Background and Aims: Occult breast cancer (OBC) is a rare clinical entity, and its biological behaviors are some different from those of common breast cancers. There is a lack of evidence of clinical research with large sample size for this kind of patients, and also the prognostic value of radiotherapy is still unclear. The purpose of this study was to evaluate the prognostic value of radiotherapy in OBC patients. 
Methods: The clinical data of OBC patients diagnosed between 2010 and 2015 were extracted from the SEER database. A total of 693 patients were enrolled after the screening according to the inclusion and exclusion criteria. Patients were divided into the radiotherapy group and the non-radiotherapy group according to whether they had received radiotherapy. The breast cancer specific survival (BCSS) and overall survival (OS) were compared between the two groups of patients, the prognostic factors for BCSS and OS in OBC patients were determined, and the impacts of radiotherapy on OBC patients with different clinicopathologic characteristics were also analyzed.
Results: Among the 693 patients, radiotherapy group included 366 cases (52.8%) and non-radiotherapy group included 327 cases (47.2%). The median follow-up time was 59 months. In radiotherapy group and the non-radiotherapy group, the estimated 5-year BCSS was 89.33% and 82.08%, respectively, and the estimated 5-year OS was 87.75% and 75.75%, respectively, and both differences were statistically significant (χ2=7.33, P=0.007; χ2=15.61, P<0.001). Results of multivariate prognostic analysis by Cox proportional hazard mode showed that receiving radiotherapy or not, lymph node stage and hormone receptor status were independent influencing factors for BCSS of the patients, and receiving radiotherapy or not, age, lymph node stage and hormone receptor status were independent influencing factors for OS of the patients (all P<0.05); the BCSS (HR=0.46, 95% CI=0.30–0.70, P<0.001) and OS (HR=0.47, 95% CI=0.33–0.67, P<0.001) in OBC patients were significantly improved by radiotherapy. Stratified analyses based on clinicopathologic characteristics showed that radiotherapy significantly improved the BCSS in patients with estrogen receptor (ER)-negative expression (HR=0.33, 95% CI=0.19–0.59, P<0.001), progesterone receptor (PR)-negative expression (HR=0.33, 95% CI=0.19–0.55, P<0.001), HER-2-positive expression (HR=0.39, 95% CI=0.01–0.89, P=0.037), and without previous breast surgery (HR=0.30, 95% CI=0.16–0.54, P<0.001), or receiving chemotherapy (HR=0.39, 95% CI=0.24–0.64, P<0.001); radiotherapy significantly improved the OS in patients with ER-negative expression (HR=0.33, 95% CI=0.20–0.55, P<0.001), PR-negative expression (HR=0.34, 95% CI=0.22–0.54, P<0.001), and without previous breast surgery (HR=0.32, 95% CI=0.20–0.52, P<0.001) or undergoing chemotherapy (HR=0.39, 95% CI=0.26–0.60, P<0.001).
Conclusion: Radiotherapy can improve the BCSS and OS in OBC patients, especially in those with negative hormone receptor expression, having chemotherapy or without history of breast surgery. The local control value of radiotherapy in OBC patient remains to be studied.