Radiation treatments affect all cells that are targeted. This means where normal healthy cells are targeted along with tumor cells, there will be injury to the healthy cells. The Merck Manual states the following:
Radiation Injury to the Nervous System: The nervous system can be damaged by radiation therapy. Acute and subacute transient symptoms may develop early, but progressive, permanent, often disabling nervous system damage may not appear for months to years. The total radiation dose, size of the fractions, duration of therapy, and volume of [healthy brain] nervous tissue irradiated influence the likelihood of injury. Considerable variation in individual susceptibility complicates the effort to predict safe radiation doses. (Source: The Merck Manual of Diagnosis and Therapy, Section 14, Neurologic Disorders.)
Side-effects of radiation are caused by the radiation treatment’s affect on normal cells with some being minimal and other being permanent. Additionally, the effects may occur quickly (acute) or months and years after treatment.
Acute reactions occur during or immediately after radiation. They are normally caused by swelling and can be easily controlled with medications. Delayed or late reactions are normally permanent and can be progressive. They can vary from mild to severe and may include decreased intellect, memory impairment, confusion, personality changes among other changes. All symptoms would be dependent on the amount of healthy tissue targeted with radiation.
Oncogenesis, the development of another tumor from the radiation treatment to the brain, is now a recognized, although rare, possible long-term side-effect of radiation to the brain. When another tumor occurs it is rare, and is most often associated with whole brain radiation or with fractionated radiotherapy. Each of these target more healthy brain tissue than one-session radiosurgery.
The most important component of whether there is radiation injury to the brain is dependent upon the amount of healthy tissue that is targeted. Additional factors will be the radiation dose that the healthy tissue receives and how it receives it. The type of instrument that is available to an individual for treatment will be the deciding factor in radiation damage. Today patients are fortunate in that there are choices for treatment that limit radiation to healthy brain tissue to small amounts or none with one-session radiosurgery. The most common machine for this type of treatment is the neurosurgical instrument the Gamma Knife®. The Gamma Knife severely restricts the radiation of healthy tissue by targeting exactly where the neurosurgeon directs the radiation to the tumor bed with negligable overlap to healthy tissue.
Linear accelerator technology can also be used for one session treatment or for fractionated radiotherapy.
With fractionated radiotherapy, a larger path of healthy tissue is targeted than with one-session treatments due to limitation with the type of linear accelerator machines utilized. In the past , it has been felt that some of the healthy cells that are radiated within the brain will have time to heal if the treatments are given over time (fractionated). Since the brain does not regenerate like other body cells, there is much debate over that value of fractionation within the brain. Additionally, fractionated radiotherapy may allow the faster growing tumor or cancerous cells within the brain to recover between treatments as tumor cell do regenerate quickly unlike normal brain tissue. Machines that do fractionated treatments are linear accelerator based. It should be noted that with daily treatments over time there is less accuracy that with one session radiosurgery as the skull can not be targeted in exactly the same place (repositioned) and manner with each subsequent treatment as it was in the first treatment. IGRT (Image guided radiation therapy) allows for each session to be reimaged before the treatment that can provide more accuracy than without the imaging. All high level linac machines are considered high-level and provide IGRT imaging including the X-Knife®, Trilogy®, SynergyS®, Novalis®, and CyberKnife®. These machines would be considered comparable in effectiveness of treatment and outcomes.
However, the most precise, lowest cost and accurate treatment is still with one session radiosurgery.
Whole brain radiation therapy (WBRT) may target wide areas of the brain resulting in more neurotoxicity. Significant neurotoxicity has been reported with the use of WBRT. Acute effects include hair loss (alopecia), nausea, vomiting, lethargy, otitis media and severe cerebral edema. Though some of these effects can be transient, dermatitis, alopecia, and otitis media can persist for months after irradiation. Chronic effects are even more serious, and these include atrophy, leukoencephalopathy, radiation necrosis, neurological deterioration and dementia.
Reports of development of severe radiation induced dementia have varied between 11% in one-year survivors to 50% in those surviving two years. The time involved in this therapeutic intervention frequently is two to six weeks, in itself a burden to many patients.
We now know that new tumors may again ‘seed’ to the brain within a few months of having completed whole brain radiation. Thus the treatment may only helps for one point in time. Unlike radiosurgery or conformal radiotherapy there is a limit to how much whole brain radiation therapy a person may have. This is usually 6000 gray.
WBRT is the most damaging of all types of radiation treatments and causes the most severe side effects in the long run to patients. In the past, patients who were candidates for whole brain radiation were selected because they were thought to have limited survival times of less than 1-2 years and other technology did not exist. Patients in good survival status (more than 18 months) may need to question the use of WBRT as a first line of defense where one-session radiosurgery or multi-session stereotactic radiotherapy can be repeated for original tumors or used for additional tumors with little or no side effects from radiation to healthy tissues. Increasingly, major studies and research have shown that the benefits of radiosurgery and stereotactic radiotherapy can be as effective as WBRT without the side effects.
Where one-session radiosurgery or stereotactic radiotherapy can not be performed, fractionated radiation treatments should be the next line of treatment considered. A patient's case is always specific to them and must be discussed with their treating physician as to what may be appropriate an treatment for them. If in doubt, the patient should consider a second or third opinion. Survival with good quality of life is the most desirable outcome.