Rare variants of solitary fibrous tumor.

Objective: Some cases of solitary fibrous tumor (SFT) exhibit unusual histologic features that may cause diagnostic difficulty, such as fascicular monotonous spindle cells accompanied by hyalinized blood vessels and numerous evenly distributed mast cells, and features mimicking myxoid liposarcoma. Awareness of these features is important for reaching correct diagnosis of similar cases.

Methods: Three cases of SFT with the above unusual features were retrieved from our consult files for review, including H&E slides and immunohistochemical stains. In addition, FISH analysis for SS18-SSX (SYT), DDIT3 and MDM2 were performed. Furthermore, formalin-fixed paraffin-embedded (FFPE) tissue sections were tested for 8 fusion variants of NAB2-STAT6 by qualitative endpoint reverse-transcriptase (RT)-PCR.

Results: Neoplastic cells from all 3 cases are positive for CD34, CD99, and STAT6 immunohistochemically. In addition, the tumors are positive for NAB2-STAT6 fusion gene. Mast cells from the first case possess nonneoplastic phenotype and are positive for CD117 and tryptase staining but negative for CD25.

Conclusions: The three cases studied here represent rare types of SFT, which differ from classical "pattern-less" pattern of SFT. Correct diagnosis required a combination of CD34 and STAT6 immunostaining and NAB2-STAT6 fusion gene analysis.