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Intramuscular juvenile xanthogranuloma in an adult: a case report with immunohistochemical study.

Juvenile xanthogranuloma (JXG) is a self-limited cutaneous lesion that most often occurs in infancy. Approximately 10% to 30% of JXGs occur in adult patients, where most cases present as cutaneous papulonodular lesions, with only rare cases involving extracutaneous sites. Intramuscular JXG is extremely rare and has received little attention. On review of the literature, all of the 6 previously reported intramuscular JXGs were noted in the pediatric population. The authors hereby describe a case of adult intramuscular JXG that occurred in a woman who initially had a dermal JXG in the nasal skin at the age of 48 years and then developed a slow-growing painless intramuscular JXG in the right forearm 4 years later. Both the dermal and intramuscular lesions revealed similar histologic features and consisted of diffuse infiltrates of histiocytes with eosinophilic and foamy cytoplasm, lymphocytes, eosinophils, and Touton giant cells in varying proportions. However, central fibrosis and a focal storiform arrangement of spindled fibroblast-like cells in the intramuscular lesion resulted in a histologic pattern reminiscent of a fibrous histiocytoma. Immunohistochemically, the intramuscular JXG was positive for CD68, factor XIIIa, CD31, and vimentin. This case underscores the fact that intramuscular JXG can also involve adult patients and its morphologic variation is more likely to be time dependent rather than site specific or age related.