Abstract

Mycosis fungoides (MF) and granuloma annulare (GA) can clinically resemble one another, as both morphologically can manifest erythematous annular plaques and papules; however, their histopathologic features are distinct. Pathologically, while granulomatous MF is well-established, MF demonstrating granulomatous histopathology mimicking GA is rare. Herein we present three such cases: 

1. 71-year-old female with a nine-year history of MF stage IB developed new annular erythematous plaques on her back and right flank. Biopsies showed epidermotropic, small to medium-sized, CD8-positive atypical lymphocytic infiltrates, with associated predominant histiocytic inflammation in the superficial to mid dermis forming palisaded granulomas. Topical corticosteroids and natural sunlight have effectively kept her skin lesions stable over the past eight years. 

2. 70-year-old female with Fitzpatrick type 5 skin presented with hypopigmented patches on her inner thighs spreading to hips and buttocks. Biopsies showed an interstitial dermal infiltrate of small to medium-sized CD8-positive lymphocytes with mild atypia, loosely palisading in regions, and focally exhibiting epidermotropism. T-cell gene rearrangement studies (TGR) identified a monoclonal T-lymphocytic population.  Her condition has remained quiescent with topical corticosteroids. 

3. 86-year-old female presented with a six-month history of a pruritic erythematous papular eruption on her back. Biopsy showed an epidermotropic, lichenoid, and perivascular, predominantly CD4-positive-lymphocytic infiltrate and accompanying focally palisading granulomatous inflammation including occasional giant cells. TGR detected two dominant peaks. At six-month follow-up, the patient’s cutaneous disease was stable with topical corticosteroid therapy.

MF exhibiting GA-like histopathology highlights an under-recognized intersection between two classically distinct entities, underscoring careful clinicopathologic integration for accurate diagnosis.