Metastatic Breast Cancer to a Dedifferentiated Solitary Fibrous Tumor Arising from a PTEN Hamartoma of Soft Tissue

Germline mutations or postzygotic mosaicism in <i>PTEN</i> cause PTEN hamartoma tumor syndrome. The hamartomas can occur at the mucocutaneous sites and superficial or deep soft tissue. PTEN hamartomas of soft tissue were initially described in the pediatric population, and therefore, they were considered pediatric lesions. Breast cancer is one of the major diagnostic criteria for Cowden syndrome. We report a 67-year-old female patient with a germline pathogenic variant of <i>PTEN</i> and a remote history of breast cancer who developed metastatic breast carcinoma into a dedifferentiated solitary fibrous tumor (SFT) that arose within a long-standing intramuscular PTEN hamartoma of soft tissue. Clinically, the dedifferentiated SFT was painful and grew rapidly within the hamartoma. Grossly, the fatty hamartoma partially encircled the fleshy dedifferentiated SFT. Upon light microscopy examination, the SFT was largely the conventional type, showing variably cellular bland fibroblasts randomly arranged in fibrous to myxoid background (patternless pattern) with branching thin-walled vessels. Within the conventional SFT, there were minute foci of metastatic ductal carcinoma of the breast, as well as distinct areas of dedifferentiated SFT consisting of mitotically active, hypercellular epithelioid and plump spindle cells with necrosis. In the surrounding fibrofatty tissue, there were increased numbers of vessels, including malformed arteries and veins, and lymphoid follicles, features described in PTEN hamartoma of soft tissue in children. Our report demonstrates that PTEN hamartoma of soft tissue in adult patients can be asymptomatic, detected incidentally, and associated with other neoplasms. Awareness of this rare phenomenon assists in arriving at a correct diagnosis.