DP06 Decoding histology: γδ T-cell lymphoma vs. tumour-stage mycosis fungoides

Abstract We describe the case of a 78-year-old accountant, with a background of rectal and bladder cancer, who presented with a history of rapid evolution of widespread cutaneous lesions. These started as red-to-purplish plaques and nodules on the arms, spreading to the trunk and lower limbs, enlarging rapidly to dermal lumps 5–6 cm in diameter. A trial of flucloxacillin produced no improvement, and an urgent ultrasound revealed nonspecific skin thickening, prompting a skin biopsy. Histopathological examination demonstrated a thin epidermis with basal vacuolation and a dense dermal infiltrate predominantly composed of small lymphocytes mixed with histiocytes. Immunohistochemistry confirmed the atypical lymphoid infiltrate to be a T-cell phenotype with CD3+, CD4+, CD8+, CD7+. Notably, CD8+ cells lacked CD5 and CD2 while retaining CD7, with a Ki-67 proliferation index of approximately 30%. The features were consistent with cutaneous T-cell lymphoma, possibly representing CD8+ mycosis fungoides (tumoral stage) or γδ T-cell lymphoma. Clonality studies were awaited and skin-directed therapies commenced, pending an opinion from the supraspecialist multidisciplinary team. Lymphocytes were negative for T-cell receptor (TCR)-β, raising the possibility of a clonal proliferation of mature, activated γδ T cells. The presence of JAK3 and MAPK1 mutations along with rearrangements of TCR-γ and TCR-δ T-cell receptors in conjunction with the morphological impression confirmed the diagnosis of primary cutaneous γδ T-cell lymphoma (PCGDTCL). Staging computed tomography scans revealed no systemic involvement. An excellent response was achieved with total-skin electron beam therapy, with complete clearance. PCGDTCL is a rare entity, representing < 1% of all cutaneous lymphomas, with a cumulative incidence of 0.4 per 10 million individuals. It is characterized by rapidly progressing, disseminated plaques, ulceronecrotic nodules, and tumours on the extremities, often accompanied by B symptoms. Although involvement of lymph nodes, spleen or bone marrow is uncommon, this aggressive lymphoma may occasionally be complicated by haemophagocytic lymphohistiocytosis with panniculitis-like tumours. The clinical and histopathological overlap with tumour-stage MF, the primary differential in this setting, posed a significant diagnostic challenge, influencing appropriate management options. In this case, the absence of systemic involvement on imaging and the atypical immuno­phenotypic features underscore the complexity of distinguishing between tumour-stage MF and PCGDTCL. This case highlights the role of detailed histopathological evaluation of rare cutaneous lymphomas and emphasizes the need for a multidisciplinary approach to design both local and systemic treatment strategies for individual patients.