Interstitial mycosis fungoid: a rare variant of mycosis fungoids. Two cases

Mycosis fungoids can present with various clinical and histological features, with only a few of them being recognized as distinct entities in the current WHO and EORTC classifications. Histologically, mycosis fungoids (MF) usually show a superficial perivascular or band-like lymphocytic infiltrate with epidermotropism. We here report two cases of a rare histological variant of MF, called interstitial in the literature. Our first patient, a 71-year-old male, had a previously diagnosed MF, which clinically evolved towards nodules, showing histologically an interstitial lymphocytic infiltrate without epidermotropism and without large cell transformation. The second patient was a 64-year-old female with widespread plaques and nodules. Histologically, a dense dermal interstitial infiltrate was observed, with foci of epidermotropism, without large cell transformation. At relapse after treatment, she presented with plaques, papules and nodules, histologically showing a slight interstitial lymphocytic infiltrate that resembled granuloma annulare or inflammatory morphea. In both patients, clinical aspect suggested MF and a dominant T-cell clone was found in lesional skin. Nodules in MF are not always the hallmark of large cell transformation, but may correspond to unusual interstitial lesions. Diagnosis of such rare variant may be difficult and requires a good clinical pathological correlation together with the search for foci of epidermotropism on skin biopsy and for a dominant cutaneous T-cell clone.