Identification of poorly differentiated synovial sarcoma: a comparison of clinicopathological and cytogenetic features with those of typical synovial sarcoma

AIMS: Poorly differentiated areas in synovial sarcomas (SS) are known to be associated with a poorer prognosis. The aim of our study was to describe the morphological spectrum of poorly differentiated synovial sarcomas (PDSS) and refine the criteria for their recognition. METHODS AND RESULTS: The clinicopathological features of 28 PDSS were compared with those of 26 classic SS. Common cell types in PDSS included epithelioid, spindle and Ewing sarcoma-like small round cells. Unusual features included presence of desmoplastic small cell tumour-like areas and extraskeletal myxoid chondrosarcoma-like areas. The presence of necrosis (P = 0.002), a mitotic rate over 10/10 high-power fields (P < 0.001), a haemangiopericytomatous vascular pattern (P < 0.001) and vascular invasion (P = 0.003) were significantly associated with PDSS, while mast cells (P < 0.001), calcification (P < 0.001) and hyaline bands (P < 0.001) were significantly associated with classic SS. Poorly differentiated areas showed increased proliferative activity with Ki67. PDSS showed a tendency to be larger (P = 0.008) and to be located in proximal more than distal sites (P = 0.025). Three entirely poorly differentiated tumours were diagnosed by demonstration of the t(X;18)(p11;q11) translocation. PDSS showed additional cytogenetic abnormalities. CONCLUSIONS: Poorly differentiated synovial sarcomas show a spectrum of histological features, which may simulate other malignant neoplasms. The diagnosis of entirely poorly differentiated synovial sarcomas requires cytogenetic analysis.