Cutaneous histiocytic malignancy. Immunohistochemical re‐examination of cases previously diagnosed as cutaneous “histiocytic lymphoma” and “malignant histiocytosis”

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Abstract
True histiocytic lymphoma (THL) and malignant histiocytosis (MH) have been defined by clinical and histologic findings and enzyme histochemistry. We reviewed cases previously diagnosed as cutaneous histiocytic lymphoma (HL) and MH with cutaneous lesions. These cases had been classified as “histiocytic” on the basis of previous enzyme histochemistry profiles of frozen tissue. Cutaneous tumor cells were reevaluated using a panel of immunohistochemical stains in formalin‐fixed, paraffin‐embedded tissue in correlation with histopathologic examination. The antibodies used in this study were directed against CD45 (leukocyte common antigen [LCA]), CD20 (L26) for B cells, CDS and CD45RO (UCHL‐1) for T cells, CD68 (KP‐1) and lysozyme for histiocytes, as well as CD30 (BerH2) for Ki‐1 positive cells. On re‐evaluation, the seven cases originally classified as HL were reclassified as one case of THL with neoplastic cells positive for CD68 (KP‐1) and lysozyme, two cases with immunohistochemical features of Ki‐1 lymphoma (including one of T‐cell lineage), three cases of T‐cell lymphoma, and one case of B‐cell lymphoma, all associated with variable degrees of reactive histiocytosis. The four cases originally classified as MH were reclassified as two cases of MH and two cases of uncertain lineage. Although rare, histiocytic malignancies do exist. However, the diagnosis of histiocytic malignancy should be made only after careful correlation of atypical tumor cells in histopathologic sections and sections stained immunohistochemically. Erroneous classification of reactive histiocytes as neoplastic histiocytes using only enzyme histochemistry in frozen sections is a pitfall to be avoided.