46-year-old woman. Excisional biopsy of subcutaneous tumor of the chest wall.  
 
HISTO: encapsulated tumor in the subcutis fat tissue, with prominent storiform pattern, composed of wavy spidle cells with monomorphic elongated nuclei and eosinophilic or retracted/clear cytoplasm. Vaguely nodular zonation with intervening more fibrotic zones. Mitotic count - 22 / 50 HPF.  
Focus in the periphery of the tumor with hyalinized stroma and scattered multinucleated foreign body giant cells due to prior intervention/unsuccessful attempt to excise the tumor.  
 
IH:  
•Positive: TLE1+ (diffuse, strong), ERG+ (diffuse, weak), Beta-Catenin+ (diffuse, weak), CD99+ (diffuse, modetare), EMA+/- (focal, weak), DOG1-/+ (focal, weak).  
•Negative: PanCK; Asm Actin/Desmin; CD34; CD68; CD117; CD10; ALK; PanTRK; S100; SATB2; MDM2; MUC4; STAT6; CD21/CD23; CD56; CD57; FXIIIa.  
•Ki67 proliferative index up to 5%.  
 
PROPOSED DIAGNOSIS: Deep benign fibrous histiocytoma with high mitotic count vs other fibrohistiocytic tumor.  
 
UPDATE: NGS showed YAP1-KMT2A t(11;11)(q22;q23) rearrangement. CORRECTED proposed diagnosis is Low-grade fibromyxoid sarcoma VS Sclerosing epithelioid fibrosarcoma (MUC4(-)).  
 

hurwitz 2021-01-21 16:35

There has been a delay for technical reasons on our part. Meanwhile the case has been solved by the submitter. Since it is easier to recognize, I am copying the data given be the submitter in the section clinical data:     UPDATE: NGS showed YAP1-KMT2A t(11;11)(q22;q23) rearrangement. CORRECTED proposed diagnosis is Low-grade fibromyxoid sarcoma VS Sclerosing epithelioid fibrosarcoma (MUC4(-)).