Abstract P3-19-15: Prognostic impact of radiation therapy in mucinous carcinoma of the breast

Abstract Purpose/Objectives Mucinous carcinoma (MC) of the breast is an uncommon variant of breast cancer which has a favorable prognosis. Current guidelines for ER/PR-positive, HER2-negative pure-MC state that endocrine therapy (ET) is only required for tumors ≥3cm and node-positive patients while chemotherapy (CTX) is not required but can be considered for node positive-patients. While radiation therapy (RT) currently remains a part of breast-conserving therapy (BCT) for this histology, the prognostic impact of RT in pure MC remains unclear. Materials/Methods The National Cancer Database (NCDB) was queried (2004-2017) for patients with non-metastatic, histologically confirmed MC who received breast-conserving surgery. Kaplan-Meier analysis was used to evaluate overall survival (OS). Univariate (UVA) and multivariate (MVA) analyses were conducted using Cox proportional hazard models to determine which clinical and treatment factors were prognostic for overall survival. Results A total of 24,837 patients with median age 70 (IQR 61-78) were included in this study. A total of 66% of carcinomas were grade 1 and 34% were grade 2. A total of 79% were pT1, 20% were pT2, and 1% were pT3-4. A total of 96% were pN0 and 4% were pN1-3. A total of 99% were ER+, 92% were PR+, and 96% were HER2-. A total of 61% received RT, 70% received ET, and 8% received CTX. The 10-year OS was 73% for those who received RT and 41% for those who did not. On UVA, RT (HR 0.31, p< 0.0001; CI 0.29-0.33), ET (HR 0.52, p< 0.0001; CI 0.49-0.55), CTX (HR 0.40, p< 0.0001; CI 0.35-0.46), and black race (HR 0.87, p= 0.006; CI 0.79- 0.96) were positive prognostic factors for OS while older age (HR 1.10, p< 0.0001; CI 1.09-1.10), CDCC comorbidity score (HR 1.93, p< 0.0001; CI 1.81- 2.06), and tumor size (HR 1.01, p< 0.0001; CI 1.01-1.02) were negative prognostic factors. On MVA, RT (HR 0.64, p< 0.0001; CI 0.59-0.69) and ET (HR 0.85, p< 0.0001; CI 0.78- 0.91) remained positive prognostic factors while older age (HR 1.08, p< 0.0001; CI 1.08-1.09), CDCC comorbidity score (HR 1.61, p< 0.0001; CI 1.50- 1.73), and tumor size (HR 1.02, p< 0.0001; CI 1.01- 1.02) remained negative prognostic factors; chemotherapy (HR 1.19, p= 0.039; CI 1.01- 1.39) became a negative prognostic factor and black race (HR 0.99, p= 0.92; CI 0.89- 1.12) was no longer prognostic. Conclusion This is the largest study to date on mucinous carcinoma of the breast and the role of RT after breast-conserving surgery. Use of postoperative RT after lumpectomy is associated with improved OS in patients with breast MC, suggesting that post-lumpectomy RT should remain standard of care. Citation Format: Neil Chevli, Kaidi Wang, Waqar Haque, Mary R. Schwartz, Julie Nangia, Jennifer Sasaki, Andrew M. Farach, Sandra S. Hatch, E. Brian Butler, Bin S. Teh. Prognostic impact of radiation therapy in mucinous carcinoma of the breast [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-19-15.