Sebaceous Carcinoma of the Breast in a Japanese Male with a Pathogenic Variant: Report of an Exceedingly Rare Case and Review of the Literature.

Introduction: Sebaceous carcinoma (SC) is a malignant neoplasm differentiated from the mature sebocyte and occurs mainly in the periorbital area. However, SC of the breast is extremely rare. We report a case of sebaceous breast carcinoma in a Japanese man with a pathogenic variant.

Case Presentation: A 77-year-old Japanese man had been aware of a mass in his right breast for about a year and had visited his previous physician for a follow-up. Over the next year, the mass grew, and the last doctor he visited referred him to our hospital for further examination and treatment. Physical examination revealed a palpable 3-cm-large mass of the right breast. There was no skin invasion, and core needle biopsy revealed invasive ductal carcinoma cT2N1M0 cStage IIB, estrogen receptor (+)/progesterone receptor (+)/HER2/ (-)/Ki-67 labeling index: 27.8%. His daughter had a history of breast cancer, and he had a pathogenic variant. The patient underwent a total right mastectomy and axillary lymph node dissection. Pathological diagnosis was primary SC of the breast, pT2N0M0, pStage IIA. His postoperative clinical course was good. Postoperatively, the patient received endocrine therapy and S-1 for 1 year and is currently receiving endocrine therapy alone. One year and 5 months have passed since the operation, with no recurrence or metastasis noted.

Conclusions: The prognosis of SC of the breast has not been elucidated. As cases of SC with a pathogenic variant are exceedingly rare, it will be necessary to continue accumulating cases in the future to understand this disease further. This research is essential to elucidating SC.