Phakomatous choristoma of the orbit with inferior oblique muscle involvement

Purpose: We report a case of phakomatous choristoma presenting as an orbital tumor with involvement of the inferior oblique muscle. Case summary: A 2-month-old male infant presented to our clinic with a right orbital mass that had been present since birth. Magnetic resonance imaging demonstrated a homogenous enhanced well-defined mass located in the inferomedial portion of the right orbit without bone erosion. By transconjunctival orbitotomy, the orbital tumor invading the inferior oblique muscle was identified and resected. Histopathology showed a thick basement membrane surrounding pseudoglandular structures embedded in a collagenous stroma, psammomatous calcific foci in the stroma, and eosinophilic material in the lumen. Immunohistochemical analysis showed positive staining for S-100 and cytokeratin. On histopathological evaluation, the tumor was diagnosed as phakomatous choristoma. Conclusions: To our knowledge, this is the first report in South Korea of phakomatous choristoma of the orbit with involvement of the inferior oblique muscle. Although rare, phakomatous choristoma should be included in the differential diagnosis of tumors occurring on the inferomedial side of the orbit.