Retinoblastoma is the most common primary intraocular malignancy of childhood. The incidence is approximately 1 in 18,000 live births, and it is estimated that 300 children develop retinoblastoma in the United States annually. There is no race or gender predilection. Approximately 80% of children are diagnosed before the age of 3 years. In contrast to less than 10% of older children, more than half of patients diagnosed in the first year of life present with bilateral disease.
Retinoblastoma occurs sporadically in the majority of affected children. Of all cases, approximately 60% are unilateral and 40% are bilateral. All bilateral cases and 15% of unilateral cases are hereditary. Only 10% to 20% of hereditary cases have an affected parent (Fig. 457-1).
Retinoblastoma develops only when both alleles of the tumor suppressor gene RB1, located on 13q14, are absent or defective. In hereditary retinoblastoma, every cell in the body inherits 1 absent or defective gene. During normal development, a mutagenic “hit” to the remaining normal allele results in the loss of normal cellular growth regulation and tumor development. In sporadic, nonhereditary retinoblastoma, a single retinal cell must receive 2 mutagenic hits before a tumor develops. Point mutations of the RB1 gene are the most common events.
Variable deletions of the long arm of chromosome 13 are not commonly found in patients with retinoblastoma, and patients who are developmentally delayed should not be presumed to have the chromosome 13q deletion syndrome. However, patients with a 13q deletion syndrome should be screened for retinoblastoma.
Genetic counseling for patients with retinoblastoma and their families is complex, but a few generalizations can be made. Patients with the hereditary form are at risk for other cancers later in life, including soft tissue and bone sarcomas and melanoma. This risk is increased in patients treated with radiotherapy. A survivor of hereditary retinoblastoma has a 50% chance of transmitting the gene to his or her offspring. With no previous family history, the risk for a sibling is 2% if the affected child has bilateral disease and 1% if the affected child has unilateral disease. These empiric risk estimates are based on the possibility of gonadal mosaicism. In addition, it is estimated that 10% of individuals who carry an abnormal RB1 gene do not develop retinoblastoma because the second event did not occur in any cell; however, they are still at risk of developing other cancers during their lifetime.
CLINICAL MANIFESTATIONS AND DIFFERENTIAL DIAGNOSIS
The most common presenting sign of retinoblastoma is leukocoria (white pupil; Fig 457-2). When a tumor is small and at the macula, the initial sign may be sensory strabismus. When disease is very advanced, presenting symptoms may include pain due ...