Retinoblastoma

Incidence is estimated as 1 in 18,000 - 30,000 live births. Diagnosis is usually made between birth and 3 years of age. The most common clinical presentation is leucokoria and "cats eye reflex". The tumour arises in the retina and may grow either under the retina causing retinal detachment or over the retina into the vitreous humour. It may then extend through the sclera into the orbital lymphatics or through the optic nerve intracranially. Cross-sectional imaging shows the tumour as an enhancing tissue mass and has both a mapping and a diagnostic task. Several conditions may mimic retinoblastoma, the most frequent being Coat's disease, retinopathy of prematurity, persistent hyperplastic vitreous humour and Toxocara canis endophthalmitis. Unlike all these conditions, retinoblastoma presents with calcifications which are thus an important element of differential diagnosis at least before 3 years of age (later other lesions may also occasionally present calcifications), and thus make CT the primary imaging modality (Fig.1). Calcification in retinoblastoma may be clumped or punctate. It should be noted that in the extraocular component of retinoblastoma calcification is rarely present. MRI of the orbit and brain is usually performed in most study protocols of suspected retinoblastoma because of its superiority in delineating optic nerve and intracranial extension as well as eventual associated pinealoblastoma in bilateral lesions. On MR retinoblastoma appears slightly or moderately hyperintense in relation to normal vitreous humour on T1- and PD-weighted images and markedly to moderately hypointense on T2-weighted images and these signal patterns are distinctive of retinoblastoma as compared to its main differential diagnosis. MR after contrast provides the highest sensitivity for small retinoblastoma lesions.

FS