Real-world treatment patterns and survival outcomes in acinic cell carcinoma of the breast: A National Cancer Database study.

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Background: Acinic cell carcinoma (ACC) of the breast is a rare subtype characterized by serous acinar differentiation resembling that of salivary gland ACC. Although often classified within the triple-negative breast cancer spectrum, breast ACC has been reported to exhibit relatively favorable outcomes, however evidence is largely limited to case reports and small case series. Methods: The National Cancer Database (NCDB) was queried for patients with invasive ACC of the breast between 2004 to 2023. Baseline demographic, tumor, and treatment characteristics were summarized descriptively. Overall survival (OS) was assessed using Kaplan-Meier analysis and compared with log-rank tests. Multivariable Cox regression analysis was used to evaluate independent associations with OS. Results: A total of 149 patients with breast ACC were identified. The mean age at diagnosis was 57.8 ± 14.3 years, and the median tumor size was 2.0 cm (IQR 1.5–3.0). Median follow-up was 58.3 months (IQR 40.0–116.1), during which 27 deaths occurred. Tumors were frequently high-grade with 44.6% poorly differentiated or anaplastic, and node-positive disease was present in 27.0% of patients. Most patients presented with early-stage disease (Stage I–II, 86.7%). Hormone receptor expression was common (ER 57.1%, PR 50.7%), whereas majority of tumors were HER2 negative (93.5%). Surgical resection was performed in 92.4% patients, with equal utilization of breast-conserving surgery (BCS) and mastectomy (46.2% each). Radiation was administered in 54.5%, chemotherapy in 56.6%, and hormonal therapy in 54.1%, while immunotherapy use was uncommon (12.8%). On survival analysis, older age ( p =0.0001), advanced stage ( p =0.0006), and nodal positivity (p=0.04) were significantly associated with worse OS, whereas receptor status was not associated with OS. Surgical resection was associated with improved survival (p=0.0054), while radiation, chemotherapy and hormonal therapy were not associated with OS. Patients who did not undergo surgical resection had significantly worse survival compared with those treated with either BCS or mastectomy (HR 4.41 and 4.32, respectively; both p <0.01), with no survival difference observed between BCS and mastectomy. On multivariable analysis adjusting for age, tumor size and comorbidity burden, surgical resection remained independently associated with improved survival (HR 0.19, 95% CI 0.04–0.80, p =0.024). Conclusions: In this largest national population-based study to date, breast ACC demonstrated heterogeneous clinicopathologic features including frequent high-grade disease and nodal involvement. Surgical resection was independently associated with improved survival, while systemic therapies were not associated with OS. These findings provide important real-world data to inform clinical management of this rare breast cancer subtype.