Initially, the term primitive neuroectodermal tumor (PNET) encompassed a wide variety of previously individually named tumors which all seemed to share certain pathologic features suggesting origin from a common progenitor cell in the subependymal matrix (primitive neuroectodermal cells) (although the actual cell of origin is unknown). They are histologically indistinguishable but genetically distinct 1).
Now, the recommendation is to call these “embryonal tumors”, 2) but the term PNET is entrenched. These tumors include: retinoblastoma, pineoblastoma, neuroblastoma, esthesioneuroblastoma.
Medulloblastoma (MB) is more than just a PNET of the posterior fossa, as alterations involved in evolution of MBs such as beta-catenin and APC mutations are absent in pineoblastomas and supratentorial PNETs (sPNETs). At least some MBs originate from the external granular layer (EGL) of the cerebellum.
Embryonal tumors most commonly arise in the cerebellar vermis (medulloblastoma), but also occur in cerebrum, pineal, brainstem or spinal cord. Primary spinal cord PNETs are extremely rare (approximately 30 cases reported by 2007 3)).
Dissemination: Embryonal tumors (ETs) may disseminate via the CSF spontaneously, 4) or iatrogenically (following surgery or shunting, the latter is a rare cause of tumor dissemination 5)). Thus, all patients with ETs require spinal axis evaluation (gadolinium enhanced MRI is about as sensitive as water-soluble myelography) and cytologic examination of CSF. Prophylactic craniospinal XRT is indicated following surgical removal, but cranial XRT is avoided if at all possible before 3 years of age to avoid intellectual impairment and growth retardation. Extraneural metastases can also occur.
Collin ’s law: AKA period of risk of recurren ce (PRR) is often applied to children who have been treated for embryonal tumors (especially medulloblastoma) but may also be used with any tumor thought to arise from a gestational event. It states that PRR is equal to the age at diagnosis plus 9 months 6). Patients that remain free of recurrence beyond the PRR have a much lower risk of recurrence, however recurrence beyond this time has been reported in a small number (≈ 1.4%) of cases, 7) and other tumors may occur e.g. as a result of induct ion by XRTused to treat the initial tumor.
A mass of rapidly growing cells that begins in embryonic (fetal) tissue. Embryonal tumors may be benign or malignant, and include neuroblastomas and Wilms tumors. Also called embryoma.