Proliferative Clonal T-Cell Infiltrate Mimicking a Cutaneous T-Cell Lymphoma Arising in Active Regression of Melanoma.

Complete melanoma regression is an uncommon phenomenon involving a complex interplay of the tumor microenvironment and host immune response. We report a case of an 84-year-old woman with a history of colon and breast cancers who presented with a right forearm tumor, which was found to be a nodular melanoma; focal features of regression were noted in the biopsy. Approximately 6 weeks later, surgical resection of the site revealed no gross evidence of tumor, and histologic sections showed an extensive lymphoid infiltrate with prominent epidermotropism. Rare residual melanoma cells were present in the dermis, best visualized on immunohistochemical stains. T cells predominated in the infiltrate with an inverted CD4:CD8 ratio at approximately 1:2. There was no appreciable loss of pan‒T-cell antigens. T-cell receptor beta and gamma gene rearrangements were performed by polymerase chain reaction and demonstrated clonality in each assay. Although a synchronous cutaneous T-cell lymphoma was considered, the overall clinicopathologic features are more in line with an exaggerated host immune response leading to near complete regression of the tumor.