It’s gastric and spindled, but it’s not a GIST… Plexiform fibromyxoma is a benign tumor that presents as a multinodular intramural mass in the stomach. The nodules can be fibrous or myxoid and have a delicate capillary pattern. #BSTPath, #UmichPath, #BSTpathGoBlue

Dealing with an unusual fibromyxoid spindle cell neoplasm (that is MUC4 negative)? Consider PLAG1! #BSTPath, #UmichPath, #BSTpathGoBlue

Interestingly, the cells in this case also showed staining for OCT2, which has historically been associated with other histiocytic-type proliferations, such as Rosai-Dorfman disease, some Langerhans cell histiocytosis, and juvenile xanthogranulomas.

ALK-positive histiocytosis of the breast is a rare tumor that can mimic other inflammatory neoplasms or processes, such as inflammatory myofibroblastic tumor, cellular spindled histiocytic pseudotumor (among others). #BSTPath, #UmichPath, #BSTpathGoBlue

Although not typically associated with a “round cell sarcoma” differential, solitary fibrous tumor can sometimes mimic small round cell type sarcomas such as Ewing sarcoma. #BSTPath, #UmichPath, #BSTpathGoBlue

Cellular spindled histiocytic pseudotumor can sometimes be confused for inflammatory myofibroblastic tumor (or other spindled cell neoplasms). The spindled cells exhibit irregular grooved nuclei which suggest the tumor’s histiocytic nature. #BSTPath, #UmichPath, #BSTpathGoBlue

Of note, familiarity with your lab’s beta catenin stain is important as this stain can have variable sensitivity/specificity for desmoid fibromatosis (this case was negative for nuclear staining). Sequencing for CTNNB1 can be helpful when the staining pattern is suspect.

Desmoid fibromatosis can sometimes exhibit histologic features which overlap with nodular fasciitis, including focal areas of a more haphazard spindle cell arrangement. #BSTpath, #UMichPath, #BSTpathGoBlue