Abstract Background: Patients diagnosed with non-invasive breast lesions such as DCIS, LCIS, ADH, and ALH are at increased risk of developing invasive breast cancer (BC). The TAM-01 study found that low-dose tamoxifen (LDT) for chemoprevention resulted in an approximate 50% risk reduction in BC events compared to placebo. A recent study at our institution (PMID: 39837695) showed that the uptake of LDT was 19%. The objective of our present study was to evaluate the recurrence patterns for patients who initiated LDT. Methods: We performed a retrospective review of patients with DCIS, LCIS, ADH, and/or ALH who started LDT from January 2019 to December 2021. We evaluated rates of LDT completion and recurrence characteristics. A time-dependent Cox regression model was used to evaluate the association between early discontinuation of LDT and risk of recurrence, accounting for changes in LDT use over time. Results: 88 eligible patients were identified and 7 were excluded due to lack of follow up. Of the remaining 81 patients, 67% (N=54) completed 3 years of LDT and 30% (N=24) stopped early due to adverse events (AEs); the remaining 3% stopped due to recurrence (N=1), to attempt pregnancy (N=1), and diagnosis of another malignancy (N=1). Overall, the mean age of our population was 54 years (SD 12) and 39% (N=32) were postmenopausal. 39% (N=32) of patients had DCIS, and 61% (N=49) had either LCIS, ADH, and/or ALH. At a median follow up time of 4.74 years, the overall recurrence rate was 10% (N=8). Among the 27 patients who discontinued LDT early, 19% (N=5) recurred: 4 after discontinuing LDT due to AEs and 1 recurred while on LDT. Among 54 patients who completed LDT, 6% (N=3) recurred: 1 during treatment and 2 after 3 years of LDT (Table 1). Based on the time-dependent Cox model, the estimated 5-year probability of recurrence was 8% for patients who completed LDT and 15% for those who discontinued early. Early discontinuation was associated with nearly a two-fold increased risk of recurrence, although this was not statistically significant due to the low event rate HR: 1.94; 95% CI: 0.42-9.07; p=0.40. All recurrences were in patients who had LCIS, ADH, and/or ALH. Most recurrences (75%) were in premenopausal patients, and the 3 patients who recurred after completing 3 years of LDT were all premenopausal. Except for 1 patient who developed HR-, HER2+ IDC and DCIS while on LDT, all other recurrences, irrespective of completion of 3 years of LDT, were atypia or LCIS. Conclusions: In patients initiated on LDT for chemoprevention, at a median follow up of 5 years, the estimated probability of recurrence was 8% in those who completed 3 years of LDT and 15% in those who stopped early. Most recurrences were seen in premenopausal patients consistent with a subgroup analysis of the TAM-01 study suggesting decreased efficacy of LDT in premenopausal women. Citation Format: K. Rupani, E. Moshier, C. Jin, D. Jaber, N. Casasanta, C. Wu, A. Tiersten, R. Patel. Recurrence Following Initiation of Low Dose Tamoxifen for Breast Cancer Prevention: A Real-World Study abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-05-16.
Rupani et al. (Tue,) studied this question.