Primitive Neuroectodermal Tumor
Intracranial neoplasms known as primitive neuroectodermal tumors (PNETs) develop in some patients with bilateral retinoblastoma; PNETs represent the intracranial component of trilateral retinoblastoma (see Fig 19-11). The ectopic focus usually is located in the pineal gland or the parasellar region and traditionally is known as a pinealoblastoma. This tumor affects up to 5% of children with a germline RB1 mutation. In rare cases, the PNET arises prior to ocular involvement. More commonly, this independent malignant tumor presents months or years after treatment of intraocular retinoblastoma.
Several observations support the concept that an intracranial PNET constitutes a primary tumor, as opposed to intracranial spread of retinoblastoma. In some patients with terminal retinoblastoma, CT findings have shown that intracranial tumor is anatomically separate from the ocular tumor(s). These intracranial tumors are not associated with metastatic disease elsewhere in the body and—unlike metastatic retinoblastoma—often have features of differentiation, such as Flexner-Wintersteiner rosettes (see Chapter 11, Fig 11-44A). Embryologic, immunologic, and phylogenic evidence of photoreceptor differentiation in the pineal gland further support the concept of trilateral retinoblastoma.
It is recommended that all patients with retinoblastoma undergo baseline neuroimaging studies to exclude intracranial involvement. Experts disagree on the role of serial imaging in screening for PNETs. Investigators have shown that the incidence of PNETs decreases over time. It is unclear whether this is due to the prophylactic effect of systemic chemotherapy or a decrease in the use of radiation therapy.
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Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.