CNS tumors account for about 20% to 25% of all pediatric malignancies. The incident rate in the 0 to 19 year old age group is 48.47/million. The highest among children 1-4 years and lowest for agest 10-14 years. Only 2-5% are genetic, the largest being NF1, NF2, tuberous sclerosis, nevoid basal cell (Gorlin's) syndrome, familial adenomatous polyposis, and Li-Fraumeni syndrome. A smaller precentage has been attributed to diagnostic or therapeutic ionizing radiation.
There has been significant changes in management of pediatric CNS tumors with improvements in imaging, functional imaging, and physical examinations, improved neuropathology, and molecular diagnostics. Therapeutics has improved along with diagnostics. Neurosurgical techniques have advanced remarkably, as have radiation therapy techniques and chemotherapeutics. Survival has improved as a result from 62.9% in 1980-89 to 75.3% for 2000-2006.
For CSI techniques where indicated, see the Duke University report which can be found in the adult discussion. Treating patients in the supine position can be more readily achieved with modern treatment planning and imaging than in the past.
For most tumor types the target is best defined with CT simulation and CT_MRI fusion. PET has a lesser role, certainly in CNS but also in pediatrics in general due to the higher metabolic activity of brown fat, which is much more prevalent in pediatric patients than in adults. If anatomy has changed as a result of intervention, this must be accounted for, particularly in diseases such as craniopharyngioma which can change volume rapidly.
The ventricles are covered most frequently with CNS germ cell tumors. Subependymal spread is common, and the target volume include the lateral ventricles, the third and fourth ventricles all with a 1 - 1.5 cm margin. Better sparing of normal brain parenchyma can be achieved with IMRT with image guidance.
CSI covers the whole brain and spinal cord including the meninges. Standard (conventional-preCT days) techniques included setting up the patient prone with the neck extended to avoid exit dose through the mouth and jaw, the brain was treated with opposed laterals with blocks and a match to a PA spine field, orthogonal to the lateral fields. For longer spines, the accelerator beam port was not large enough to cover the field, resulting in a second inferior PA field which had to be matched to the superior field, resulting in two matches: 1. The PA/orthogonal opposed laterals and 2.) the inferior and superior PA fields. A skin gap and collimator rotation calculation had to be performed to account for lateral beam intersection with the divergent PA superior beam and the table needed to be rotated to account for lateral beam divergence to match the divergent inferior portion of the lateral field with the superior divergence. (See CSI setup for more details).
The target volume definition is now done with 3D conformality using CT imaging and where appropriate, MRI. CT simulation is now considered necessary to insure full coverage of the cribriform plate. Formerly this was done with plain film/fluoroscopy on lateral images and is now done with CT. Blocks on the lateral ports have been used to protect the lenses and facial structures, but with the advent of CT simulation, it was found that it was imposible to shield the lens while still covering the entire cribriform plate in the sub-frontal region. Where compromise is needed, the PTV coverage should take precedence.
CT is also useful for defining the lateral aspects of the meninges of the spine and important organs at risk (kidneys, lungs, heart) making the field potentially narrower in the thoracic region and wider in the lumbar region, while protecting the gut and gonads. The traditional lower border was anatomically set to S2, but MRI imaging demonstrates the end of the thecal sac varies between L5 and S3, thus allowing individual radiation field adjustments. Generally photons are used to treat the CSI, but with improved treatment planning of electrons (monte carlo) some centers are using electrons and others who have them available are using protons.
Care must be taken with imaging based treatment planning that the fields cover all of the at risk areas, including extensions of the CSF space such as along the optic nerves and into the internal auditory canals.
Conventional dose for pediatric CNS tumors is generally 1.8 Gy/day, the dose typically delivered is between 54 Gy to 55.8 Gy. When treating a primary tumor of the spinal cord, a lower dose of 50.4 Gy is commonly used. Dose is further reduced in infants under age 3 to reduce the risk of neurocognitive deficits. For intracranial germinomas, the dose is reduced further with 1.5 Gy/fraction to 30 Gy - 45 Gy when tumors are highly radiosensitive.
Radiation therapy is an essential part of treatment for many children with CNS tumors but carries a substantial risk of long term sequelae. These are multi-factoral, but radiation has serious consequences on the developing child. Many treatment strategies over the past decades have been used at reducing the risks associated with radiation therapy. Improved tumor/at-risk tissue targeting, newer techniques and modalities all offer better outcomes and improved therapeutic gain.
Quality of life in children with brain tumors is frequently compromised by long term sequalae, particularly neurologic defects. Radiotherapy, either alone or in concert with surgery and chemotherapy is responsible for many late effects. Neurocognitive sequelae have been better characterized and it is know known that myelinization and functional maturation of the CNS continue until well into young adulthood. Radiotherapy through its effect on microvasculature, oligodendrocyte precursor cells that produce myelin, disrupts neurogenesis and causes cortical atrophy. Patients fail to develop new knowledge and skills at an age-appropriate rate and show a progressive decline in IQ over time. The magnitude is a direct function of age at the time of treatment. Other factors, include the dose and volume of radiation, use of adjuvant chemotherapies, location, hydrocephalus also play a role.
Endocrine deficits are also quite common. Radiotherapy is responsible for growth hormone deficiencies as a function of increasing dose to the pituitary/hypothalamus axis. Primary hypothyroidism is seen after axial radiation, there are direct and indirect effects on the skeleton, muscloskeltal development, and increased risk of cardiovascular complications.