- Participants must have demonstrated ability to tolerate endocrine therapy by prior successful completion of at least 1 month of tamoxifen or aromatase inhibitor (AI) therapy without significant adverse events, and in the opinion of the treating physician any ongoing toxicity does not preclude ability to continue on tamoxifen or AI for at least a projected 2 year continuous duration. Ongoing use of any endocrine therapy, including tamoxifen, letrozole, anastrozole, or exemestane, is allowed. Patients may enroll within 2 years of beginning endocrine therapy, as long as there is a plan for at least 2 more years of adjuvant endocrine therapy.