The effect of adjuvant radiotherapy on M0 adenoid cystic carcinoma of the breast: a retrospective cohort study based on SEER data.

Background: Adenoid cystic carcinoma of the breast (ADCC) is a rare histologic subtype of breast cancer. Currently, there are no universally accepted guidelines for the treatment of ADCC. This study aimed to evaluate the association between adjuvant radiotherapy (RT) and the survival outcomes, including overall survival (OS) and breast cancer-specific survival (BCSS), of patients with non-metastatic (M0) ADCC following breast-conserving therapy (BCT). Methods: We identified patients diagnosed with breast ADCC between 2000 and 2021 using the Surveillance, Epidemiology, and End Results (SEER) database. The inclusion criteria were as follows: (I) an International Classification of Diseases for Oncology-3 histology code of 8200/3; (II) complete clinicopathological data; (III) diagnosis at the M0 stage; (IV) documented tumor size; and (V) recorded details of the surgical approach. The exclusion criteria included a lack of surgical treatment, an unknown surgical status, diagnosis at the distant metastasis (M1) or unknown metastasis status (Mx) stage, and/or an unknown tumor size. The following prognostic variables were evaluated: age, race, tumor size, nodal status, type of surgery, and receptor status [estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2)]. Follow-up adhered to the SEER protocols with a focus on OS and BCSS. Kaplan-Meier analysis and multivariable Cox proportional hazards models were used to compare the RT and non-RT groups, adjusting for clinicopathological covariates. Results: A total of 716 patients with ADCC were included in the study, of whom, 350 (48.9%) received RT and 366 (51.1%) did not receive RT. There were no significant differences between the two groups in terms of age, sex, race, primary tumor (T) stage, nodal status, and chemotherapy (CT) (all P>0.05). Overall, the patients who received RT showed significantly improved OS compared to those who did not receive RT (P=0.002). Subgroup analyses revealed that RT was associated with improved OS in patients undergoing BCT (P=0.001), particularly in those with tumors >10 mm (P=0.002), but not in those with tumors ≤10 mm (P=0.22) or those treated with mastectomy (P=0.75). No significant RT benefit was observed in terms of BCSS in the overall cohort (P=0.10). Conclusions: Adjuvant RT did not improve the OS and BCSS of the M0 ADCC patients with tumors ≤10 mm who had undergone BCT. Therefore, adjuvant RT may not be necessary for M0 ADCC patients with tumors ≤10 mm who have undergone BCT.