- Optional Letrozole Registration Program for patients who have not yet completed 5 years of hormonal therapy. (Note: As of September 5, 2008, the optional NSABP B-42 Registration Program closed to patient enrollment. Accrual and data collection for the NSABP B-42 randomized treatment trial continues as planned.) In order to have a predominantly letrozole-treated population for B-42 study entry, patients who have had a minimum of 2 years of hormonal therapy and who are currently on tamoxifen (for up to 3 years) or an AI may be offered letrozole at no cost until they complete 5 total years of initial adjuvant hormonal therapy.