Primary cutaneous gamma‐delta T‐cell lymphoma masquerading as leukemia cutis in a patient recently diagnosed with small lymphocytic lymphoma: Clues to the diagnosis

A 54‐year‐old man recently diagnosed with small lymphocytic lymphoma (SLL) had waxing and waning, indurated, erythematous plaques on his legs, with leukopenia and anemia disproportionate to the SLL burden in his marrow and pelvic lymph nodes. Punch biopsy of a plaque performed to evaluate for leukemia cutis revealed a lymphocytic lobular‐panniculitis‐like infiltrate resembling lupus panniculitis, but a preponderance of CD8+/Ki‐67+ T‐cells surrounding adipocytes raised concern for subcutaneous panniculitis‐like T‐cell lymphoma (SPTCL). Additional immunohistochemistry (IHC) studies showed that the adipotropic T‐cells expressed TCR‐gamma, supporting the rare, unexpected diagnosis of Primary cutaneous gamma‐delta T‐cell lymphoma (PCGDTCL). The patient subsequently met diagnostic criteria for hemophagocytic lymphohistiocytosis (HLH). PCGDTCL is an aggressive, HLH‐associated lymphoma requiring different management than SPTCL and SLL. This case illustrates how PCGDTCL can co‐exist with B‐cell lymphoma and resemble panniculitis on biopsies. PCGDTCL and SPTCL should enter the differential diagnosis whenever patients present with the constellation of lobular panniculitis and unexplained cytopenias. In the present case, close clinicopathologic correlation and judicious use of IHC on a small sample allowed for a prompt diagnosis.