Abstract
Interdigitating dendritic cell sarcomas (IDCS) are rare and aggressive dendritic cell tumors which typically occur within lymph nodes. Extranodal IDCS is less common, and fewer than ten cases of primary cutaneous IDCS have been reported in the literature. IDCS present histologically as proliferations of atypical spindle cells with irregular nuclei, and thus may be difficult to distinguish from nodular melanoma, follicular dendritic cell sarcoma, and Langerhans cell tumors based on H&E examination alone. Herein, we present a case of an 88-year-old-male with recent resection of an invasive melanoma on the neck, who presented with a new growth on the left cheek. Physical exam showed a 1.5-centimeter erythematous nodule with central erosion. Histopathologic analysis with H&E showed an atypical dermal lymphohistiocytic infiltrate around blood vessels and adnexal structures, as well as atypical-appearing cells in the epidermis forming small nests along the dermal-epidermal junction. Immunohistochemical analysis showed strongly positive S100 staining, with negative SOX10 and Melan A staining, supporting a histiocytic proliferation. Fascin and lysozyme were also strongly positive, while CD1a, langerin, CD21, and CD23 were negative. This staining pattern supported the diagnosis of IDCS, while arguing against the diagnoses of Langerhans cell histiocytosis and follicular dendritic cell sarcoma. In light of this diagnosis, the patients prior biopsy showing invasive melanoma was revisited, and similar immunohistochemical analysis ultimately identified the lesion as cutaneous IDCS misdiagnosed as melanoma. Imaging with PET scan and MRI showed no definitive evidence of lymphadenopathy or metastasis. This case highlights an extremely rare cutaneous presentation of IDCS, as well as the potential pitfall of its misdiagnosis as melanoma, due to the overlapping H&E features and S100 positivity similar to melanoma.
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