Solitary Indeterminate Cell Histiocytosis Presenting as a Reddish Nodule on the Abdomen of a 4‐Year‐Old Boy: Clinicopathologic Correlation and Electron Microscopy Findings

Indeterminate cell histiocytosis (ICH) is a clonal proliferative disorder of mononuclear phagocyte cells that shows features of both dendritic and histiocytic cells. Only around 100 cases of ICH were reported, and among them, only 24% of reported cases involve pediatric patients, with a mean age at diagnosis of 50 years. We report an exceptionally rare case of solitary ICH in a 4‐year‐old boy. A 4‐year‐old visited our department presenting a solitary asymptomatic bean‐sized reddish nodule on upper abdomen for 1 year. Initially thought to be keloid or dermatofibroma, an excisional biopsy was performed. Histopathological examination revealed dense cellular infiltration in the dermis, characterized by medium‐sized mononuclear histiocytes displaying abundant glassy eosinophilic cytoplasm and indented or folded nuclei. Immunohistochemistry studies confirmed positivity for S‐100 protein and CD1a, coupled with negativity for CD207/Langerin. Electron microscopy revealed the absence of Birbeck granules, leading to the diagnosis of ICH. With no remarkable findings in laboratory and imaging studies, the patient is undergoing regular follow‐up to monitor any potential recurrence or development of related conditions. While most cases of skin‐restricted ICH follow an indolent course, recognizing the reported association between ICH and other hematologic neoplasms is crucial. Therefore, awareness of ICH is essential, and in cases of suspicion, a skin biopsy should be performed for an accurate diagnosis.