Interstitial mycosis fungoides, a variant of mycosis fungoides resembling granuloma annulare and inflammatory morphea.

BACKGROUND Interstitial mycosis fungoides (IMF) is a rare variant of mycosis fungoides that resembles the interstitial form of granuloma annulare and inflammatory morphea. IMF has received little attention in the literature. METHODS Clinical, histological, immunophenotypical, and genotypical findings of five cases of IMF were reviewed. The histological and immunophenotypical findings were compared with those of eight cases of interstitial granuloma annulare and six cases of inflammatory morphea. RESULTS Five patients with IMF presented with non-indurated, erythematous macules; ill-defined erythematous plaques with slight scale; and nodules on the trunk and proximal limbs. Two of five patients had a prior diagnosis of mycosis fungoides. Skin biopsies revealed a striking dermal interstitial infiltrate of lymphocytes with rare histiocytes that resembled the interstitial form of granuloma annulare or inflammatory morphea. Epidermotropic lymphocytes were present at least focally in all cases. A band-like lymphocytic infiltrate was observed in two of five cases. In contrast, many plasma cells and histiocytes were observed in cases of inflammatory morphea and interstitial granuloma annulare, respectively. With Movat-pentachrome stains, increased dermal mucin deposition was observed in two of five IMF cases, in all cases of interstitial granuloma annulare, and in one of six cases of inflammatory morphea. There was focal loss of elastic fibers in all cases of inflammatory morphea. Immunohistochemical studies of IMF highlighted a dominant population of T cells (CD3+) in the dermis and epidermis. In contrast, moderate numbers of B cells (CD20+) were admixed with T cells and plasma cells in inflammatory morphea. Almost equal numbers of histiocytes (CD68+) and T cells comprised the infiltrate of interstitial granuloma annulare. In two of five IMF cases, a clonal T-cell population was detected by PCR T-cell gamma gene rearrangement analysis. CONCLUSION Mycosis fungoides occasionally presents as an interstitial lymphocytic infiltrate that mimics granuloma annulare and inflammatory morphea. Hematoxylin & eosin (H&E) findings alone can sometimes distinguish the three disorders. Immunophenotyping and genotyping may be helpful in difficult cases.