Reconsidering the Diagnostic and Prognostic Utility of LN-2 for Undifferentiated Pleomorphic Sarcoma and Atypical Fibroxanthoma

The topic of distinguishing atypical fibroxanthoma (AFX) from undifferentiated pleomorphic sarcoma (UPS), formerly malignant fibrous histiocytoma, is highly controversial. Although their clinical behavior is disparate, AFX and UPS commonly appear nearly identical on routine histopathologic examination. Although conceptually useful, subcategorization of UPS into superficial (confined to the dermis and subcutaneous tissue) and deep (involvement of fascia and deeper structures) types has not improved our ability to differentiate UPS from AFX. Numerous authors have purported LN-2 (CD74) immunopositivity as able to distinguish UPS from AFX and to predict those rare AFX likely to behave aggressively, although only a single prior study has been dedicated to evaluating this marker. We performed LN-2 staining of 14 AFX, 8 superficial UPS, and 65 deep UPS specimens using an identical protocol as described by prior authors. Of the 73 total UPS specimens, only 1 (1.4%) stained strongly with LN-2, as compared with 3 of 14 (21%) AFX (P = 0.012). One of 2 (50%) clinically aggressive AFX tumors that later exhibited both local recurrence and metastasis stained strongly for LN-2, whereas 2 of 12 (17%) of the more indolent tumors stained strongly with this marker (P = 0.40). Our data do not replicate prior reports of LN-2 as a sensitive and specific marker for UPS, or as indicative of prognosis for AFX, and therefore does not support the use of LN-2 as either a diagnostic or prognostic marker.