A Very Rare Case of Predominantly Well-Differentiated Primary Male Breast Liposarcoma With Focal Dedifferentiation and Histologically Confirmed Pectoral Muscle Invasion.

Primary breast liposarcoma is a rare subtype of breast sarcoma. In men, it is extremely rare and has generally been reported in the literature as case reports. An 82-year-old man presented with a painless, slowly growing mass in his left breast that had been present for 5-6 months. On examination, the mass measured approximately 20 cm and extended into the axilla. Mammography, breast MRI, and PET-CT demonstrated a fat-containing mass extending from the breast into the axilla. The mass had an indistinct border with the pectoral muscle, several solid areas, and mild FDG uptake (SUVmax, 2.5). Similar hypermetabolic areas were observed in the pectoral muscle and axilla. Core needle biopsy suggested dedifferentiated liposarcoma. The patient underwent simple mastectomy with resection of the pectoral muscle and axillary lymph node sampling. Final pathology of the surgical specimen showed a predominantly well-differentiated liposarcoma with two small foci of dedifferentiation and a low mitotic index, with no necrosis or pleomorphism identified, and confirmed histological infiltration of the pectoralis major muscle fibers. No metastasis was observed in the axillary lymph node sampling. The patient recovered well, and no adjuvant therapy was recommended. There was no recurrence or metastasis at 10 months of follow-up. This case demonstrates that pectoral muscle invasion, although rare, can occur in breast liposarcoma. It also demonstrates that the extent of dedifferentiation identified on biopsy may differ from that identified on final pathology. Finally, this case highlights that a definitive diagnosis requires comprehensive histopathological examination.