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Renal Inflammatory Myofibroblastic Tumors (IMT) are rare. Withen the GU tract, they are more common in the bladder. Three histologic patterns have been described. 1) loosely organized spindle cells admixed with small blood vessels and inflammatory cells in a myxoid background, similar to nodular fasciitis. 2) spindle cell proliferation admixed with variable amounts of dense collagen and aggregates of lymphocytes, often forming follicles, and plasma cells. 3) hypocellular fibrous tissue with dense “keloid-like” fibrosis and sparse inflammatory cells. Nuclear atypia is usually not seen and mitosis are infrequent. This particular tumor demonstrated a phenotype predominantly of the 3rd (keloid-like) pattern with focal pattern 2 and 1 areas. Of note, in one of the largest studies on these tumors, increased ALK expression was not found via immunohistochemistry. Similar to their splenic and lymph node capsular counterparts, Renal IMTs may represent a non-neoplastic lesion or alternatively, a tumor with an activation mechanisms involving something other than the ALK translocation.
Kapusta Am J Surg Pathol 27(5): 658–666, 2003
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