A Case of Adenocarcinoma of the Retinal Pigment Epithelium: An Immunohistochemical and Electron Microscopic Study.

Purpose Retinal pigment epithelium (RPE) adenocarcinoma is a very rare malignant intraocular tumor. Herein we describe the histopathological features of RPE adenocarcinoma. Case A 36-year-old male was referred to our clinic because of floaters in his left eye. The initial diagnosis was malignant melanoma of the choroid. We resected the tumor and studied it histopathologically. The tumor tissue was investigated by light microscopy including immunohistochemistry using antibodies against S-100, HMB-45, EMA, and AE-1. Electron microscopic examination was also performed. Results The tumor arose from the RPE and contained intracytoplasmic vacuoles and abundant melanin pigment. There were no nevoid cells in the choroid. A small part of the tumor cells showed tubular or lobular proliferation and choroidal invasion. Immunohistochemistry revealed positive staining in tumor cells with 4 antibodies. Tight cellular junctions specific to the RPE were confirmed by electron microscopy. The final diagnosis was RPE adenocarcinoma. Conclusions Most pigmented intraocular tumors are nevus and malignant melanomas of the choroid. It is easy to misdiagnose a RPE adenocarcinoma as a malignant melanoma of the choroid. An exact differential diagnosis should be determined by immunohistopathological and electron microscopic examination.