A challenging diagnosis of Sézary syndrome without erythroderma and epidermotropism

Summary: Sézary syndrome (SS) is a rare, aggressive cutaneous T cell lymphoma involving the peripheral blood that is challenging to diagnose. It clinically presents as erythroderma that is often indurated and pruritic. It is usually evaluated by skin biopsy, which is frequently non-diagnostic. SS has a CD3+/CD4+ immunophenotype; the absence of CD2, CD3, CD5, and CD7 expression supports the diagnosis. Herein, we report a 92-year-old man who presented with an erythematous and morbilliform rash on the trunk extending down to his knees, which felt to represent a hypersensitivity eruption possibly. A punch biopsy of the right abdomen revealed a mild to moderate perivascular lymphocytic infiltrate with minimal epidermal involvement. The lymphocytes diffusely expressed CD3, CD4, CD5, and PD-1, without significant loss of CD7 expression. Only scattered CD8(+) lymphocytes were identified. PCR demonstrated clonal T-cell receptor gamma and beta gene rearrangements. Flow cytometry showed 85% CD4+/CD26(-) T cells (absolute count >2900), with clonal TRBC1, confirming a diagnosis of SS. This case highlights that SS can present without erythroderma or epidermotropism, requiring flow cytometry to confirm the diagnosis. This case also illustrates the value of immunohistochemistry (including PD1) in assessing the dermal lymphocytic infiltrate for CTCL.