Answer: D - Solitary Fibrous Tumor 

The tumor reveals a low-grade spindle cell tumor with myxoid background, branching vessels and haphazardly arranged tumor cells. There is focal extension into adipose tissue at the periphery of the mass. No necrosis or prominent/atypical mitosis is noted. The tumor shows expression for CD34 (strong), Claudin-1 (focal) and STAT6. All other markers (S100, EMA, CD31, ERG, Desmin and MUC4) are negative. NAB2-STAT6 fusion is detected in the tumor, supporting the diagnosis of solitary fibrous tumor (SFT).

SFTs can show variable morphology depending on the relative proportion of cells and fibrous stroma. Cells can form short fascicles or no specific arrangement (pattern-less pattern). The stroma usually contains prominent vasculature of variable size, often with a characteristic staghorn appearance. Abundant collagen bundles are common, surrounding the vessels and extending into the interstitium. Myxoid change can be seen seen and may mimic myxoid liposarcoma; however, presence of thick-walled and staghorn vessels in SFT is helpful in the differential diagnosis. Presence of nuclear expression of STAT6 and NAB2-STAT6 fusion are diagnostic for SFT.

Spindle cell lipoma can show myxoid change and strong CD34 expression, but the location of current tumor and STAT6 expression do not support a spindle cell lipoma.

PNSTs (neurofibroma and perineurioma) are among the differential diagnoses (especially with Claudin1 expression); however, lack of S100 and EMA expression do not support PNST.

Hemangioendotheliomas commonly reveal intracellular vacuoles and express CD31 and ERG, both absent in the current tumor.

Reference:

Enzinger and Weiss’s Soft Tissue Tumors.7th ed. JR Goldblum, AL Folpe, and SW Weiss, eds. Philadelphia, PA:  Elsevier,  2020