Prognostic value of SS18-SSX fusion type in synovial sarcoma; systematic review and meta-analysis

SS18-SSX (formerly called SYT-SSX) fusion gene has been established clinically as a molecular diagnostic test for synovial sarcoma, but the prognostic value of the fusion gene variant for survival is controversial. The objective of this systematic review is to provide an up-to-date and unprecedented summary of the prognostic impact of SS18-SSX fusion type in synovial sarcoma. Studies evaluating SS18-SSX fusion type as a prognostic marker in synovial sarcoma were systematically searched for in MEDLINE, EMBASE, and Web of Science. Comparative analysis of the pooled hazard ratios (HR) between fusion types was carried out, in order to assess the likelihood of overall survival (OS), disease-specific survival (DSS), progression-free survival (PFS), and metastasis-free survival (MFS). A total of 10 studies comprising 902 patients with synovial sarcoma were considered for the meta-analysis. The pooled HR for eight eligible studies evaluating for OS or DSS was 1.28 (95% confidence interval: 0.81-2.00), suggesting no significant difference between SS18-SSX1 and SS18-SSX2 (P = 0.29). For seven studies which evaluated for PFS or MFS, the presence of SS18-SSX1 may indicate a lower survival probability than that of SS18-SSX2, although the effect did not reach a level of statistical significance (P = 0.09). There was no significant difference in OS or DSS between SS18-SSX1 and SS18-SSX2, but there were indications of SS18-SSX1 being an unfavorable prognostic factor of PFS or MFS. Further studies including cohorts with a longer follow-up period are needed.