Objective:To determine the efficacy of letrozole in suppressing estradiol levels during ovarian stimulation in cancer patients.Methods: A retrospective chart review of cancer patients undergoing ovarian stimulation for fertility preservation between 2014-2019 at a private university-affiliated fertility clinic in Canada was conducted. Ovarian stimulation was completed？with？no letrozole (Group A, n = 10), and adjuvant daily letrozole use at 5.0？(Group？B, n = 34) or 7.5 mg (Group C, n = 61). The primary outcomes were peak？estradiol levels and oocyte yield. ANOVA with a post hoc two-tailed？t-test assuming equal variance was utilized as a statistical method. Result(s): Patient age and AFC count were not different between groups. The yield of mature eggs was not different at each letrozole dose; 9.2 ± 6.0, 13.9 ± 6.5 and 12.7 ± 7.2 for Groups A to C respectively (p = 0.18). Mean estradiol levels(pmol/L) were reduced in a dose-dependent manner; 7432 ± 4553 for Group A, 2072 ± 1656 for Group B, and 1445 ±1238 for Group C (A vs. C,
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