Abstract

Indeterminate dendritic cell tumors (IDCTs) are rare neoplasms with phenotypes similar to indeterminate cells, thought to be precursors of Langerhans cells. Although this neoplasm immunophenotypically and morphologically resembles Langerhans cells, it is distinguished by the lack of Langerin expression and the absence of Birbeck granules on electron microscopy. Here we present an unusual S100 negative variant of IDCT. A 62-year-old female presented with a pruritic rash that began on her face and spread to cover her neck, trunk, and bilateral upper/lower extremities. Her history was significant for anal squamous cell carcinoma s/p chemoradiation. Skin examination of the trunk and upper extremities revealed pink, round papules, with a few showing hemorrhagic crusts. In contrast, the lower extremities showed dark erythematous to violaceous round, occasionally umbilicated papules. Punch biopsies taken from the upper extremities revealed an atypical mononuclear infiltrate diffusely involving the dermis. It was composed of atypical cells with ovoid, indented to folded nuclei with vesicular chromatin, inconspicuous nucleoli, and abundant eosinophilic cytoplasm with scattered mitotic activity. There was a background of scant lymphocytic infiltrate but no eosinophils. The atypical cells were positive for CD1a, CD45, CD4, CD33, CD68 (dim), CD163 (partial) and CD56. They were negative for Langerin, S-100, CD123, CD23, CD34, CD117, lysozyme, MPO, and CD15. The differential included involvement by leukemia cutis, but a bone marrow biopsy was within normal limits. Next-generation sequencing found an ETV3-NCOA2 fusion supporting the diagnosis of IDCT. This case adds to the small number of documented cases of S100 negative IDCTs.

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