Retinoblastoma is the most common intraocular malignancy of childhood (melanoma is the most common in adults). The prevelance is about 1/16,000 live births.
Aetiology and Pathogenesis
About 40% of retinoblastomas occur due to germline mutation of the RB1 gene. The majority of these patients present with bilateral (or trilateral) retinoblastoma but about 25% present with unilateral disease. Children with germline mutation of RB1 tend to present at an earlier age.
The disease often spreads within the orbit. Bilateral disease and multifocal disease occurs with germline mutation of RB1. Local destruction of the eyeball and spread into the orbit can occur. Metastatic disease develops within 4 months of diagnosis and is fatal within 1 year.
The most common presenting finding is leucocoria which is an abnormal white reflex when light is shone into the eye. About 20% of patients present with strabismus. The size of the retinoblastoma is often given by its relation to the optic disc. Imaging demonstrates an intraocular mass, often with calcification. MRI of the orbit and brain is essential to rule out trilateral disease (bilateral retinoblastoma with associated primitive neuroectodermal tumour of the pineal or suprasellar region).
Tumour/Normal Tissue Features
The tumour is typically white to grey. The tumour may exhibit endophytic (growth into the eyeball layers) or exophytic growth (growth into the vitreous). Seeding can occur throughout the vitreous. Retinal detachment may also be seen.
The tumour is a small round blue cell tumour. Well differentiated areas form Flexner-Wintersteiner rosettes around a central core.
Staging / Classification
Retinoblastoma is staged according to an International Classification:
- A: < 3 mm in size, solitary tumour
- B: > 3 mm in size, or located close to the optic disc or macula (< 3 mm)
- C: Vitreous seeds < 3 mm from tumour
- D: Vitreous seeds > 3 mm from tumour
- E: Extensive tumour (50% of globe), glaucoma, or invasion of choroid or optic nerve