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.