RESUMEN
Synovial sarcoma (SS) is a rare mesenchymal tumor, accounting less than 10% of soft tissue sarcomas. We report a case of intraarticular SS mimicking nodular synovitis and lateral meniscus rupture. Due to clinical and radiological presentation, arthroscopic synovectomy was performed, and histology confirmed nodular synovitis. After four years the lesion recurred and new arthroscopic biopsy was performed, revealing a monophasic SS with SYT/SSX1 translocation. Repeated histology of the first specimen confirmed appearance of a nodular synovitis microscopically, with no morphological criteria for a sarcoma, but molecular analysis showed positive SYT/SSX1 translocation. Wide extraarticular knee resection and reconstruction with a tumor megaendoprosthesis-allograft composite was performed with a negative tumor margins. This case report showed that in a case of benign histological appearance, underlying sarcoma is possible and could be identified in early stages only with an advanced pathology methods. LEVEL OF EVIDENCE: Level IV historical case.