• Ying Jin, Lirong Bi, Ruming Yang, Tinghan Jiang, Xiaoxiao Zhang, and Sijie Li.
• Department of Breast Surgery, First Hospital of Jilin University, Changchun, Jilin, People's Republic of China.
• Medicine (Baltimore). 2021 Nov 24; 100 (47): e27908.

RationaleMalignant 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 ConcernsA 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.DiagnosesBreast osteosarcoma originating from a malignant phyllodes tumor was confirmed by histopathologic and immunohistochemical evaluation.InterventionsThe patient underwent a wide local excision.OutcomesThe patient recovered uneventfully and was discharged after the operation. The 6-month postoperative follow-up assessment revealed no evidence of recurrence.LessonsDiagnosing 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.Copyright © 2021 the Author(s). Published by Wolters Kluwer Health, Inc.

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