Retina pathology of a failed external beam-radiated group Vb retinoblastoma

We report on a 7-month-old boy who suffered from retinoblastoma with presentation of a white pupillary reflex in his right eye. Initial examination showed a large subretinal and intraretinal mass nasally with extensive vitreous seeding of tumor cells (Group Vb in the Reese-Ellsworth classification). External beam radiation therapy (EBRT) was applied in hopes of preserving the eye, and significant regression with disappearance of vitreous seedings and a prominently decreased tumor mass with localized calcification were documented. Unfortunately total retinal detachment was subsequently identified 3 months after irradiation. The patient ultimately underwent enucleation, and histopathology revealed significant calcification within the residual tumor without marked necrosis; a preretinal fibrous membrane with focal vascular thickening was noted, which implied a partial but incomplete effect of EBRT for this group Vb retinoblastoma. We describe the histopathological findings of the failure of irradiation for a group Vb retinoblastoma, and emphasize the importance of early application of EBRT treatment for a retinoblastoma.