Several lines of evidence implicate granule neuron precursors (GNP) in the external granule layer (EGL) of the developing cerebellum as likely cells of origin for certain classes of medulloblastomas.
1). For example, cells that compose a preneoplastic stage of medulloblastoma colocalize with GNPs in the EGL and they express molecular markers of immature granule neurons ( 2). Another possible medulloblastoma cell of origin has been identified: a neural progenitor located in the cerebellar white matter and expressing both nestin and prominin ( 3). Signal transduction pathways that stimulate proliferation and inhibit differentiation of GNPs and other neural progenitor cells during development have been implicated in medulloblastoma. Thus, understanding the mitogenic functions of these pathways will yield insights into medulloblastoma formation.
The overexpression of proteins that normally stimulate proliferation of neural progenitor cells may initiate medulloblastoma formation. Two known mitogens for neural progenitors are the c-Myc oncoprotein and Sonic hedgehog (Shh), a crucial determinant of embryonic pattern formation in the central nervous system.
Several genes have been implicated in the development of medulloblastoma in children, including Patched-1 and Smoothened. The protein products of these genes function within the sonic hedgehog molecular signaling pathways, which are important in neural development and disease.
Combined activation of the Shh/Ptc and IGF signaling pathways is an important mechanism in MB pathogenesis 2).
Both pathways are essential regulators of granule neuron precursors (GNP) proliferation during cerebellar development. In cultured GNPs, IGF signaling stabilizes the oncogenic transcription factor N-myc by inhibiting glycogen synthase kinase 3beta-dependent phosphorylation and consequent degradation of N-myc. However, determinants of Shh and IGF tumorigenicity in vivo remain unknown
Activation of the Sonic hedgehog (Shh)/Patched signaling pathway in the postnatal cerebellum is sufficient to induce medulloblastoma in mice. Activation of the phosphatidylinositol 3-kinase (PI3K) signaling pathway by insulin-like growth factor-II, inactivation of the p53 tumor suppressor protein, loss of DNA damage repair mechanisms, and ectopic expression of Myc oncoproteins cooperate with Shh/Patched signaling to enhance tumor formation in mice. Ectopic expression of alpha and beta interferons in the developing brain also induces Shh-mediated medulloblastoma formation, suggesting a possible role for antiviral response in the genesis of medulloblastoma 3).
Cerebrospinal fluid (CSF) dissemination to the cranio-spinal axis occurs in 30% to 40% of cases 4).
Usually arises from the floor of the 4th ventricle
Typically squeezes out the foramen of Luschka
Genomics-based classification has identified four major subgroups and provides greater opportunity for developing targeted therapies more successful than current conventional therapy.
Surgical resection is undertaken with the goal of gross total resection. Postoperative neuroimaging studies are compared with preoperative studies to determine the amount of residual disease.
The prognostic benefit of increased extent of resection for patients with medulloblastoma is attenuated after molecular subgroup affiliation is taken into account. Although maximum safe surgical resection should remain the standard of care, surgical removal of small residual portions of medulloblastoma is not recommended when the likelihood of neurological morbidity is high because there is no definitive benefit to gross total resection compared with near-total resection 10).
Cerebrospinal fluid is obtained from a lumbar puncture done at the conclusion of the surgical resection or 2 weeks after surgery in order to determine microscopic leptomeningeal spread. Children are enrolled, when possible, in open clinical trials.
Chemotherapy and radiation are given as per protocol. The goal of current treatment approaches is to tailor therapy based on clinical risk factors, with intensification of treatment for children with high-risk disease and reduction of radiation therapy for those with standard-risk disease.
Chemotherapeutic trials have been developed to assess the safety and efficacy of various multi-agent therapies to improve the poor results of high-risk patients and to allow reduction in the dose of radiation needed to cure standard-risk patients, which may allow a decrease in late cognitive sequelae. Currently, it is policy to evaluate all children with posterior fossa tumors characteristic of medulloblastoma with preoperative, staging neuroimaging studies of the craniospinal axis.
Lack of standard response criteria in clinical trials for medulloblastoma and other seeding tumors complicates assessment of therapeutic efficacy and comparisons across studies. An international working group was established to develop consensus recommendations for response assessment. The aim is that these recommendations be prospectively evaluated in clinical trials, with the goal of achieving more reliable risk stratification and uniformity across clinical trials. Current practices and literature review were performed to identify major confounding issues and justify subsequently developed recommendations; in areas lacking scientific investigations, recommendations were based on experience of committee members and consensus was reached after discussion. Recommendations apply to both adult and pediatric patients with medulloblastoma and other seeding tumors. Response should be assessed using MR imaging (brain and spine), Cerebrospinal fluid cytology, and neurologic examination. Clinical imaging standards with minimum mandatory sequence acquisition that optimizes detection of leptomeningeal metastases are defined.
Warren et al. recommend central review prior to inclusion in treatment cohorts to ensure appropriate risk stratification and cohort inclusion. Consensus recommendations and response definitions for patients with medulloblastomas and other seeding tumors have been established; as with other RANO recommendations, these need to now be prospectively validated in clinical trials 11).
A 63-year-old woman with an atypical medulloblastoma in the cerebellum and a lesion in the suprasellar area that did not appear to be a metastasis of the medulloblastoma. The patient underwent a subtotal resection of the cerebellar medulloblastoma which was classified histologically as classic subtype and molecularly as non-WNT/non-Sonic hedgehog (SHH) subtype in the World Health Organization Classification of Tumors of the Central Nervous System 2016. Then she underwent postoperative chemotherapy followed by radiotherapy. We administered chemotherapy to facilitate therapeutic diagnosis of the suprasellar lesion. The combination treatment resulted in the disappearance of the cerebellar medulloblastoma with treatment toxicity well tolerated, additionally the suprasellar lesion remains under control.
Even in adults over 60 years of age, medulloblastoma should be included in the differential diagnosis of a cerebellar mass, and chemotherapy for adult medulloblastoma has the potential to be efficacious and tolerable 12).