Malignant sarcomatous transformation of calvarial fibrous dysplasia: illustrative case.

backgroundFibrous dysplasia (FD) is a nonheritable genetic disorder characterized by abnormal osteogenesis, resulting in benign bone lesions in one or multiple bones. Despite their predominantly benign nature, these lesions can transform into malignant neoplasms, resulting in pain, swelling, disfigurement, and even death. The authors report a case of malignant sarcomatous transformation in an adult patient with a history of craniofacial FD. OBSERVATIONS: A 61-year-old male with a history of systemic FD presented with a rapidly enlarging cranial mass that had recently started becoming painful to touch. Magnetic resonance imaging of the brain displayed extensive FD changes throughout the calvarium and a large dysplastic mass in the right parietal bone with extension into the underlying epidural space, resulting in severe mass effect. The patient subsequently underwent radical resection and complex soft tissue reconstruction. The pathology confirmed high-grade undifferentiated pleomorphic sarcoma, for which the patient subsequently underwent an adjuvant chemotherapy regimen. LESSONS: Malignant sarcomatous transformation of craniofacial FD most often results in the formation of an osteosarcoma in the maxilla, with the temporal and sphenoid bones also being common sites of such transformations. Although treatment begins with radical resection, the efficacy of adjuvant chemotherapy and radiation remains controversial. https://thejns.org/doi/10.3171/CASE24537.