Malignant ossifying fibromyxoid tumor of the calvaria: illustrative case.

BACKGROUND: Ossifying fibromyxoid tumor (OFMT) is a rare entity of soft tissue tumor that most commonly occurs in the subcutaneous tissues of trunk or extremities with occasional cases involving the head and neck; however, primary involvement of the skull has not been reported. While historically considered slow-growing benign to intermediate malignant, few cases of atypical or malignant features have been described. OBSERVATIONS: Herein, the authors present a case of malignant OFMT with primary skull and transcranial extension. The tumor caused lytic calvarial destruction with intra- and extracranial soft tissue components. Gross total resection was performed, and histopathology revealed malignant OFMT with 40 mitoses per 50 high-power fields and moderate nuclear atypia. LESSONS: OFMT can rarely occur in the head and neck and, as reported herein, may involve the skull with intracranial extension. While no uniformly recognized histological criteria for malignancy exist, a three-tiered classification has been proposed: typical, atypical, and malignant, based on features such as hypercellularity, mitotic activity, infiltrative growth, and/or nuclear atypia. Malignant variants should be considered along the high-grade sarcoma spectrum with elevated risk for recurrence or metastatic spread. Routine adjuvant radiotherapy is not typically recommended; however, surveillance imaging is advised.