medulloblastoma

Medulloblastoma is a malignant primary brain tumor that arises in the cerebellum. It is the most common malignant brain tumor in children and accounts for a substantial proportion of pediatric posterior fossa tumors; it is much less common in adults.

Clinical presentation reflects cerebellar dysfunction and disturbed CSF flow. Symptoms include morning headaches, nausea or vomiting, and signs of increased intracranial pressure, as well as truncal ataxia, imbalance, or dysmetria. Leptomeningeal spread may occur, and metastasis can be present at diagnosis or develop later.

Pathology: Medulloblastomas are composed of small, round, blue cells with hyperchromatic nuclei and scant cytoplasm. Homer Wright rosettes may be seen. The tumor often arises in the cerebellar vermis and has desmoplastic/nodular variants. It is thought to originate from granule neuron precursor cells of the cerebellum.

Molecular subgroups: Current classification identifies four main subgroups: WNT-activated, SHH-activated, and Groups 3 and 4. These subgroups differ in biology, age distribution, pattern of spread, and prognosis, with WNT-activated tumors generally having the best outcome and Group 3 often associated with a higher risk of metastasis.

Diagnosis typically combines magnetic resonance imaging with histopathologic confirmation from tumor tissue. MRI characteristically shows a midline cerebellar mass that can extend to the fourth ventricle, potentially causing hydrocephalus. Staging includes evaluation of the neuraxis for metastatic spread, using MRI and CSF cytology.

Treatment is multimodal. It usually starts with maximal safe surgical resection, followed by craniospinal irradiation with a boost to the posterior fossa. Chemotherapy is commonly added, particularly for younger children or higher-risk disease, and treatment is adjusted based on molecular subgroup and metastatic status to balance survival with long-term effects.

Prognosis varies by age, extent of resection, metastatic spread, histology, and molecular subgroup. Overall survival has improved with risk-adapted therapy, with the best outcomes in WNT-activated tumors and poorer outcomes in some high-risk groups such as certain Group 3 tumors.