Retinoblastoma.

Retinoblastoma is a rare eye tumor of childhood that arises in the retina and represents the most common intraocular malignancy of infancy and childhood 1. It may occur at any age but most often it occurs in younger children, usually before the age of two years.

The incidence is 1 in 15,000–20,000 live births. In 60% of cases, the disease is unilateral and the median age at diagnosis is two years. Of these cases, 15% are hereditary. Retinoblastoma is bilateral in about 40% of cases with a median age at diagnosis of one year. All bilateral and multifocal unilateral forms are hereditary.

Leukocoria (white reflection in the pupil) and strabismus are the most frequent clinical manifestations of retinoblastoma (Figure 1). Leukocoria is initially inconstant, visible only at certain angles and under certain light conditions. This sign may be seen on flash photography. Strabismus, when present, becomes rapidly constant, reflecting impairment of the vision. These signs are still all-too-often overlooked and justify an ophthalmological consultation with ocular fundus examination. Some other signs may be observed, including iris rubeosis, hypopyon, hyphema, buphthalmia, orbital cellulitis, and exophthalmia. Some children with retinoblastoma may have no symptoms. Screening in case of familial history or dysmorphic syndrome with a 13q14 deletion 2 may lead to diagnosis of retinoblastoma. Most affected children are diagnosed before the age of five years.

Retinoblastoma is the first disease for which a genetic etiology of cancer has been described and the first tumor suppressor gene identified. Knudson in 1971 developed the hypothesis that retinoblastoma is a cancer caused by two mutational events 3. This led to the understanding that there are two forms of retinoblastoma, germinal and non germinal. Loss or mutations of both alleles of the retinoblastoma gene RB1, localized to chromosome 13q1.4 4, are required to develop the disease. In hereditary cases (representing approximately 55% of the cases) patients carry a germline inactivated RB1 allele present in all cells in the body and somatic loss of the second allele in retinal cells. Germinal RB1 mutations with a high penetrance rate (> 90%) concern all patients with bilateral retinoblastoma as well as 15% of patients with the unilateral form. In non hereditary cases (45% of all patients) both RB1 alleles are inactivated somatically in a single developing retinal progenitor cell. In these cases, retinoblastoma is always unilateral and unifocal.

The RB1 gene, composed of 27 exons, encodes for a 110 kD nuclear phospoprotein (pRB). pRB, a tumor suppressor gene, is a regulator at the cell cycle check point between G1 and entry into S phase. In the knock-out mouse model of retinoblastoma, Zhang et al. demonstrated that pRB is required for appropriate exit from the cell cycle of retinal progenitor cells and for rod development 5. Numerous studies indicate that other molecular events, in addition to the loss of pRB, are necessary for tumorigenesis (chromosomal gain +1q, +6p; chromosomal loss -16, -16q, -17, -17p) 67.

A novel theory about the potential mechanism of retinoblastoma tumor development was proposed in response to the observation that spontaneous unilateral retinoblastoma may be more frequent in non industrialized countries. Orjuela et al., in a study evaluating the presence of oncogenic human papilloma virus (HPV), concluded that pRB inactivation could be caused in part by the oncoprotein HPV E7 8. Environment (low folate intake during pregnancy) was also postulated to play a role in the risk of retinoblastoma occurrence because of the increased incidence of unilateral retinoblastoma in developing countries 9.

An examination of the ocular fundus under general anesthesia leads to diagnosis. The lesion appears as a white tumor with angiomatous dilatation of the vessels (Figure 2).

Ocular ultrasound demonstrates a mass more echogenic than the vitreous, with fine calcifications. Retinal detachment may also be observed in exophytic forms (Figure 3).

Magnetic resonance imaging (MRI) is the imaging modality of choice to assess the local extension. The mass has a signal equivalent to or a slightly more intense than that of the vitreous on T1-weighted sequences, with a relatively low-intensity signal on T2-weighted sequences (Figure 4). Fine calcifications are not visible. MRI can depict extensions to the optic nerve, anterior chamber and orbital fat. MRI may be useful for distinguishing retinoblastoma from pseudotumoral conditions such as Coats disease or eye malformations and to diagnose rare cases of trilateral retinoblastoma (third tumor in the pineal gland or parasellar region) (Figure 5) 10.

Computed tomography (CT) typically shows an intraocular mass with a higher density than the vitreous body, calcified in 90% of cases and moderately enhanced after iodine contrast agent injection.

The rare, invasive, diffuse forms of retinoblastoma, usually not calcified, are very difficult to diagnose due to their atypical and radiological features, mimicking an advanced form of Coats disease and Toxocara Canis eye infection 10. Coats disease corresponds to a unilateral retinal disease with telangiectasia and subretinal exudates. Persistence of the primitive vitreous may also be mistaken for retinoblastoma 11.

Genetic counseling should be proposed to every parent having a child with retinoblastoma and to patients having a familial history of retinoblastoma. During this consultation, the progress of management of retinoblastoma and anticipated examination schedule for testing should be presented to the family. An analysis of the child's personal and familial history, and direct and indirect molecular studies should be performed by the geneticist.

Young age at diagnosis, multifocal nature of the tumor, presence of psychomotor retardation and a malformative syndrome, and presence of a familial history of retinoblastoma and retinoma suggest a genetic predisposition for the disease. The risk of transmission is a function of the familial history and the type of retinoblastoma. In case of hereditary retinoblastoma, the risk of transmission is 50%. In case of unilateral, unifocal, non familial retinoblastoma, the risk of transmission is 5%.

Genetic analysis in affected children may include the following molecular tests:

• Direct search for a constitutional mutation of the RB1 gene performed on the constitutional DNA. The mutation detection rate is very high in hereditary forms 12. No preferential mutation or "hot spot" has been identified in the RB1 gene 13141516.

• Indirect demonstration of the allele carrying the mutation in cases of familial history. This test consists of identifying intragenic or RB1 flanking markers common to all affected members of the family.

• Tumor loss of heterozygosity evaluation. This technique requires tumor material and allows determination of which allele is remaining and carrying the mutation.

During this consultation, patients should be informed of the risks of transmission and of second primary tumor development 1718. At present, there is no evidence that a particular abnormality of the RB1 gene is associated with a higher risk of second cancer.

Antenal diagnosis may be proposed in case of hereditary form with an identified RB1 mutation. Indirect detection of the allele carrying the mutation may be used.

Several groups have started to investigate the use of preimplantation genetic diagnosis (PGD) for families who have children with germinal retinoblastoma. Abramson et al. recently reported the first live birth of a healthy newborn to a couple in which the father had an history of germinal retinoblastoma 19.

Treatment of retinoblastoma is based on the bilateral or unilateral characteristics of the retinoblastoma, on the staging performed by the fundoscopy and the extension of the disease.

Vital prognosis, related to retinoblastoma alone, is now excellent in patients with unilateral or bilateral forms of retinoblastoma, with a cure rate of 95% in industrialized countries. The preservation of visual function depends on ocular preservation, the initial tumor volume, the anatomical relationships of the tumors with the macula and the optic disk and the adverse effects of the treatments (cataract, vitreous hemorrhage). Long term survival in the hereditary form is threatened by the risk of occurrence of second tumors 1750. A recently published study reported a cumulative incidence for developing a new cancer at 50 years after diagnosis of retinoblastoma of 36% for hereditary and 5.7% for non hereditary patients 18. Patients treated for hereditary retinoblastoma are at an increased risk of developing non-ocular malignancies due to a mutation in the second RB1 allele in different tissues. External beam radiation, when administered before the first year of life, and chemotherapy may also increase the risk of development of second neoplasms 2951. The most frequent tumors encountered are osteogenic sarcomas of the skull and long bones, soft tissue sarcomas, cutaneous melanomas, brain tumors, and lung and breast cancer. Although external beam radiotherapy is less widely used than in the past, the risk of development of a second tumor is still significant and patients should be informed and carefully followed-up 52.

Despite the identification of the RB1 gene and the current insight into the function of pRB, the understanding of the sequence that leads to human retinoblastoma is still incomplete. Recent development of new animal models of retinoblastoma will increase the knowledge on tumorigenesis and provide an opportunity to develop treatment strategies 5. Although retinoblastoma has a good prognosis in industrialized countries, mortality due to development of a second tumor remains high. Development of a non-mutagenic therapy (such as photodynamic therapy) could be interesting, particularly in case of hereditary retinoblastoma 535455. Gene therapy for treatment of retinoblastoma is under evaluation 56.

In developing countries, retinoblastoma is unfortunately accompanied by a high mortality rate due to a significantly delayed diagnosis made at advanced stages of the disease.