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Indeterminate dendritic cell tumor (IDCT) is a rare disease composed of so-called indeterminate cells, a dendritic cell subset displaying histological and some ultrastructural and immunophenotypic features of Langerhans cells, but lacking Birbeck granules. We report a case of cutaneous IDCT occurring in a patient with chronic myelomonocytic leukemia (CMML) successfully treated with UV-A phototherapy. Next-generation sequencing studies of the CMML demonstrated mutations in TET2, ASXL1, and ZRS2 genes, also detected in the IDCT, demonstrating a clonal relationship between both tumors and confirming IDCT as a specific subtype in the spectrum of CMML-related cutaneous lesions.

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Atypical fibroxanthoma is a rare mesenchymal skin tumor of intermediate malignancy that typically occurs on sun-damaged skin of elderly patients. Histologically, it is composed of pleomorphic cells with hyperchromatic nuclei and abundant cytoplasm, commonly arranged in a spindle cell pattern. Different histologic variants have been described during the past years. We present a case of atypical fibroxanthoma containing a dense inflammatory infiltrate, which in conjunction with the existence of immunoblast-like and Reed-Sternberg-like neoplastic cells could be misinterpreted as a lymphoid neoplasm. Immunohistochemical studies revealed strong positivity of tumor cells for CD10 and negativity for cytokeratins, p63, p40, S100, SOX10, ERG, actin, desmin, B and T-cell markers, BCL6, CD15, and CD30. The inflammatory infiltrate contained a mixed reactive T- and B-cell population with negative T-cell receptor and immunoglobulin heavy rearrangements. We discuss the differential diagnosis of this entity in which clinical, immunohistochemical, and molecular features are essential to avoid the diagnosis of a lymphoproliferative disease.