Herein, we present a patient with a lipidized fibrous histiocytoma, an underrecognized variant of dermatofibroma (cutaneous fibrous histiocytoma). Our patient presented with a nodule on the ankle that showed foamy histiocytes and hyalinized collagen bundles on histology. This case highlights a classic presentation and features of lipidized fibrous histiocytoma, raising further awareness of this distinctive variant of dermatofibroma that should be distinguished from xanthoma and xanthogranuloma.

Eruptive collagenoma is a rare entity, with unknownetiology, considered to be a type of connective tissuenevus composed of collagen. It is usually reported inyoung adults occurring predominantly on the trunkand extremities. Systemic findings and family historyof a similar condition are not typically associated andthe prognosis is excellent. There are few pediatric casesreported in literature. Herein we report an uncommoncase of eruptive collagenoma in a 12-year-old childand present a brief review of the literature.

Non-Langerhans cell histiocytosis (NLCH) is a histiocyte disorder comprised of dermal dendritic histiocytes with a characteristic staining pattern. Erdheim-Chester disease (ECD) is a subset of NLCH in which patients experience bone pain with corresponding changes on imaging. In addition, these patients show other evidence of systemic involvement, which can also be identified with imaging. This disease can occasionally present with cutaneous findings. We present a case of generalized eruptive histiocytosis (GEH), misdiagnosed as ECD, found to have an NTRK1 gene rearrangement. This is the first report of an NTRK1 kinase fusion with NLCH. The implication is unclear and further studies are warranted.

Generalized eruptive histiocytosis (GEH) is an exceedingly rare type of Factor XIIIA-positive histiocytopathy, characterized by symmetric distribution of red-brown papules on the trunk, proximal extremities, and face. It is diagnosed by the integration of the clinical features, light microscopic findings, and phenotypic profile. Herein, we report an unusual presentation of GEH in a 23-year-old man, who had a striking papular and erythrodermic component to his lesions.