Medulloblastoma (WHO grade IV) is a malignant, invasive embryonal tumor of the cerebellum that occurs primarily in children, has a predominantly neuronal differentiation, and has a tendency to metastasize via CSF pathways (Roberts et al. 1991; Giangaspero et al. 2000).
Medulloblastoma is the most common childhood central nervous system tumour, but is a relatively rare tumour in adults. About 25 per cent of all medulloblastomas occur in adults (age >16 years). The annual incidence is 0.5 per 100,000 children younger than 15 years accounting for 20–25% of all intracranial tumors in children. In adulthood, 80% of medulloblastomas occur in people aged 21–40 years. These types of tumors rarely occur beyond the fifth decade of life.
Presentation is usually either related to a cerebellar ataxia, or raised intracranial pressure related to obstructive hydrocephalus. Cranial nerve presentations (especially diplopia) also occur.
Imaging and Location
The tumor is the one who most likely tend to metastasize within and outside the neuroaxis, not uncommon to bone marrow or lymph nodes. Medulloblastomas normally occur in the midline, characteristically filling the fourth ventricle. This is also the characteristic appearance on MRI: an intraventricular mass lesion in a midline or paramedian location with isointense signal to gray matter on T2-weighted imaging (Maleci et al. 1992; Koci et al. 1993; Bourgouin et al. 1992).
Medulloblastomas usually arise in the midline and are most commonly found in the cerebellum. The tumour has low signal on T1-weighted MRI scans and high signal on T2-weighted scans and there is usually gadolinium enhancement on T1-weighted images. MRI scan of the spine to look for gadolinium-enhancing nodules can be performed any time after surgery. Thirty to 40 per cent of children will have CSF dissemination, although this may be greater in children under 5 years of age (Deutsch 1988).
The enhancement is intense with some heterogeneity and sometimes ring enhancement. Subarachnoid seeding is common in medulloblastomas, occurring in up to 33% of all patients at the time of initial diagnosis (David et al. 1997). Some investigators believe that the prevalence of CSF seeding may actually be much higher and perhaps present in all patients with the disease.
The management of acute obstructive hydrocephalus is controversial. Surgery for medulloblastoma is performed in the prone position, to avoid the risks of air embolus and pneumocephalus or systemic hypotension when operating in the seated position. A repeat MRI scan at 48 hours may be helpful for assessing the extent of tumour resection.
Radiotherapy to the tumour improves survival in children with medulloblastoma. In addition, there is good evidence that craniospinal irradiation (CSI) reduces the risk of recurrence from CSF dissemination. This study was closed prematurely after an interim analysis at 16 months demonstrated a significant number of relapses in the low-dose CSI treatment group. Craniospinal irradiation also improves the number of 10-year survivors (Castro-Vita et al. 1980; Landverg et al. 1980).
Ventriculoperitoneal shunt involvement is common (20% of cases) and may lead to metastatic spread in the abdominal cavity. Numerous studies have shown that patients with evidence of CSF spread have a poorer prognosis compared with those in whom it is absent (Meyers et al. 2000). Therefore, its detection is crucial to optimal patient management, and those who review these imaging studies must be aware of its imaging manifestations.
The bad prognostic features for medulloblastoma appear to be age less than 3 years, CSF dissemination, and, possibly, extent of resection (Evans et al. 1990). Approximately 10–20 per cent of patients will eventually have metastatic spread outwith the central nervous system (especially lungs and bone). With treatment, approximately 33–66 per cent of children are alive at 5 years and 25–50 per cent are alive at 10 years, depending on the patient selection for the survival analyses (Bloom et al. 1991). Although it was previously thought that adults did less well, a review of cases treated between 1971 and 1981 suggests that survival is no different, or perhaps slightly better (76 per cent alive at 10 years) (Bloom and Bessell 1990).
- M.F. Reiser, W. Semmler, H. Hricak (Eds.) "Magnetic Resonance Tomography", Springer 2008