Synovial sarcoma in knee joint, mimicking low-grade sarcoma confirmed by molecular detection of SYT gene split.

A 10-year-old boy underwent arthroscopic curettage for an intra-articular mass in knee joint. The tumor was diagnosed as low-grade fibrous sarcoma. Five years later, the patient presented with a recurrent tumor. The patient underwent a marginal excision with knee joint preservation and without adjuvant therapy. Two years after the last surgery, the patient is thriving with no evidence of recurrent or metastatic disease. The final diagnosis was synovial sarcoma confirmed via a SYT gene split performed with fluorescent in situ hybridization (FISH), although the tumor appeared as a low-grade fibrous type in a hematoxylin-eosin section. The first curetted specimen was also confirmed to bear a SYT gene split. Synovial sarcoma has been conventionally recognized as a high-grade sarcoma. Our patient had a tumor that exhibited the characteristics of both a histologically and clinically low-grade tumor. From the present case, we consider that low-grade variants of synovial sarcoma do exist although their existence remains controversial.