Retinoblastoma SDL

Learning Objectives 1. Discuss epidemiology of retinoblastoma. 2. Compare and contrast mechanisms of hereditary and nonhereditary retinoblastoma. 3. List the most common clinical sings of retinoblastoma. 4. Describe the cell of origin and microscopic features of retinoblastoma. 5. Compare and contrast Flexner-Wintersteiner rosettes, pseudorosettes, fleurettes and Homer-Wright rosettes 6. Describe the cell of origin, genetics, histopathologic and clinical features of retinocytoma.

Definition Retinoblastoma is a malignant tumor of the retina. o The result of a double (‘two-hit’) oncogenic mutation occurring between the start of the third month post-conception and the age of 4 years. The cell of origin is most likely a precursor cone photoreceptor cell or a multipotent retinoblast. Rb1 was the first tumor-suppressor gene to be identified, leading to the discovery of a whole new class of anti-oncogenes.

Epidemiology Incidence is 1 in 20,000 – 30,000 live births. The most frequent primary eye malignancy in children: it represents 80% of all primary ocular cancers in children up to 15 years old. Third most common intraocular tumor in all ages (the most common intraocular tumor is choroidal melanoma, with a mean age at diagnosis of 55 years). The disease occurs in utero and up to the age of 4 years, the average age at appearance of first signs being 13 months for bilateral cases and 24 months for unilateral cases.

Genetics The pivotal genetic event in all retinoblastoma tumors is the inactivation, due to mutations or deletions, of both copies of Rb1 at the chromosomal locus 13q14. Rb1 protein is a regulator at the cell-cycle checkpoint between G1 and entry into the S-phase.

Genetics: Hereditary One allele has already been inactivated in the germline (usually inherited from one parent) and every cell of the body is primed to enter the carcinogenesis process as soon as the other allele is lost. If the loss occurs in a retinal cell, the carcinogenesis process will follow the direction that leads to retinoblastoma. Other cells in the body can give rise to malignancies in the absence of Rb1 function, such as osteosarcoma. These patients are prone to additional primary retinoblastoma in the same or contralateral eye.

Genetics: Non-Hereditary Truly nonhereditary retinoblastoma (60% of cases) is unilateral and unifocal and occurs when the same cell in the developing retina sequentially loses both Rb1 functional alleles.

Pathogenesis The tumor arises from photoreceptor precursors. Horizontal interneurons (fully mature neurons of the retina) are the specific cell that gives rise to retinoblastoma. o Retinoblastoma can arise from fully matured neurons.

Clinical Presentation Leukocoria is the most common presenting sign in patients with retinoblastoma (90%). The second major presenting sign is strabismus (20%).

Pathology: Gross Findings Soft and friable tumors. On gross section, they appear as a cream-colored mass arising from the retina, containing chalky- white calcified flecks scattered within necrotic zones.

Pathology: Microscopic Findings Undifferentiated retinoblastoma is composed of immature retinoblasts, which are neuroblasts: small, round or polygonal, densely packed cells with large, hyperchromatic nuclei and scant cytoplasm. Marked proliferation with necrosis and calcification due to outgrowth of blood supply. The classic histopathologic description of retinoblastoma is “islands of blue cells in a sea of pink necrosis”.

Microscopic Findings

Photoreceptor Differentiation The Flexner-Wintersteiner rosettes are specific for retinoblastoma and are seen in 70% of tumors. o Clusters of cuboidal or short columnar cells arranged around a central lumen with the nuclei displaced away from the lumen. Do not confuse with pseudo-rosettes!

Microscopic Findings Fleurettes, flower-like groupings of tumor cells, may represent a more advanced recapitulation of retinal epithelium with photoreceptor differentiation, and are more characteristic of well- differentiated tumors. The Homer-Wright rosettes are not specific to retinoblastoma and are seen frequently in other neuroblastic tumors such as neuroblastoma and medulloblastoma

Homer-Wright Rosettes

Two Growth Patterns Two Growth Patterns 1) Tumors arising from the inner layers of the retina exhibit endophytic growth by seeding the vitreous cavity, and eventually the anterior chamber, where the tumor cells infiltrate the iris stroma. 2) Tumors that arise from the outer retinal layers exhibit exophytic growth and cause retinal detachment which is often high and bullous in configuration, and the retina typically is plastered against the posterior surface of the lens. Eventually, tumor cells breach Bruch's membrane and invade the choroidal stroma. The uveal tract's rich blood supply nurtures the proliferating tumor cells and can serve as a major route for distant hematogenous dissemination.

Dissemination Pathways The optic nerve is the most common avenue for extraocular extension by retinoblastoma. o Optic nerve invasion is an extremely important prognostic factor in retinoblastoma. o Mortality rates correlate directly with the depth of optic nerve invasion. Hematogenous dissemination can result in widespread metastases to the lungs, bones, and other organs.

Second Non-Occular Primary Tumors 1. Trilateral retinoblastoma: consists of unilateral or bilateral hereditary retinoblastoma associated with an intracranial midline neuroblastic tumor in the pineal region (pineoblastoma). 2. Non-ocular second malignant tumors in survivors of retinoblastoma. The Rb1 mutation present in all cells of a patient with retinoblastoma predisposes them to developing second non-ocular malignant tumors. The most common sites of second nonocular tumors are, in order of decreased frequency: soft head tissue i.e. in the irradiated field (24%), bone (18%), skin (15%), and brain (8%).

Treatment Small intra-retinal tumors away from fovea are treated with external beam radiation therapy. Tumors larger than 3 mm are treated with systemic chemotherapy. Enucleation is performed when there is no chance of preserving useful vision in an eye.

Retinocytoma Definition Retinocytoma is a rare benign tumor of the retinal neurons occurring in patients with RB1 gene mutation. The incidence is 1% of that of retinoblastoma. The median age at diagnosis is 15 years.

Genetics Similar to retinoblastoma, with autosomal dominant inheritance involving a mutation in the Rb1 gene locus on chromosome 13q14. Both Rb1 alleles are mutated, but do not show proliferation, aneuploidy or gene expression patterns characteristic of retinoblastoma, suggesting that additional genetic events are necessary to convert a RB1– / – cell to a retinoblastoma cell.

Clinical Presentation Histologically, retinocytomas are composed of well- differentiated, benign-appearing retinal cells forming fleurettes. Mitoses and necrosis are characteristically absent.

Fleurettes

Clinical Course Retinocytomas can undergo malignant transformation into retinoblastoma in about 4% cases and thus require periodic follow-up. Malignant transformation is characterized by enlargement in size and vitreous seeding. Retinocytoma does not require aggressive treatment, but regular periodic follow-up.