Rapidly Progressive Primary Cutaneous Gamma Delta T-Cell Lymphoma With FYN Gene Alteration.

Primary cutaneous gamma delta T-cell lymphoma (PCGDTCL) is a rare type of non-Hodgkin lymphoma accounting for <1% of primary cutaneous T-cell lymphomas. The exact cause of PCGDTCL is not known, however, it is thought that chronic antigen exposure in the skin may lead to immune dysregulation at the site, resulting in abnormal proliferation of mature, post-thymic cytotoxic gamma delta T cells. Mutations are the most common genetic alteration seen in PCGDTCL, while structural abnormalities such as gene fusions are not common. We report a case of PCGDTCL with atypical immunophenotypic features, including expression of CD5 with lack of cytotoxic marker expression, and a structural alteration leading to FYN deletion at exon 8. Recently, it was described that a deletion of the area between FYN exon 8 and TRAF3IP2 intron 2 results in a novel FYN::TRAF3IP2 fusion in peripheral T-cell lymphoma, not otherwise specified. We describe our patient's clinical course, differential diagnosis, and potential implications of FYN deletion on disease pathogenesis. To our knowledge, this is the first report of an FYN structural alteration to be described in PCGDTCL.