Abstract PS4-08-23: Reassessing Systemic Therapy in Breast Adenoid Cystic Carcinoma: Insights from a Large Retrospective Cohort

Abstract Background: Adenoid cystic carcinoma (ACC) comprises <1% of all breast cancers. It is characterized by dual epithelial and myoepithelial cell populations and frequently harbors MYB gene rearrangements. Due to its rarity, consensus management guidelines are lacking, leading to significant variability in treatment. This study aims to describe patient demographics, tumor characteristics, treatment modalities, and clinical outcomes among breast ACC patients seen at our institution. Methods: Retrospective cohort study including 51 patients diagnosed with breast ACC and treated at our NCI-Designated Cancer Center between 2000-2025. Data on demographics, tumor characteristics, treatment modalities, and clinical outcomes were analyzed. Results: Most patients presented with disease limited to the breast n = 47 (92%) of which n= 5 (9.8%) had axillary lymph node (ALN) involvement at diagnosis. Histologic subtype was documented in 71% of cases; 24% were solid. All patients with localized disease underwent surgery (58% segmental mastectomy, 42% mastectomy), while ALN dissection was performed in only 4% (n=2). Triple-negative phenotype was most common (83%). While low ER or PR positivity (1-10%) was observed in 18% and 16% of patients respectively, no tumors exhibited > 10% expression. Two patients received neoadjuvant chemotherapy; one had distant progression during treatment and one had distant recurrence <6 months after surgery. Among the 7 patients (15%) who received adjuvant systemic therapy-- endocrine therapy alone (n=1), combined endocrine/chemotherapy (n=1), and chemotherapy alone (n=5), 5 developed recurrence. Adjuvant radiotherapy was administered to 51% of patients. Among patients with localized disease, 15 (32%) developed distant recurrence with recurrence rates similar across histologic subtypes (33% solid only, 47% non-solid only, 56% mixed). ER-positive tumors had numerically lower recurrence rate in this small sample size (11% vs 37% for ER-negative). Notably, 60% of patients with recurrence had received systemic therapy (either neoadjuvant or adjuvant). For localized disease, median time to recurrence was 35 months, with 1 and 5-year survival rates of 98% and 66%, respectively. Conclusions: This represents the largest single-institution cohort of breast ACC reported to date. Our findings highlight significant variability in management and relatively high recurrence rates despite systemic therapy, underscoring the absence of evidence-based treatment guidelines. While selection bias may contribute to these findings, limited efficacy of chemotherapy—consistent with ACC at other primary sites—suggests that systemic therapy in the curative-intent setting should be applied judiciously. Citation Format: A. B. Newman, F. Bonini, C. Albarracin, S. Damodaran, L. Licitra, R. Ferrarotto. Reassessing Systemic Therapy in Breast Adenoid Cystic Carcinoma: Insights from a Large Retrospective Cohort [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 PS4-08-23.