Coexistence of patch stage mycosis fungoides and interstitial granuloma annulare in the same patient: a pitfall in the clinicopathologic diagnosis of mycosis fungoides.

Several clinical and histopathologic variants of mycosis fungoides (MF) have been well described, including the often elusive interstitial MF. Differentiation from other inflammatory disorders, such as interstitial granuloma annulare (GA) and inflammatory morphea, may be extremely difficult. We report a case of MF and GA coexisting in a 54-year-old woman who initially presented to clinic in 2000 with slightly scaly patches on the trunk and extremities, histopathologically diagnostic of MF. A second biopsy taken a few months later revealed an interstitial infiltrate that was initially interpreted as interstitial MF. Over the following 10 years, additional biopsies revealed features of conventional MF. In 2009, a new biopsy showed unequivocal features of interstitial GA. Reevaluation of the original biopsy, diagnostic of "interstitial MF," revealed that this, too, could be better classified as interstitial GA than interstitial MF. Our case illustrates that MF and interstitial GA may coexist simultaneously, thus representing a pitfall in the histopathologic diagnosis of MF. Given the similarities in clinicopathologic presentation, dermatologists and dermatopathologists should be cautious not to inadvertently misinterpret GA as interstitial MF.