Malignant phyllodes tumor of the breast with osteosarcoma component: a diagnostic dilemma - a case report

Osteosarcoma component in the breast is rare and can present as a rapidly growing mass with calcification in imaging studies. Differential diagnosis includes metaplastic carcinoma, malignant phyllodes tumor with osteosarcoma component, and exceptionally rare osteosarcoma. Distinguishing these tumors is critical for determining therapeutic strategies, especially in differentiating metaplastic carcinoma, as only the former is indicated for adjuvant chemoradiotherapy. Diagnosis can be challenging, however, particularly when the dominant stromal and/or sarcomatous components mask the epithelial component. Here, we present a rare case of breast tumor with osteosarcoma component determined as malignant phyllodes tumor after extensive pathological examination. A 55-year-old woman presented with a left breast mass rapidly growing over the past 3 months. FDG-PET/CT revealed a calcified tumor in the left breast, nearly exposed to the skin, as well as an incidentally found small right breast tumor. Bilateral total mastectomy with sentinel and left axillary lymph node dissection was performed. Microscopic examination of the left breast tumor revealed osteosarcoma components with the atypical cells producing osteoid/neoplastic bone, chondrosarcoma-like components, and massive overgrowth of atypical spindle cells. Extensive pathological examination revealed a small amount of compressed mammary glands without atypia. Collectively, the diagnosis of malignant phyllodes tumor with osteosarcoma component, was made. The patient was discharged without major postsurgical complications and initiated on anastrozole for the concurrently identified hormone-receptor-positive, HER2-negative right breast invasive ductal carcinoma. Breast tumor with osteosarcoma component warrants not only diligent pathological examination to determine the indication for chemoradiotherapy, but also multidisciplinary collaboration to develop effective therapeutic strategies.