Synovial sarcoma with extensive calcification and ossification is a rare variant, the ultrastructural, cytogenetic and molecular analysis of which has not been reported previously.

A large mass in the shoulder of a 20‐year‐old male patient led to a deformity of the chest wall, thus supporting the hypothesis that this is a slowly growing variant of synovial sarcoma. Nevertheless, the patient developed metastatic lung disease 7 months after resection. On histology, the monophasic spindle cell proliferation was in several areas obscured by the massive calcification and ossification. Immunohistochemistry showed keratin, epithelial membrane antigen, vimentin and CD99 expression. The cytogenetic analysis revealed a single t(X;18)(p11.2; q11.2), typical for synovial sarcoma. Additional fluorescence in‐situ hybridization revealed SSX2 involvement. At the ultrastructural level, prominent needle‐shaped intramitochondrial crystals were present, both in the cytoplasm and in the extracellular matrix.

The presence of the t(X;18) with SSX2 involvement definitively characterizes this tumour as a variant of synovial sarcoma. In addition, the needle‐like mitochondrial calcifications give a possible clue to the pathogenesis of the extensive metaplastic ossification and calcification.