Abstract PO4-01-09: The Prognostic Impact of Adjuvant Endocrine Therapy by Age for Patients with Small Estrogen Receptor-Positive (ER+), Human Epidermal Growth Factor 2 Negative (HER2-) Breast Cancer

Abstract Backgrounds: We previously reported an analysis of the impact of adjuvant endocrine therapy (ET) for patients with ER+ and HER2- T1a/bN0M0 breast cancer (BC) at ASCO 2023 (Abstract #538). For the whole cohort, adjuvant ET significantly reduced distant metastases, but the absolute overall survival (OS) difference was small. The presence of medical comorbidities related to treatment toxicity and the estimated life-expectancy differ based on age, affecting the clinical impact of ET. There may be settings where comorbidities result in a sufficiently short, expected OS such that the benefit of ET will not outweigh risk. In multivariate analysis, age (55 vs 55) was not an independent risk factor for distant metastasis. Here we evaluate the impact of adjuvant ET for ER+/HER2- T1a/bN0M0 breast cancer by age-specific subgroups, focusing on younger ( 40) and older (65) population. Methods: This is a multicenter retrospective cohort study that evaluated the impact of adjuvant ET for patents with ER+/HER2- T1a/bN0M0 BC who underwent primary breast cancer surgery between 2008 and 2012 in 47 institutes of the Japan Clinical Oncology Group (JCOG). We analyzed distant metastatic-free survival (DMFS) and OS using Kaplan-Meier estimates with log-rank test in patients treated with and without ET in each of three age-specific subgroups (40, 41-64 and 65 years of age). Results: Median follow-up was 9.2 years; the median age of the entire cohort was 55 years. Of 4756 eligible patients, 417 patients were age 65 (1025 (82%) with ET and 227 (18%) without ET). Of all 3989 (84%) patients with ET (2053 (51%) with Tamoxifen, 2142 (54%) with Aromatase inhibitor, 595 (15%) with LH-RH analog and 76 (2%) with others, includes duplicates). In the 40 subgroup, 9-year DMFS was 97.2% with ET and 91.0% without ET (p=0.023) and 9-year OS was 97.8% with ET and 97.0% without ET (p=0.954). In the 65 subgroup), 9-year DMFS was 92.6% with ET and 84.0% without ET (p 0.01) and 9-year OS was 93.8% with ET and 84.9% without ET (p 0.01). In those aged 41-64, no significant difference was observed in both 9-year DMFS and OS between patients with and without ET (p=0.328 and p=0.731, respectively) See Table for details. Conclusions: Adjuvant endocrine therapy was associated with improved distant metastatic-free survival in the younger (40) and older (65) population in this large Japanese dataset and was associated with improved overall survival in those 65. Our findings suggest clinically relevant benefit from ET in both younger and older unselected patients with very small ER+ BC. The lack of benefit in patients aged 41-64 is likely due to individual tumor and treatment factors, which will be evaluated and described in detail at the meeting. Table. The 9-year DMFS and OS of each age-specific three subgroups. *They are analyzed by using Kaplan-Meier estimates with log-rank test between patients treated with and without ET. Citation Format: Yuko Takahashi, Shinsuke Sasada, Naoto Kondo, Hiroya Hashimoto, Kaori Terata, Kumiko Kida, Yasuaki Sagara, Yoichi Naito, Keisei Anan, Takayuki Ueno, Akihiko Suto, Chizuko Kanbayashi, Mina Takahashi, Rikiya Nakamura, Toshiyuki Ishiba, Michiko Tsuneizumi, Seiichiro Nishimura, Yukinori Ozaki, Akihiko Shimomura, Takehiko Sakai, Tsuguo Iwatani, Hideo Shigematsu, Kenji Tamura, Tomomi Fujisawa, Takashi Hojo, Fumitaka Hara, Tadahiko Shien, Hiroji Iwata. The Prognostic Impact of Adjuvant Endocrine Therapy by Age for Patients with Small Estrogen Receptor-Positive (ER+), Human Epidermal Growth Factor 2 Negative (HER2-) Breast Cancer 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 PO4-01-09.