Breast osteosarcoma originating from a malignant phyllodes tumor: A case report of a rare neoplasm.

Rationale: Malignant phyllodes tumors with osteosarcomatous transformation are exceedingly rare. The clinical manifestations are similar to those associated with benign giant calcification, resulting in nonspecific and complex clinical symptoms.

Patient Concerns: A 59-year-old woman presented with a firm, painless, movable, 5.0 × 4.0 cm lump in the lower inner quadrant of the left breast that she had detected 1 month prior.

Diagnoses: Breast osteosarcoma originating from a malignant phyllodes tumor was confirmed by histopathologic and immunohistochemical evaluation.

Interventions: The patient underwent a wide local excision.

Outcomes: The patient recovered uneventfully and was discharged after the operation. The 6-month postoperative follow-up assessment revealed no evidence of recurrence.

Lessons: Diagnosing malignant phyllodes tumors with osteosarcomatous transformation requires a high level of suspicion and awareness by both surgeons and pathologists. They should be aware of the extent of such disease, which might be mistaken as benign giant calcification. Medical history and imaging findings are important for accurate diagnosis. Phyllodes tumor with an osteosarcomatous component is an aggressive neoplasm associated with distant metastasis. Delayed diagnosis and insufficient excision might negatively impact both treatment and survival.