Abstract PO2-11-03: A long-term follow-up of patients who interrupted endocrine therapy to become pregnant following breast cancer surgery

Abstract Introduction: Regarding discontinue adjuvant endocrine therapy to attempt pregnancy, Pregnancy Outcome and Safety of Interrupting Therapy for Women with Endocrine Responsive Breast Cancer (POSITIVE) clinical trials have revealed temporary interruption of endocrine therapy to attempt pregnancy did not confer short-term risk of breast cancer events. However, that report involved a short observation period, longer-term follow-up is required. This study was aimed to evaluate the safety of endocrine therapy interruption with long-term followed-up data base. Methods: This was a retrospective study using surveying of medical chart data between June 2007 and November 2015 at St. Luke’s International Hospital. The subjects were 65 patients with estrogen-receptor positive primary breast cancer (clinical stage I-III) who had been performed fertility preservation prior to starting breast cancer treatment. Adjuvant endocrine therapy completion was defined as 120 months (10 years) and discontinuation was defined as 120 months. Using the definition, we classified the patients into an endocrine therapy interruption-group and a continuation-group. The primary outcome was the breast cancer free interval (BCFI). We defined breast cancer events as local, regional, or distant recurrence of invasive breast cancer or new contralateral invasive breast cancer during follow-up. Results: Median follow-up period was 110 (range, 36–181) months. The number of adjuvant endocrine therapy interruption-group was 30 and continuation-group was 35. Among 65 patient, the mean patient age in the interruption vs. continuation groups were as follows: 38(range, 26-48) years vs. 36.5 (range, 26–45) years at breast cancer diagnosis and HER2 positive breast cancer was identified in 4 (11.4%) vs 4 (13.3%) patients. Clinical stages were Stage I, 9 (30%) vs. 11 (31.4%) patients; Stage II, 18 (60%) vs. 18 (51.4%) patients; and Stage III, 3 (10%) vs. 6 (17.1%) patients. Chemotherapy was administered in 17 (56.7%) and 23 (65.7%) patients, respectively. The median duration of endocrine therapy before interruption was 25.5 months: 3 (10%) patients received endocrine therapy for 0–18 months, 16 (53.3%) for 18–30 months, and 11 (36.7%) for 30 months. The BCFI was 124 and 119 months in the interruption and continuation groups, respectively (p = 0.16). There were 6 (20%) and 9 (25.7%) breast cancer events in the interruption-group and continuation-group, respectively (p = 0.4). All the pregnancies were in the endocrine therapy interruption group (17 pregnancies, 16 deliveries), with one pregnancy involving the delivery of twins. Three pregnancies were spontaneous. Endocrine therapy was resumed in 16 (59.3%) patients in the interruption-group (median time to resumption, 20.5 months). One patient developed distant metastasis during endocrine therapy interruption period. Conclusion: Among patients who performed fertility preservation prior to starting breast cancer treatment, our long-term follow-up data, almost 9 years, suggested that the BCFI was not statistically different between endocrine therapy interruption-group and continuation-group. A limitation of this study is that only three cases of Stage III breast cancer were included in the interruption group. Therefore, we consider that careful decisions should be made regarding interruption of endocrine therapy. Citation Format: Risa Kasahara, Atsuko Kitano, Fumi Akitani, Kyoko Shiota, Kumiko Kida, Junko Takei, Atsushi Yoshida. A long-term follow-up of patients who interrupted endocrine therapy to become pregnant following breast cancer surgery abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-11-03.