**2. Epidemiology of brain tumors in children**

The incidence of pediatric CNS tumors varies worldwide with an average of 4 cases per 100,000 children with the highest occurrence is in the United States [3]. By age groups, the highest incidence is in adolescents (15–19 years, 6.38/100,000), followed by a group of children under 1 year (6.2/100,000). Subsequently, it is slightly declining, with 5.5/100,000 children aged between 1 and 4 years; in 5–14 years, the incidence is 5.1/100,000 [2, 5]. About 25–30% are in supratentorial localization, followed by the cerebellum (15–20%), the brain stem (10–12%), the pituitary and suprasellar region (10–15%), cranial nerves (6–7%), brain ventricles (5–6.4%), spinal cord (4.3–4.6%) and 2.6–2.9% are tumors of meninges [2, 6], as shown in **Figure 1**. Also a typical localization and histological type occur in certain age group. By 1 year of age, tumors most often occur at multiple locations in the brain and in ventricles. At the age of 1–4 years, the most common site is the cerebellum, cerebral hemispheres and brain stem. In the age range from 5 to 9 years, tumors occur frequently in the cerebellum, brain stem and cerebral hemispheres. At the age from birth to 9 years, the most common tumors are gliomas and embryonal tumors (up to 1 year is the most common embryonic atypical teratoma/rhabdoid tumor (AT/RT), in the older children it is medulloblastoma (MB)). In 10–14 year age group, the most typical site is cerebral hemisphere and most frequently occurring are gliomas, tumors of the pituitary region and embryonal tumors. Adolescents (aged 15–19 years) most often have pituitary tumors, then astrocytomas and neoplastic tumors, but the incidence of meningiomas also increases. Typical localizations in this age group are the pituitary and suprasellar regions, followed by cerebral hemispheres and cerebellum. In general, brain tumors are more common in boys [2, 5].

diseases [7], congenital anomalies [8], higher birth weight and larger head circumference [9]. Interestingly, the results of studies suggest lower incidence of CNS tumors in children with

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The new WHO classification of CNS tumors from 2016 for the first time uses molecular genetic characteristics for tumor classification in some cases. In this new edition, some new tumor entities appeared, some have been merged while others were excluded. Changes involving childhood tumors include: inclusion of epithelial glioblastoma, removal of the term brain gliomatosis, reclassification of the diffuse intrinsic pontine glioma to diffuse midline glioma, *H3 K27–*mutant, inclusion of the ependymoma, *RELA* fusion–positive as a separate unit and inclusion of the diffuse leptomeningeal glioneuronal tumor. The change also affected the classification of medulloblastomas, where genetically defined classification was added. The term primitive neuroectodermal tumors (CNS PNET) was eliminated while an embryonal tumor

Childhood brain tumors are also classified according to their localization into: infratentorial, supratentorial tumors and tumors of parasellar region. Most frequently observed histological

**Infratentorial** localization is typical for cerebellar astrocytoma (pilocytic astrocytoma – PA, but also diffuse less frequently anaplastic astrocytoma and glioblastoma), medulloblastomas, ependymomas, brain stem gliomas (most commonly diffuse midline glioma, *H3 K27–*

**Supratentorial** localization are common: low-grade glioma—LGG (PA, diffuse astrocytoma, oligodendroglioma, mixed oligoastrocytoma, subependymal giant cell astrocytoma— SEGA, pleomorphic xanthoastrocytoma—PXA), high-grade glioma—HGG (anaplastic astrocytoma, anaplastic oligodendroglioma and glioblastoma), neuronal and mixed neuronal-glial

allergies and asthma [10, 11].

**3. Classification of brain tumors in children**

with multilayered rosettes, *C19MC*–altered was included [12].

types of pediatric CNS tumors are shown in **Figure 2**.

**Figure 2.** Most frequent histological types of CNS tumors in children [5].

mutant, PA), AT/RT and choroid plexus tumors.

There are many risk factors for brain tumor development, but to date only some hereditary syndromes (type 1 and 2 neurofibromatosis, tuberous sclerosis, Li-Fraumeni syndrome, Gorlin syndrome, Turcot syndrome, Cowden syndrome, Rubinstein-Taybi syndrome and hereditary retinoblastoma) and ionizing radiation have been verified. Other possible risk factors include: a personal history of previous cancer treatment, a family history of the CNS tumor, the parent age at the time of conception, the later contact of the child with common childhood infectious

**Figure 1.** Localization of CNS tumors in pediatric population [5].

diseases [7], congenital anomalies [8], higher birth weight and larger head circumference [9]. Interestingly, the results of studies suggest lower incidence of CNS tumors in children with allergies and asthma [10, 11].
