Solitary Fibrous Tumor Developing Two Years After Primary Cranial Synovial Sarcoma: A Rare Case

Synovial sarcoma is an aggressive and rare malignant soft tissue neoplasm, typically affecting young adults and predominantly arising near major joints. Primary cranial involvement is extremely uncommon. In rare circumstances, patients may develop multiple soft tissue tumors either synchronously or metachronously, raising a challenging differential diagnosis between metastatic disease and a second primary malignancy. We report the case of a woman diagnosed with a primary high-grade cranial synovial sarcoma, treated with wide surgical resection followed by adjuvant radiotherapy, who remained disease-free for two years. She later presented with multifocal skeletal lesions. Biopsies taken from two distinct bone lesions demonstrated diffuse STAT6 positivity and the absence of SS18 rearrangement by molecular analysis. These features, confirmed by independent pathological reviews and a multidisciplinary tumor board, were consistent with a multifocal solitary fibrous tumor rather than metastatic disease. This rare case highlights the diagnostic complexity of multifocal skeletal lesions developing after primary cranial synovial sarcoma and emphasizes the importance of thorough pathological, molecular, and multidisciplinary evaluation.