Retinoblastoma

ABSTRACT

• Retinoblastoma is the most common intraocular malignancy in children, with a worldwide incidence between 1 in 13,500 and 1 in 25,000 live births. The presenting signs and symptoms include leukokoria, strabismus, low-vision orbital cellulitis, unilateral mydriasis, and heterochromia. The disease can be unifocal or multifocal and unilateral or bilateral. The average age of diagnosis is 12 months for bilateral and 18 months for unilateral cases, and 90 percent of affected individuals are diagnosed before age 3 years. Unusual manifestations of this disease include late onset retinoblastoma, 13q-deletion syndrome, retinoma, trilateral retinoblastoma, and second-site primary tumors, including osteosarcoma, Ewing sarcoma, leukemia, and lymphoma.

• Early diagnosis and treatment are of primary importance in the survival of retinoblastoma patients. A variety of diagnostic approaches are used, including computed tomography (CT), magnetic resonance imaging (MRI), ultrasonography, and fine-needle aspiration biopsy (FNAB). Each has advantages, and when used in combination, they can establish the proper disease classification. Effective methods for the treatment of retinoblastoma tumors include enucleation, external-beam irradiation, episcleral plaques, xenon arc and argon laser photocoagulation, cryotherapy, and chemotherapy. The choice of treatment depends on several factors, such as multifocal or unifocal disease, site and size of the tumor, diffuse or focal vitreous seeding, age at diagnosis, and histopathologic findings.

• Retinoblastoma has served as the prototypic example of a genetic predisposition to cancer. It is estimated that 60 percent of cases are nonhereditary and unilateral, 15 percent are hereditary and unilateral, and 25 percent are hereditary and bilateral. A model encompassing these findings suggests a requirement for as few as two stochastic mutational events for tumor formation. The first of these events can be inherited through the germ line or can be somatically acquired, whereas the second occurs somatically in either case and leads to a tumor that is doubly defective at the retinoblastoma locus. Cytogenetic analyses have demonstrated the involvement of a genetic alteration in a gene for negative growth regulation at chromosome band 13q14. This model has been tested and confirmed using restriction-fragment-length polymorphisms (RFLP) for loci on chromosome 13. These studies have shown that the second wild-type retinoblastoma allele may be lost by several somatic mutational mechanisms, including mitotic nondisjunction with loss of the wild-type chromosome, mitotic nondisjunction with duplication of the mutant chromosome, mitotic recombination between the RB1 locus and the centromere, and other regionalized events, such as deletion and point mutation.

• The 200-kb genomic locus for RB1 has been isolated, and its exon/intron structure has been characterized. Current molecular technology has allowed the identification of a variety of aberrations in this locus in retinoblastoma patients and their tumors at the DNA, RNA, and protein levels. RB1 alterations also have been detected in a variety of clinically related second-site primary tumors and nonrelated tumors, including osteosarcoma, breast carcinoma, and small-cell lung carcinoma. The ability to detect mutations in RB1 coupled with the isolation of polymorphic sequences within the gene locus, has further extended the prenatal risk assessment for this pediatric tumor.

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