Retinoblastoma and Pseudoglioma

Retinoblastoma

General Information

I. Retinoblastoma, along with leukemia and neuroblastoma, is one of the most common childhood malignancies and is the most common childhood intraocular neoplasm. Nevertheless, the tumor is rare. There are 250–350 cases annually in the United States, representing only 4% of pediatric malignancies.

II. It is third to uveal malignant melanoma and metastatic carcinoma as the most common intraocular malignancy in humans of any age.

III. The incidence is approximately one in 18,000 live births in the United States, with a trend toward a higher prevalence than historically found (because of increased survival rate).

A. The average annual incidence of retinoblastoma is 5.8 per one million for children younger than 10 years and 10.9 per one million for children younger than five years of age. No significant race or sex predilection exists.

B. The two clinical classification systems most commonly used for retinoblastoma are the Reese–Ellsworth classification (REC) (Table 18.1) and the International Classification (IC) (Table 18.2).

The REC is based on tumor(s) size and location, the presence of vitreous seeding, and the response to external beam radiation. The IC is based on tumor(s) size, subretinal fluid, vitreous or subretinal tumor seeding, and response to chemotherapy. The two classifications have different goals. The REC was designed to be predictive of globe salvage for the affected eye. The IC was designed to be applicable to modern therapeutic modalities including chemoreduction. In fact, it is predictive of chemoreduction outcome. No doubt modifications and refinements of the IC will continue.

TABLE 18.1

Reese–Ellsworth Classification for Intraocular Tumors

Group	Tumor Characteristics	Prognosis
Group I	A. Solitary tumor, smaller than 4 disc diameters (DD), at or behind the equator	Very favorable for maintenance of sight
	B. Multiple tumors, none larger than 4 DD, all at or behind the equator sight	Very favorable for maintenance of sight
Group II	A. Solitary tumor, 4–10 DD at or behind the equator	Favorable for maintenance of sight
	B. Multiple tumors, 4–10 DD behind the equator	Favorable for maintenance of sight
Group III	A. Any lesion anterior to the equator	Possible for maintenance of sight
	B. Solitary tumor, larger than 10 DD behind the equator	Possible for maintenance of sight
Group IV	A. Multiple tumors, some larger than 10 DD	Unfavorable for maintenance of sight
	B. Any lesion extending anteriorly to the ora serrata	Unfavorable for maintenance of sight
Group V	A. Massive tumors involving more than one half of the retina	Very unfavorable for maintenance of sight
	B. Vitreous seeding	Very unfavorable for maintenance of sight

(Modified from National Cancer Institute: Retinoblastoma classification. Available at http://www.cancer.gov/cancertopics/pdq/treatment/retinoblastoma/HealthProfessional/page3.)

TABLE 18.2

International Classification for Intraocular Retinoblastomas

Group A	Small intraretinal tumors away from the foveola and disc
	• All tumors are 3 mm or smaller in greatest dimension, confined to the retina AND • All tumors are located farther than 3 mm from the foveola and 1.5 mm from the optic disc
Group B	All remaining discrete tumors confined to the retina
	• All other tumors confined to the retina not in group A • Tumor-associated subretinal fluid less than 3 mm from the tumor with no substantial seeding
Group C	Discrete local disease with minimal subretinal or vitreous seeding
	• Tumor(s) are discrete • Subretinal fluid, present or past, without seeding involving up to of the retina • Local fine vitreous seeding may be present close to discrete tumor • Local subretinal seeding less than 3 mm (2 DD) from the tumor
Group D	Diffuse disease with significant vitreous or subretinal seeding
	• Tumor(s) may be massive or diffuse • Subretinal fluid present or past without seeding, involving up to total retinal detachment • Diffuse or massive vitreous disease may include “greasy” seeds or avascular tumor masses • Diffuse subretinal seeding may include subretinal plaques or tumor nodules
Group E	Presence of any one or more of the following poor prognosis features
	• Tumor touching lens • Tumor anterior to anterior vitreous face involving ciliary body or anterior segment • Diffuse infiltrating retinoblastoma • Neovascular glaucoma • Opaque media from hemorrhage • Tumor necrosis with aseptic orbital cellulitis • Phthisis bulbi

(Modified from National Cancer Institute: Retinoblastoma classification. Available at http://www.cancer.gov/cancertopics/pdq/treatment/retinoblastoma/HealthProfessional/page3.)

IV. Bilaterality occurs in 20–35% of all cases and is a useful marker for patients with hereditary retinoblastoma.

V. The eye is a normal size at birth or slightly smaller, but later it may become phthisical or buphthalmic.

Comparing eyes with unilateral retinoblastoma to the fellow eye, the tumor-containing eye is statistically smaller in axial length, equatorial diameter, and eye volume in comparison to the normal fellow eye. Moreover, the larger the tumor volume, the smaller the eye until buphthalmos ensues. A rare exception to the normal or smaller size at birth is a microphthalmic eye that contains both a retinoblastoma and persistent hyperplastic primary vitreous [PHPV (persistent fetal vasculature)]. Microphthalmia, retinoblastoma, and 13q deletion syndrome can also occur and have been accompanied by colobomas.

VI. Age at initial diagnosis

A. Average age is 13 months, with 89% diagnosed before three years of age.

B. Approximately 8.5% of patients are older than five years of age at the time of diagnosis. It is quite rare after the age of 20 years. Nevertheless, it has been reported in patients past 50 years of age, and it has even occurred during pregnancy.

VII. Children with retinoblastoma may have other genetic and congenital abnormalities such as the 13q deletion (deletion of chromosomal region 13q14—all children with 13q14 deletions should have an ophthalmologic examination to rule out retinoblastoma) syndrome, 13qXp translocation, 21 trisomy, 47,XXX, 47,XXY, PHPV, the Pierre Robin syndrome, or hereditary congenital cataracts.

Axenfeld–Rieger anomaly and retinoblastoma have been diagnosed in a two-month-old female with developmental delay, dysmorphic features, and chromosome 13q deletion. Chromosome 13q deletion syndrome is characterized by growth retardation, cognitive delays, and organ and musculoskeletal deformities. Sectoral iris heterochromia and retinal pigment variation may also be found. It has been associated with the Cornelia de Lange syndrome phenotype having retinoblastoma.

A. Peters’ anomaly and retinoblastoma have occurred in the same neonate.

VIII. Genetic cases have an increased prevalence of nonocular cancers, especially of the pineal gland (bilateral retinoblastoma plus pineal tumor comprise trilateral retinoblastoma), and sarcomas.

IX. Trilateral retinoblastoma

A. The association of a midline intracranial neoplasm with bilateral retinoblastomas is known as trilateral retinoblastoma. It is found in 4–8% of patients with hereditary retinoblastoma.

B. The intracranial neoplasm is most often an undifferentiated neuroblastic pineal tumor, or suprasellar or parasellar neuroblastoma.

Pineal cysts are significantly more common in patients who have bilateral retinoblastoma than in those with unilateral tumor, thereby suggesting a benign variant of trilateral retinoblastoma or possibly a hereditary influence.

C. Loss of the retinoblastoma “genes” is thought to confer an increased susceptibility to the development of an intracranial neoplasm.

D. Approximately 95% of patients who have trilateral retinoblastoma have a positive family history of retinoblastoma, bilateral retinoblastoma, or both.

X. Retinoblastoma, which behaves as an autosomal-dominant trait with 90% penetrance, represents a prototype of a class of human cancers characterized by a loss of genetic information at the constitutional or tumor level. Other cancers in this class include Wilms’ tumor, neuroblastoma, small cell carcinoma of the lung, pulmonary carcinoid, breast and bladder cancer, osteosarcoma, and renal cell carcinoma.

XI. Prenatal screening of peripheral blood of fetal cultured amniocytes for RB1 gene mutations is possible.

XII. Human papillomavirus infection may be a cofactor in the development of retinoblastoma.

Heredity

I. The retinoblastoma (Rb) gene was the first tumor suppressor gene cloned. It is a negative regulator of the cell cycle and acts through its ability to bind the transcription factor E2F, thereby repressing the transcription of genes required for the cell proliferation S phase of the cell cycle and for cell survival. In addition, more than 100 other proteins have been reported to interact with Rb.

II. “Two-hit” model
Knudson’s two-hit model states that retinoblastoma arises as a result of two mutational events involving the RB1 gene (see discussion of chromosomal region 13q14, later).

A. If both mutations occur in the same somatic (postzygotic) cell, a single, unifocal, unilateral retinoblastoma results. Because the mutations occur in a somatic cell, the resultant condition is nonheritable.

The nonheritable form arises through two unrelated events occurring at homologous loci in a single neural retinal cell. Double such sporadic mutations are highly unlikely; hence, a unifocal, unilateral retinoblastoma results.

B. In the hereditary form, the first mutation occurs in a germinal (prezygotic) cell (which, therefore, would mean that the mutation would be present in all resulting somatic cells), and the second mutation occurs in a somatic (postzygotic) neural retinal cell, resulting in multiple neural retinal tumors (multifocal in one eye, bilateral, or both), as well as in primary tumors elsewhere in the body (e.g., pineal tumors and sarcomas).

1. The probability that in the inherited form the tumor will develop in the patient (i.e., genotypically and phenotypically abnormal) is 90 in 100 (penetrance is estimated at 90%).

2. Occasionally, a generation may be skipped, and the retinoblastoma may be transmitted to a genotypically abnormal but phenotypically normal family member.

Retinoblastoma is usually said not to be associated with overgrowth syndromes; however, it has been suggested that macrocephaly cutis marmorata telangiectatica congenita (M-CMTC) may be a tumor-prone syndrome. Similarly, there is an increased incidence of multiple cutaneous malignant melanomas among individuals with retinoblastoma and dysplastic nevus syndrome and also among their family members. It has been recommended that survivors of inherited retinoblastoma and their families be screened for dysplastic nevus syndrome.

III. The chromosomal region 13q14 (the retinoblastoma gene—Rb gene) regulates the development of normality (i.e., the region acts as an antioncogene).

Tumor suppressor genes, of which the Rb gene is one, are the “opposite numbers” of oncogenes because their normal role is to inhibit cell growth; oncogenes, conversely, stimulate cell growth.

A. If both chromosomal 13q14 regions are normal, no retinoblastoma will develop.

B. If one of the two 13 chromosomes has a 13q14 deletion, duplication, or point mutation (a heterozygous condition), a retinoblastoma will still not result.

C. If both 13 chromosomes have a 13q14 deletion, duplication, or point mutation (a homozygous condition), the potential for retinoblastoma results.

D. Therefore, retinoblastoma is inherited as an autosomal-recessive trait at the cellular level; nevertheless, retinoblastoma behaves clinically as if it has an autosomal-dominant inheritance pattern with 90% penetrance.

E. Gain of chromosome 1q31–1q32 is found in >50% of retinoblastoma and is also common in other tumors.

F. “Fragile-site” loci probably contribute synergistically to the development and progression of the cytogenetics of retinoblastoma malignancy.

G. A facial phenotype for retinoblastoma patients having interstitial 13q14 deletions has been described in Japanese and white individuals. It includes cranial anomalies, frontal bossing, deeply grooved and long philtrum, depressed and broad nasal bridge, bulbous tip of the nose, thick lower lip, thin upper lip, broad cheeks, and large ears and lobules.

H. The genes differentially expressed in RB compared to normal retina belong mainly to DNA damage-response pathways, including, but not limited to, breast cancer associated genes (BRCA1, BRCA2), ataxia telangiectasia mutated gene (ATM), ataxia telangiectasia and Rad3 related gene (ATR), E2F, and checkpoint kinase 1 (CHK1) genes. In general, the mutations associated with retinoblastoma are predominantly gene-inactivating mutations, which are single-base nonsense mutations and splice site mutations.

I. The retinoblastoma gene model may oversimplify the disease process.

The genomic instability and aneuploidy are probably responsible for the genesis of retinoblastoma, and the RB1 mutation may or may not be the critical event in the development of retinoblastoma. For example, other steps toward malignancy may include 1q32.1 gain followed by 6p22 gain that in turn is followed by 16q22 loss and 2p24.1 gain. Thus, it is probable that additional genetic and epigenetic events are required for the development of retinoblastoma. For example, oncosuppressor gene deletions are found in association with retinoblastoma. Similarly, it had been thought that retinoblastoma cells bypass the p53 pathway leading to apoptosis because they arise from intrinsically death-resistant cells. Rather, retinoblastoma cells do undergo p53-mediated apoptosis; however, increased expression of MDMX protein suppresses the p53 response in RB1-deficient tumor cells. Thus, cellular abnormalities in addition to the traditional “two-hit” hypothesis are necessary for a cell to complete the journey to malignancy.

IV. Hereditary cases (approximately 40% of cases)

A. Approximately 10% of all retinoblastomas are inherited (familial). All are multifocal in one eye, bilateral, or both.

B. Another 30% are caused by a new germline mutation (see later discussion of sporadic cases). Although most of these mutations are multifocal in one eye, bilateral, or both, unilateral retinoblastoma, lack of family history, and older age do not exclude the possibility of a germline retinoblastoma gene mutation.

C. Retinoblastoma and hypochondroplasia have occurred as two clinically distinct heritable germline mutations arising de novo in a single individual.

V. Sporadic cases

A. Approximately 90% of all retinoblastomas develop by mutation—that is, sporadic cases (the other 10% are the inherited familial cases).

The mutation rate is approximately 2 × 10⁻⁵ (one in 36,000 births), with, perhaps, a third representing a genetic mutation in a germinal cell capable of transmitting the retinoblastoma to offspring. The remaining two-thirds represent a somatic mutation incapable of transmitting the tumor.

B. The retinoblastoma in sporadic somatic mutation cases is unifocal and unilateral (~60% of the total cases); in the sporadic genetic mutation cases, it is usually multifocal in one eye, bilateral, or both (~30% of the total cases). The other 10% are the inherited familial cases.

VI. Genetic counseling

A. Healthy parents with one affected child run approximately a 6% risk of producing more affected children (the parent may be genotypically abnormal but phenotypically normal).

B. If two or more siblings are affected, approximately a 50% risk exists that each additional child will be affected.

C. A retinoblastoma survivor with the hereditary type has approximately a 50% chance of producing affected children. Phenotypically normal children of an affected parent may be genetically abnormal.

D. A patient with sporadic disease has approximately a 12.5% chance of producing affected children.

E. Preimplantation genetic diagnosis has been helpful in genetic counseling in heritable retinoblastoma.

F. Newer genetic techniques provide improved risk assessment and management options for both genetic and sporadic forms of retinoblastoma.

Clinical Features

I. Strabismus and leukokoria are the most common clinical manifestations of retinoblastoma.

II. Early lesions may present with visual difficulties or strabismus. Alternatively, they may be completely asymptomatic and present as small fundus lesions found on routine eye examination. It has been suggested that differentiated tumors present at an earlier age than poorly differentiated tumors irrespective of laterality.

III. Moderate lesions may present as:

A. Leukokoria (i.e., “cat’s-eye reflex”; Figs. 18.1 and 18.2)

The term leukokoria is derived from the Greek leukos, meaning “white,” and korē, meaning “pupil.”

1. Ocular, orbital, and central nervous system imaging studies are critical to the evaluation of the patient with leukokoria and in the prognosis and treatment strategy development for the patient with retinoblastoma. High-resolution, contrast-enhanced magnetic resonance imaging (MRI) is replacing computed tomography (CT) for these purposes. MRI provides necessary information without coincident radiation exposure, which may be particularly important for the patient with hereditary retinoblastoma. Ultrasound complements MRI for these purposes.

Fig. 18.1 Exophytic retinoblastoma. A, Clinical appearance of exophytic retinoblastoma, which grows predominantly toward the subneural retinal space, causing a neural retinal detachment that results in leukokoria. B, Gross specimen shows retinoblastoma growing into subneural retinal region, inducing a neural retinal detachment. Note multifocal growth of tumor. (A, Courtesy of Dr. HG Scheie; B, Armed Forces Institute of Pathology account No. 117729.)

Fig. 18.2 Endophytic retinoblastoma. A, Retinoblastoma grows mostly inward (endophytum). Note resemblance of tumor to brain tissue. B, Retinoblastoma grows inward from neural retina and fills vitreous compartment of eye. (A, Courtesy of Dr. HG Scheie.)

B. “Pseudoinflammation”—that is, simulating uveitis, endophthalmitis, or panophthalmitis with or without pseudohypopyon (Fig. 18.3)

1. Any childhood intraocular inflammation should be considered retinoblastoma until proven otherwise.

2. Vitreous seeding

a. Diffuse vitreous seeding is associated with 16q24 loss.

b. It also may be found in association with retinal astrocytoma.

Occasionally, cytologic examination of a vitrectomy specimen is required to diagnose retinoblastoma in cases with unusual presentation (Fig. 18.4). In such instances, cytologic characteristics such as the presence of loosely coherent atypical cells with high nuclear-to-cytoplasm ratio and a “salt-and-pepper” chromatin pattern can be helpful in the diagnosis. Immunohistochemical staining that is positive for neuroendocrine markers (neuron-specific enolase, synaptophysin, and CD56) and diffuse staining for the proliferation index marker Ki-67 also contribute to the diagnosis.

3. Orbital inflammation can occur even when the retinoblastoma is confined to the eye; signs of orbital cellulitis, therefore, do not necessarily mean orbital extension of the tumor. Such inflammation may be precipitated by autoinfarction of the tumor.

Fig. 18.3 Spread of tumor. A, Retinoblastoma cells have formed balls of tumor in the anterior chamber inferiorly, simulating a hypopyon, thus “pseudohypopyon.” B, In another case, clusters of retinoblastoma cells have spread from the neural retina into the anterior chamber and are present inferiorly as a pseudohypopyon and elsewhere float freely. (A, Courtesy of Dr. JA Shields; B, courtesy of Prof. GOH Naumann.)

Fig. 18.4 Cytology of the vitreous fluid. A, Neoplastic cells forming loosely cohesive clusters. Note the scant cytoplasm, the high nuclear-to-cytoplasmic ratio, and the “salt-and-pepper” chromatin pattern. Nuclear molding can also be appreciated as well as the presence of small single conspicuous nucleoli (cytospin preparation, Papanicolaou). B, Compared to the well-aligned normal retinal neuronal cells (right), the neoplastic cells (left) are haphazardly arranged and show larger nuclei and abundant nuclear debris (cell block preparation, H&E). Immunocytochemistry performed on the cell block of the vitreous fluid. C, Strong and diffuse staining of both the neoplastic and non-neoplastic cells for synaptophysin. D, Strong and diffuse staining of the neoplastic cells for Ki-67 (left) contrasting with no staining in the non-neoplastic cells (right). (Reprinted from Orellana ME, Brimo F, Auger M et al.: Cytopathological diagnosis of adult retinoblastoma in a vitrectomy specimen. Diagn Cytopathol 38:59, 2010.)

C. Iris neovascularization (rubeosis iridis; Figs. 18.5A and 18.5B) with a hyphema, chronic secondary closed-angle glaucoma, or both may be seen. Iris neovascularization can lead to hyphema.

Spontaneous hyphema in a child should always alert the physician to suspect retinoblastoma, medulloepithelioma, juvenile xanthogranuloma, or nonaccidental trauma.

Fig. 18.5 Secondary effects of tumor. A, Iris neovascularization prominent in eye that shows leukokoria caused by retinoblastoma. B, Left eye in another case shows leukokoria caused by retinoblastoma. Eye buphthalmic because of secondary closed-angle glaucoma. C, Proptosis of left eye caused by posterior extraocular extension of retinoblastoma. D, Enucleated eye shows retinoblastoma in the eye with massive extension posteriorly behind the eye (to left). The cornea is to the right. (A, Courtesy of Dr. JA Shields; B, courtesy of Dr. HG Schele; C, courtesy of Dr. RE Shannon.)

Vascular endothelial growth factor, secreted by hypoxic retina, may play a role in the development of iris neovascularization. The glaucoma may lead to symptoms of photophobia and, if prolonged, the development of buphthalmos.

D. Phthisis bulbi

IV. Advanced lesions may present with proptosis (see Figs. 18.5C and 18.5D), distant metastases, or both. Although proptosis as a presenting sign is rare in the United States, it is relatively common in developing countries. Similarly, age at diagnosis and signs of more advanced disease, such as globe enlargement, correlate with the socioeconomic status of the patient’s country of residence.

Histology

I. Growth pattern

A. Multifocal growth (i.e., spontaneous development from more than one region of the same neural retina; Fig. 18.6)

Fig. 18.6 Multifocal origin. A, Three small retinoblastoma (s) tumors and one large inferior tumor (l) are present in the same eye. B, Gross specimen of same eye after enucleation (see Fig. 18.10 for histology). (A, Courtesy of Dr. DB Schaffer.)

B. Bilateral involvement is itself a reflection of multifocal neural retinal involvement.

C. Exophytic retinoblastoma (see Fig. 18.1) grows predominantly toward the subneural retinal space and detaches the neural retina.

D. Endophytic retinoblastoma (see Fig. 18.2) grows predominantly toward the vitreous. The neural retina is not detached.

Most retinoblastomas have both endophytic and exophytic components. Diffuse infiltrating retinoblastoma is a rare subtype of retinoblastoma, comprising approximately 1.5% of the total.

E. Rarely, retinal neovascularization may be found with retinoblastoma.

Histology of retinoblastoma may be helpful in suggesting in vitro drug resistance. Undifferentiated tumors are more sensitive to several cytostatic drugs. Calcification and apoptosis reflect an inverse relation to in vitro drug resistance to ifosfamide and vincristine.

II. The basic cell type is the radiosensitive undifferentiated retinoblastoma cell (Fig. 18.7).

A. Retinoblastoma cells seem to be neuron-committed cells that arise from photoreceptor progenitor cells or from primitive stem cells that are capable of differentiation along both neuronal and glial cell lines.

1. Retinoblastomas contain tumorigenic retinal stem-like cells that express retinal development-related genes such as nestin, CD133, pax6, chx10, and Rx and overexpress Bmi-1, which is required for self-renewal and proliferation of stem cells. These cells can give rise to tumors histomorphologically and immunophenotypically similar to the primary “parent” tumor when transplanted to a suitable murine host.

2. Others contend that retinoblastoma arises from mature neural cells and not from tumor stem cells.

Mucin-like glycoprotein associated with photoreceptor cells is an immunohistochemical marker that is specific for retinal photoreceptor cells. Its use demonstrates photoreceptor differentiation even in apparently “undifferentiated” retinoblastomas.

B. The primitive cells may be difficult to differentiate from other soft-tissue sarcomas. Immunohistochemistry, electron microscopy, and molecular assays for specific gene fusion may all be helpful in establishing the diagnosis.

C. The cells (bipolar-like) are positive for neuron-specific enolase, class III β-tubulin isotype (hβ4), microtubule-associated protein 2 (MAP2), and synaptophysin; they are negative for glial fibrillary acidic protein and S-100 protein. Neuron-specific enolase is present in the aqueous humor of patients who have intraocular retinoblastomas.

D. The product of the retinoblastoma susceptibility gene, p110^RB1, can be identified in paraffin-embedded tissues with commercially available techniques.

E. Apoptosis is more frequently found in tumors of young patients and to be distributed within rosettes.

Fig. 18.7 Retinoblastoma. A, Characteristically, sections of retinoblastoma stained with hematoxylin and eosin and viewed under low magnification show dark blue areas surrounded by light pink areas. The dark areas represent the viable cells and calcium deposition, whereas the light areas represent tumor necrosis. B, Increased magnification shows viable (dark blue) tumor cells clustered around central blood vessels and surrounded, in turn, by a mantle of necrotic (pink) cells. Numerous Flexner–Wintersteiner rosettes are present. C, Further increased magnification shows the viable tumor cells around blood vessels (pseudorosettes), the necrotic areas, and the Flexner–Wintersteiner rosettes.

III. Rosettes are of two types:

A. Flexner–Wintersteiner rosettes (Figs. 18.8 and 18.9; see also Fig. 18.7C) are the characteristic rosettes of retinoblastoma, but they are not always present.

1. In the rosettes, the cells line up around an apparently empty central lumen.

2. Special stains show that the lumen contains a hyaluronidase-resistant acid mucopolysaccharide.

Fig. 18.8 Types of rosettes. A, A Flexner–Wintersteiner rosette consists of a central lumen lined by cuboidal tumor cells that contain nuclei positioned basally (away from the lumen). Delicate limiting membranes are seen at the apices of the cells that surround the lumen. B, Homer Wright rosettes are found more frequently in medulloblastomas and neuroblastomas than in retinoblastomas. In these rosettes, the cells line up around an acellular area that contains cobweb-like material. C, Fleurettes are flower-like groupings of tumor cells in the retinoblastoma. The cells of fleurettes show clear evidence of differentiation into photoreceptor elements.

Fig. 18.9 Types of rosettes. A, Flexner–Wintersteiner rosettes (r) show clear lumina lined by a delicate limiting membrane and cuboidal retinoblastoma cells that contain basally located nuclei. B, In this histologic section of a retinoblastoma, almost all of the cells show photoreceptor differentiation, indicated by the pale eosinophilic cellular regions. The differentiated areas are forming fleurettes (f).

B. Homer Wright rosettes (see Fig. 18.8), named after John Homer Wright, are found in medulloblastomas, neuroblastomas, and, occasionally, retinoblastomas.
In Homer Wright rosettes, the cells line up around an area containing cobweb-like material but no acid mucopolysaccharide.

IV. Pseudorosette—this very poor and confusing term is often used to refer to arrangements in the tumor that on cursory examination may resemble the aforementioned rosettes. The structures are formed by:

A. Viable tumor cells that cluster around blood vessels, giving a rather uniform cuff with a mean thickness of 98.7 µm

B. Small foci of necrotic cells between larger masses of viable tumor cells

C. Incompletely formed Flexner–Wintersteiner or Homer Wright rosettes

V. Fleurettes and retinocytoma (Fig. 18.10; see also Figs. 18.8 and 18.9)

A. Fleurettes are flower-like groupings of tumor cells in the retinoblastoma that clearly show evidence of differentiation into photoreceptor elements.

B. Fleurettes may be absent, may be present in small nodules, or, rarely, may be present as the only cells in the tumor so that the entire tumor has differentiated into photoreceptor-like elements.

1. The fully differentiated retinoblastoma is called a retinocytoma (retinoma).

2. Retinocytomas have a uniformly bland cytology, photoreceptor differentiation, abundant fibrillar eosinophilic stroma, absence of mitotic activity, and occasional foci of calcification.

3. Cells in the differentiated part of the tumor show immunoreactivity for retinal S antigen, S-100 protein, and glial fibrillary acidic protein.

Histopathology and immunohistochemistry support the concept that retinocytomas arise de novo rather than from retinoblastomas that have undergone spontaneous regression. Moreover, in patients with a family history of retinoblastoma, it has been suggested that the types of inherited mutations underlying retinoma are indistinguishable from retinoblastoma-related ones, which are largely dominated by truncating mutations.

4. Multinucleated tumor cells may be found in retinocytomas.

5. Very rarely, a retinocytoma may undergo malignant transformation. In fact, some consider it to be a premalignant lesion.

C. Those areas that show photoreceptor differentiation usually lack evidence of necrosis and show only occasional calcification.

1. Retinocytoma is most likely to be found in the inherited form, the mutation presumably taking place in a relatively mature retinoblast.

2. There may be an increased possibility of a second primary tumor following the development of retinoma.

D. Developing photoreceptors have apical adherens junctions, mitochondria-filled inner-segment regions, occasional fragments of membranous outer segments, and cilia that contain a 9 + 0 tubular arrangement.

Fig. 18.10 Fleurettes. A, Retinoblastoma shows complete photoreceptor differentiation (see Fig. 18.6 for fundus and gross appearance). B, Note arrangement (fleurettes) of cells that have undergone photoreceptor differentiation. C, Photoreceptor inner segments resemble those of cone cells. They radiate from attachment girdle (zonulae adherentes) (arrows) of external limiting membrane, partly because intervening Müller cell processes are lacking (m, mitochondria). (C, Modified from Tso MOM, Zimmerman LE, Fine BS: The nature of retinoblastoma: I. Photoreceptor differentiation. Am J Ophthalmol 69:339. © Elsevier 1970.)

VI. Other histologic features include significant areas of necrosis (Fig. 18.11; see also Fig. 18.7), necrotic retinoblastoma presenting as orbital cellulitis, and necrosis leading to spontaneous and complete regression in a shrunken, scarred, calcific eye—a rare occurrence (Fig. 18.12),

A. Calcification (see Fig. 18.11) is a frequent and important diagnostic feature. It is mainly present in areas of necrosis. Calcification begins in nonviable cells or cells that are undergoing necrosis. The calcification is intracellular and begins in the cytoplasm, probably in the mitochondria.

B. Basophilic areas around blood vessels (Fig. 18.13), lying freely within the tumor, and on the lens capsule represent deposition of DNA liberated from necrotic retinoblastoma cells.

C. Approximately 1.5% of cases have a diffuse, infiltrating type of tumor without a discrete neural retinal mass. It occurs in a slightly older age group than the usual type, tends not to be bilateral, and is frequently accompanied by a simulated hypopyon.

Fig. 18.11 Tumor necrosis. A, Retinoblastoma shows central area of necrosis and calcification. B, Electron microscopy of another case demonstrates transition between viable cell (above) and necrotic cell (below). Note nuclear chromatin clumping and focal densification of cytoplasmic component in necrotic cell. C, Radiograph of retinoblastoma-containing eye shows calcification. D, Early calcification in cytoplasm of necrotic retinoblastoma cell appears in mitochondria. Nucleus free of calcification. The arrows point to intact cell plasmalemma. Inset shows large foci of cytoplasmic calcification in another necrotic cell.

Fig. 18.12 Necrosis and spontaneous regression of retinoblastoma. A, Elevated gray masses (g) in superior and inferior fundus and adjacent posterior chorioretinal atrophy in left eye of patient who had bilateral spontaneously regressed retinoblastoma. Other eye enucleated because of pain. B, Histologic section of another eye shows mass replacing temporal neural retina (on left). Note intraocular ossification, most marked nasally (on right). C, Increased magnification demonstrates full-thickness replacement and thickening of the neural retina, abnormal blood vessels, and a sprinkling of lymphocytes, all characteristic of massive gliosis that developed in an eye with spontaneous regression of a retinoblastoma. (Case reported in Benson WE, Cameron JD, Furgiuele FP et al.: Presumed spontaneously regressed retinoblastoma. Ann Ophthalmol 10:897, 1978. Reproduced with kind permission of Springer Science and Business Media.)

Fig. 18.13 Blood vessel basophilia. A, Basophilia present around retinal blood vessels probably represents DNA. Choroid in lower right corner. B, Increased magnification of basophilic blood vessel walls.

VII. Mode of extension

A. Local spread

1. Anteriorly by seeding, into the vitreous and aqueous (see Figs. 18.2B and 18.3)

Aqueous seeding may simulate a hypopyon. Deposits may appear on the iris and in the anterior chamber angle and may produce a secondary open-angle glaucoma.

2. Posteriorly (see Figs. 18.5C and 18.5D) by direct extension into the subneural retinal space (Fig. 18.14)

After invasion into the choroid, the tumor may gain access to the systemic circulation. By spread into the optic nerve, the tumor may gain access to the subarachnoid space.

Fig. 18.14 Subneural retinal and choroidal invasion. A, Necrotic (pink) retinoblastoma present between neural retina (out of field above) and retinal pigment epithelium (RPE). Viable (blue) retinoblastoma fills the choroid. B, Increased magnification shows RPE changes over choroidal retinoblastoma.

B. Extraocular extension

1. Orbit (see Figs. 18.5C and 18.5D)

2. Brain
The most common site of metastasis is the central nervous system, and it has an extremely poor prognosis, particularly if radiotherapy is not utilized in the treatment.

C. Metastases (Fig. 18.15; see section Prognosis, later)

Fig. 18.15 Metastases. Retinoblastoma cells are present in bone marrow aspirate.

D. Invasive tumors express N-cadherin, α-catenin, and decreased E-cadherin and CD9. Thus, loss of E-cadherin and gain of N-cadherin expression are features of invasiveness.

Prognosis

Overview

The retinoblastoma five-year survival rate in the United States from 1975 to 1984 was 92.3%; it increased to 93.9% for 1985–1994 and to 96.5% for 1995–2004.