Localized episcleral ALK-positive histiocytosis: a novel adult presentation confirmed by ALK gene rearrangement.

We report the first documented case of isolated episcleral ALK-positive histiocytosis (ALK+H) in an adult. A 36-year-old woman presented with an episcleral nodule clinically mimicking nodular scleritis. Excisional biopsy revealed sheets of foamy CD68+/CD163+ histiocytes with Touton-type giant cells. Immunohistochemistry demonstrated strong cytoplasmic ALK1 expression with negative CD1a and BRAF V600E, and fluorescence in situ hybridization confirmed ALK gene rearrangement in 60% of nuclei. Whole-body PET-CT identified a suspicious inguinal node, subsequently proven reactive on biopsy. Following excision, symptomatic local recurrence developed within 3 months, confirmed by orbital MRI. Alectinib 600 mg twice daily was commenced, with symptomatic improvement after 2 months. This case expands the anatomic spectrum of ALK+H to the ocular surface, highlighting that routine ALK1 screening of atypical histiocytic infiltrates is essential for accurate diagnosis and access to targeted therapy.