A 72-year-old woman had generalized cutaneous nodules and multiple erythematous lesions. A histopathologic examination revealed the infiltration of tumor cells expressing T-cell receptor γ (TCRγ) into the dermis without epidermotropism or angiocentricity, demonstrating a diagnosis of primary cutaneous γ/δ T-cell lymphoma (PCGD-TCL) (panels A and B; original magnification ×40 and ×400, respectively; hematoxylin and eosin stain). Her disease was controlled with local irradiation and electron beam therapy, combination chemotherapies, and mogamulizumab for >2 years but eventually advanced. Medium- to large-sized abnormal cells bearing clumped to dispersed chromatin and a basophilic cytoplasm were detected in the peripheral blood (PB; 0.047-42.2 × 10⁹/L). Some showed vacuolation and a convoluted nucleus (panels C-G; original magnification ×600; May-Grünwald Giemsa stain). They subsequently proliferated in the bone marrow (BM) (panels H, red arrowheads, and I-K; original magnification ×400 for panels H-K; stains: May-Grünwald Giemsa [H], hematoxylin and eosin [I], anti-TCRβ antibody [J], and anti-TCRγ antibody [K]). Their immunophenotype by flow cytometry was CD3⁺/CD2⁺/CD5⁺/CD4⁻/CD8⁻/CD7⁻/CD56⁻/TCRγ/δ-1⁺.

PCGD-TCL accounts for only 1% of all cutaneous T-cell lymphomas. It occurs in adults and has an aggressive clinical course with B symptoms. Its 2 major cutaneous infiltration patterns are (1) the epidermis and perivascular in the upper dermis (mycosis fungoides-like) and (2) the deep dermis to subcutaneous tissue (panniculitis-like). These lesions often disseminate to extranodal sites. Their involvement in PB is occasionally detected using flow cytometry, but BM, lymph nodes, and spleen are rarely involved.