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KEY POINTS
Fetal brain tumors are extremely rare and usually remain undiagnosed through pregnancy.
When diagnosed in pregnancy the diagnoses are made in the majority of the cases late and most of them carry a poor prognosis.
Choroid plexus papillomas and lipomas have a better prognosis.
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Most neonatal brain tumors have a fetal onset, though late in pregnancy; therefore, these tumors are called congenital brain tumors (CBTs). However, the confidence with which the fetal origin of a tumor can be taken for granted weakens with advancing age of the neonate. This is why in the postnatal literature “congenital brain tumors” are defined as those that become symptomatic from within the first 2 months of life1 to those diagnosed up to 1 year of age.2
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The significant discrepancy in the time limit accounts for the differences in the reported incidence. CBTs account for 0.5% to 1.9% of all pediatric brain tumors. The incidence of CBTs ranges from approximately 0.3 cases per 100,000 live births in Japan3 to 2.9 cases per 100,000 live births in eastern Denmark.4
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The actual incidence at birth is difficult to ascertain due to possible underreporting of cases undergoing termination of pregnancy or intrauterine fetal death, but is apparently rising to reach 1.7 to 13.5 per 100,000 live births.5
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Although the cause of the development of most fetal tumors is still unknown, developments in the field of genetics demonstrate an increasing number of tumoral conditions closely related not only to DNA mutations, deletions, and insertions but also to epigenetic profiles. These new data have impacted the diagnosis, allowing for the subgrouping of heterogeneous tumor groups and leading to the complete renaming of some tumor types. For example, the designation of primitive neuroectodermal tumors (formerly known as PNETs) has been completely eliminated and replaced by subgroups defined by the absence or presence of specific chromosomal amplification.6 At the same time, medulloblastomas, diffuse astrocytomas, and ependymomas now have specific subtypes based on molecular features.6 In addition, atypical teratoid/rhabdoid tumors (ATRTs) are being subgrouped according to alterations of either INI1 or, very rarely, BRG1, which can be assessed through immunohistochemistry. Hence, since 2016 the final diagnosis needs to be reached not only through a detailed imaging workup followed by histology, but relies also on a detailed genetic assessment of the lesion.
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The type of chromosomal abnormalities appearing in pediatric brain tumors differs from that found in adult brain tumors. Genetic abnormalities detected at the chromosomal and molecular levels have been observed in several fetal brain tumors.7,8 The most common abnormality in medulloblastoma is the partial or total loss of chromosome 17, a monosomy of (17q), which is observed, according to some authors, ...