have emerged since then. The two largest follow-up studies with the longest follow-up periods showed that breast cancer patients treated with tamoxifen for 10 years showed a survival advantage compared to those in the population of patients treated with tamoxifen for only 5 years, while having a lower risk of both breast cancer recurrence and contralateral breast cancer. ASCO Clinical Practice Guideline Update: Adjuvant Endocrine Therapy for Hormone Receptor-Positive Breast Cancer This update reflects new data from five new studies on tamoxifen treatment that have emerged since the 2010 update to the guideline. According to Professor Harold J. Burstein, co-chair of the ASCO expert panel that drafted the guideline update, it has been the practice for decades to receive tamoxifen for 5 years as standard adjuvant endocrine therapy, yet we can now recommend, based on the evidence, that women with hormone-receptor-positive breast cancer be given tamoxifen for up to 10 years. Postmenopausal women also have the option of taking an aromatase inhibitor as an alternative option to tamoxifen or in sequence after tamoxifen. Aromatase inhibitors are not recommended for use in the premenopausal female population. Worldwide, hormone receptor-positive (HR+) breast cancer is the most common type of breast cancer. Approximately 60-75% of women with breast cancer are estrogen receptor positive (ER+) breast cancer, and 65% of these cancers also present with progesterone receptor positive (PgR+). Adjuvant endocrine therapy is very effective and it is indicated for almost all women with ER-positive and/or PgR-positive cancers. The ASCO expert panel conducted a series of formal systematic reviews of the relevant medical literature to revise this edition of the clinical practice guidelines. Two randomized trials (one published and the other presented at the 2013 ASCO Annual Meeting) on extended tamoxifen maintenance provided evidence for the updated recommendations. Since the last guideline update, no new evidence on prolonged maintenance with aromatase inhibitors has emerged. Key guideline recommendations: Premenopausal/perimenopausal women diagnosed with hormone receptor-positive breast cancer should receive adjuvant endocrine therapy with tamoxifen for 5 years, after which additional therapy should be given depending on menopausal status. If premenopausal, tamoxifen should be given for 10 years of continuous treatment. If postmenopausal, tamoxifen should be given for 10 years of continuous treatment or aromatase inhibitor (AI) adjuvant endocrine therapy for a total of 10 years. Women diagnosed with hormone receptor-positive breast cancer after menopause should be given one of the following adjuvant endocrine therapy regimens: tamoxifen for 10 years; aromatase inhibitor for 5 years; tamoxifen for 5 years followed by crossover aromatase inhibitor for 5 years; or tamoxifen for 2-3 years and crossover aromatase inhibitor for 5 years. Postmenopausal and female patients who are intolerant to tamoxifen or aromatase inhibitors should be given an alternative adjuvant endocrine therapy. If already treated with an aromatase inhibitor, but treatment is discontinued for less than 5 years, or tamoxifen may be given for a total of 5 years. If already receiving tamoxifen for 2-3 years, they should receive aromatase inhibitor therapy for up to 5 years, or a total of 7-8 years of adjuvant endocrine therapy. The guideline also discusses how clinicians should communicate face-to-face with patients about extending the duration of adjuvant endocrine therapy. In response, Professor Jennifer Griggs, co-chair of the ASCO expert panel, said it is important for clinician-patient communication to weigh the potential risks of side effects against the potential benefits of adjuvant endocrine therapy for up to 10 years. Many women on adjuvant tamoxifen experience side effects, and these appear to be associated with longer duration of treatment. In conclusion, these trials did not identify any new or unexpected side effects.