Retinoblastoma is rare, but nevertheless the most common intraocular malignant tumour of childhood, clinically presenting as a white mass behind the lens, resulting in the so-called cat's eye reflex. Vision in the eye is impaired, leading to strabismus. The tumour affects children younger than 5 years and can be unilateral or bilateral (30%). Retinoblastoma in adults is extremely rare. Lesions with the typical clinical presentation but are of other kinds are called pseudoretinoblastoma. They can be other tumours (astrocytic hamartomas, haeman-gioblastomas), congenital malformations or inflammatory conditions (especially solitary Toxocara granuloma). The accuracy of clinical diagnosis has been improved by radiology, ultrasonography, CT scanning and nuclear magnetic resonance, and this has brought about a considerable reduction in the number of such cases.
The retinoblastoma gene, located on chromosome 13q14 is a tumour-suppressor gene. Retinoblastoma can be inherited (bilateral tumours in the first 2 years of life) or sporadic (unilateral tumours in children aged 25 years). One-third of the patients with a sporadic retinoblastoma show a germline mutation and can transmit the disease to their offspring. Compared with the general population, carriers of germline mutations in the retinoblastoma gene, who survive retinoblastoma are at increased risk of early-onset second cancers, particularly sarcomas and brain tumours. External beam radiotherapy has been a standard treatment for medium and large, or vision-threatening, intraocular retinoblastoma, but it markedly increases the risk of cosmetic deformities and secondary cancer in children with germline mutations. For that reason primary systemic chemotherapy called "chemoreduction" has been employed to avoid radiotherapy and enucleation. The cure rate of retinoblas-toma is more than 90% in specialised centres.
Retinoblastomas are tumours originating from plu-ripotent germinal retinoblasts; the tumour can grow en-dophytically (growing into the vitreous), exophytically (growing into the subretinal space, leading to retinal detachment) or diffusely (a rare pattern with widespread
nodular thickening of the retina). The diffuse growth pattern has a bad prognosis. Trans-scleral spread of reti-noblastoma is uncommon. The tumour usually spreads into the meninges or parenchyma of the optic nerve. For this reason, it is important to take transverse blocks of the cut surface of the optic nerve, before cutting the enucleated eye. Microscopic examination will show a small blue round cell tumour with a high mitotic rate. The cells have ill-defined cytoplasm and inconspicuous nucleoli. Rosettes are frequently seen and apoptosis is common (Figs. 10.24, 10.25). Glial differentiation is rare. If the tumour was irradiated before enucleation, amorphous calcified structures will be present. Immunohis-tochemistry of retinoblastomas will show positivity for S-100, GFAP and NSE. Use of these markers can be helpful not only in identifying the tumour, but also in identifying the spread of the tumour. Choroidal invasion in particular can be hard to recognise in an H&E staining. Axonal degeneration in the optic nerve, with reactive proliferation of astrocytes has to be differentiated from real tumour spread.
From the glial tumours, only astrocytomas can be found in the retina and optic nerve. Optic pathway gliomas are frequently asymptomatic; sometimes they demonstrate rapid growth, causing considerable visual dysfunction, neurologic deficits, and endocrine disturbances. Most optic pathway gliomas are diagnosed in patients with neurofibromatosis. Children with optic pathway glio-mas associated with neurofibromatosis 1 predominantly have multifocal lesions.
Benign astrocytic tumours of the retina (astrocytic hamartomas) most frequently occur in patients with tu berous sclerosis. Retinal astrocytic hamartoma and reti-noblastoma may be very similar clinically and their differentiation in atypical cases can be difficult, even with the use of ultrasonography and computed tomography. Histologically, a well-circumscribed glial cell proliferation, sparing the outer layers of the retina will be visible.
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