Radiation Therapy for Medulloblastoma
Radiation—also called radiation therapy or radiotherapy—is a cancer treatment that uses high-energy X-rays to kill cancer cells. The radiation treatment plan will be created by a specialist called a radiation oncologist.
Because medulloblastoma can spread to the covering of the brain and the spinal cord, surgery to remove the medulloblastoma tumor is typically followed by radiation to the entire brain and spine to help prevent spread or recurrence. This is referred to as craniospinal irradiation, or CSI.
A typical radiation therapy schedule for medulloblastoma consists of a specific number of treatments given over a set period of time (usually 5-7 weeks). The last step in radiation treatment is typically an extra “boost” dose to the tumor bed, which refers to the area where the tumor was before surgery.
How Radiation Works
The X-rays used in radiation treatment deposit energy within the tumor, causing damage to the DNA of cells. The tumor cells are then unable to repair the damage, and ultimately die when the tumor cells try to divide.
It was first noted in the 1950s that patients who received radiation to their brain and spine had higher survival rates compared to patients who did not receive radiation.
Treatment for Patients Ages 5 and Under
Radiation of the whole brain in young children is associated with a decline in neurocognitive function. Because of this, patients under the age of roughly 3-5 are treated with chemotherapy-only regimens if possible, depending on subtype and facility.
The Radiation Therapy Process
Planning Session
Before starting radiation, parents of a child with a medulloblastoma consult with the radiation oncologist to discuss the planned therapy and potential side effects. After this initial consultation, the child undergoes a planning session (also referred to as a simulation). At that time, a mask will be custom-made for the child.
Radiation Mask
The mask allows accurate, consistent positioning of the head for each treatment and helps the child remain still during treatment. The mask is made out of a special type of plastic that becomes moldable when heated in a water bath. After the mask is completed, a special CT scan is performed. This entire process takes about one hour.
Defining the Radiation Area
After the planning session, the radiation oncologist uses the CT scan to define the area that corresponds to the tumor and the regions of the brain that should not receive radiation.
With the help of dosimetrists (who specialize in calculating the dose of radiation to ensure the tumor gets enough radiation) and physicists (who develop and direct quality-control programs for radiation equipment and procedures), a radiation therapy plan is developed to best treat the tumor while minimizing the amount of radiation delivered to normal brain tissues and surrounding tissues.
Delivery of Radiation
Radiation therapy is then delivered daily, Monday through Friday, for six weeks with a total dosage determined by the radiation oncologist. Smaller daily doses adding up to the total dose over weeks is intended to allow normal tissues the chance to repair some radiation-induced DNA damage while still destroying the tumor.
As radiation patients must lie still and alone on a table, some children are too young or too ill to tolerate the radiation treatments while awake. In these cases, the planning session and treatments can be performed under general anesthesia.
A commonly used anesthetic agent in radiation therapy is propofol. Propofol is an intravenous anesthetic that allows for rapid induction and recovery, and—most importantly—does not require intubation (insertion of a tube) for protection of the airway. Even with daily use, the risks of complications with propofol are very low.
Typically, the child is brought into the room awake, with the parents, and the anesthesia is initiated. After induction of anesthesia, the parents leave the room and the radiation therapy procedures are performed.
Types of Radiation Therapy
- Proton beam therapy, also called proton radiation therapy (PRT)
- External beam radiation therapy (the type of radiation is called photon radiation)
Proton Beam Therapy
Proton beam therapy is a type of radiation therapy that delivers high radiation doses directly to the brain tumor site. It is also called proton radiation therapy, or PRT.
PRT uses proton particles for treatment rather than the photons that are used for most radiation treatments. Protons have an anti-tumor effect that is very similar to conventional photon radiation. In some brain tumors, protons are safer than photon radiation because they can spare nearby normal brain tissue from the potentially harmful effects of radiation.
This type of therapy has had limited availability in the United States, but it is becoming more available and is being used more often for treatment of medulloblastoma. It is the preferred method of radiation due to its sparing abilities of surrounding tissue. Availability depends on the facility where you are treated.
External Beam Radiation Therapy (Photon Radiation Therapy)
External beam radiation therapy comes from a machine outside the body that aims radiation at a specific area of the body. It is used to treat many types of cancer.
Most external radiation machines use photon beams. These are the same types of beams used in X-rays, but X-rays use smaller amounts of photon beams. When these beams enter the body, they reach both the cancer cells as well as healthy tissue. The goal is to reach as many of the cancer cells as possible while limiting how much healthy tissue is affected.
FLASH Radiation
FLASH radiotherapy is an experimental approach to cancer treatment that delivers radiation therapy at ultra-high dose rates, typically in fractions of a second. Conventional radiotherapy delivers radiation at much lower dose rates over several minutes.
While these potential benefits are promising, FLASH radiation therapy is still in the early stages of research and clinical testing. Further studies are needed to fully understand how safe and effective it is, and how it can be used across different cancer types and patient populations. Currently, FLASH is not yet being used for pediatric brain cancer therapies.
Re-irradiation
Re-irradiation refers to a second round of radiation therapy on an area of the body that has already had radiation treatment. This was previously not recommended because of the negative impact radiation can have on the body. However, newer research and technologies have shown that re-irradiation can be performed safely and effectively for some patients.
Radiation Side Effects and Possible Complications
Side effects and complications caused by radiation therapy vary from person to person and can range from mild to severe.
Short-term side effects from radiation treatment can include being tired, feeling sick or weak, and hair loss or thinning. Radiation also sometimes causes inflammation in the brain, which can temporarily make symptoms (including headaches, nausea, and seizures) worse. Steroids are used to control inflammation when necessary.
Longer-term side effects can include problems with cognition (memory and learning), vision, hearing, growth, motor skills, and hormones.
Radiation necrosis, which is cell death of brain tissue, can sometimes be a complication of radiation therapy in patients with medulloblastoma. It can cause swelling and potentially lead to neurologic symptoms such as headache, nausea, vomiting, cranial neuropathies, and ataxia (loss of muscle movement coordination). One study found that radiation necrosis occurred in a small number of medulloblastoma cases (19-30%).
While radiation is an effective treatment for destroying cancer cells and lowering the risk of recurrence, the side effects caused by it can be significant and long-lasting.