[Fibroma of tendon sheath: a clinicopathological and genetic analysis of 134 cases]

Objective: To investigate the clinicopathological features, immunophenotypes and molecular genetics of fibroma of tendon sheath (FTS). Methods: One hundred and thirty-four cases of FTS or tenosynovial fibroma diagnosed in the Department of Pathology, West China Hospital, Sichuan University, Chengdu, China from January 2008 to April 2019 were selected. The clinical and histologic features of these cases were retrospectively reviewed. Immunohistochemistry, fluorescence in situ hybridization (FISH) and reverse transcription-polymerase chain reaction (RT-PCR) were performed on the above cases. Results: There were a total of 134 cases of FTS, including 67 males and 67 females. The patients' median age was 38 years (ranged from 2 to 85 years). The median tumor size was 1.8 cm (ranged from 0.1 to 6.8 cm). The most common site was the upper extremity (76/134, 57%). Follow-up data was available in 28 cases and there was no detectable recurrence. Classic FTS (114 cases) were well-defined and hypocellular. A few spindle-shaped fibroblasts were scattered in the dense collagenous sclerotic stroma. Characteristically elongated slit-like spaces or thin-walled vessels were observed. Most of cellular FTSs (20 cases) were well-defined and the area with increased cellularity of the spindle cells coexisted with classic FTS. There were occasional mitotic figures, but no atypical mitotic figures. Immunohistochemistry was performed in 8 cases of classic FTS and most cases were positive for SMA (5/8). Immunohistochemistry was also performed in 13 cases of cellular FTS and showed 100% positive rate for SMA. FISH was conducted on 20 cases of cellular FTS and 32 cases of classical FTS. USP6 gene rearrangement was found in 11/20 of cellular FTS. Among 12 cases of CFTS with nodular fasciitis (NF)-like morphological feature, 7 cases showed USP6 gene rearrangement. The rearrangement proportion of USP6 gene in cellular FTS without NF-like morphological features was 4/8. By contrast, 3% (1/32) of the classic FTS showed USP6 gene rearrangement. RT-PCR was performed in those cases with detected USP6 gene rearrangement and sufficient tissue samples for RT-PCR. The MYH9-USP6 fusion gene was detected in 1 case (1/8) of the cellular FTSs, while no target fusion partner was detected in the classic FTS. Conclusions: FTS is a relatively rare benign fibroblastic or myofibroblastic tumor. Our study and recent literature find that some of the classic FTS also show USP6 gene rearrangements, suggesting that classical FTS and cellular FTS are likely to be at different stages of the same disease (spectrum). FISH for USP6 gene rearrangement may be used as an important auxiliary diagnostic tool in distinguishing FTS from other tumors.