Medulloblastoma - group 3 tumors are malignant tumors of the central nervous system, and one of the most common pediatric tumors. They typically arise from the vermis of the cerebellum and are present in childhood and infancy. They have the worst prognosis of all medulloblastoma groups.
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Terminology
The nomenclature and classification of medulloblastomas are rapidly evolving. In the 5th edition (2021) of the WHO classification of CNS tumors, group 3 and group 4 medulloblastomas are grouped together under non-WNT/non-SHH tumors, which is in turn divided into 8 subgroups 5.
Epidemiology
Group 3 tumors account for approximately 27% of all medulloblastomas, and have a predilection for males, with a 2:1 male to female ratio 1,4.
They are most frequently encountered in children (4-16 years of age) and infants but are rarely seen in adults 1,2,4.
Pathology
The majority of group 3 medulloblastomas have classic or large cell/anaplastic histology 2. They are associated with MYC amplification 2,4.
Radiographic features
The radiographic features of group 3 tumors are similar to group 4 tumors (which we typically associate with medulloblastomas; midline masses arising from the vermis). Unlike group 4 tumors, group 3 tumors tend to be less well defined but have more prominent enhancement 3.
For more details on radiographic features, please refer to the general article on medulloblastomas.
Treatment and prognosis
Surgery is the first line of therapy (as is the case in all medulloblastomas) with the aim being histological proof, molecular subtyping and maximal tumor resection, with adjuvant therapy depending on an overall risk profile (see general article on medulloblastoma) 2.
The incidence of CNS metastatic disease in group 3 tumors at diagnosis is common, found in 30% of all cases, and is even more frequent in infants (47%) 1,2. This often is the case even when the tumor is small, which can be a clue to the diagnosis pre-operatively 4.
Group 3 tumors have the worse prognosis compared to other subtypes of medulloblastoma, similar to group 4 tumors, but significantly worse than SHH and WNT subtypes 1,2:
- 39% 10-year overall survival in infants
- 50% 10-year overall survival in children
Prognosis is also influenced by histological subtype.