Stereo tactic treatment can be given with a gamma knife which is dedicated tele cobalt unit having multiple pencil beams of gamma irradiation from Co60 source or X-knife which is a Linear Accelerator with a attachment of special cones to deliver pencil beams. Localization is done using special head rings fixed to the head to the patient. Dose varies from 12 -25Gy as a single fraction.  

Radiobiology:

The interaction of ionizing radiation with biological material proceeds through several stages resulting in wide variety of biological end effects. Ionizing radiation interact with molecules producing excitation and ionization. The chemical changes in irradiated molecule can be direct or indirect. Since water forms > 70% most of the indirect action involves water molecules. Radiation results in 2 types of cell death. Reproduction death occurs in dividing cells and interphase death not restricted to proliferative cells. Neurons which are not capable of cell division undergo interphase death.

Rapidly dividing cells are more radiosensitive compared to slowly dividing cells. CNS tumor with slowly dividing cells is less radiosensitive compared to epithelial tumor. Since cells show variety of sensation in different phases of cell cycle, fractionated radiotherapy is given in an attempt to attack more and more cells in the sensitive phase of cell cycle. Fractionation also helps normal cell recovering because of differential recovery of normal and tumor cells. As the number of fraction increases the total dose has to be increased because of recovery of cells from sub lethal damage resulting in wastage of radiation. But when large single dose is given the damage to normal tissue is the same as the tumor tissue and hence normal tissue cannot be included with target volume. This is the basic principle of stereo tactic radio surgery which involves giving a large single dose to the lesion resulting in necrosis of the treated area.

Effects of radiation on the brain:

Large single dose or radiation causes brain death within hours. There is wide spread increase in vascular permeability and increase in intracranial tension. At dose levels used in therapy the same phenomenon occurs to a lesser degree.

Acute reaction occur during or immediately after a course of irradiation (within 2 weeks). There may be headache due to increased pressure for first few fractions. During the acute phase, blood vessels, nerve cells and glial cells are injured directly. Vascular changes contribute to further cell degeneration. There may be reduction of conscious level and worsening of focal neurological signs. These phenomena are rarely seen today due to liberal use of Dexamathosone and due to provision of shunts.

Delayed reaction may be " early delayed" (subacute) appearing a few weeks to a few months (6-12 weeks) after radiation or "late delayed" starting month or years (4-40 months) later.

The " early delayed" (subacute) reaction is usually one of transient demyelination due to temporary depletion of oligodendroglia. The somnolence syndrome represents an early delayed radiation reaction in the brain and in characterized by transient period of exhaustion at 2 weeks, drowsiness, lethargy and anorexia at 4-6 weeks after radiation. It is usually reversible over a period of 1-2 weeks. 4-8 weeks after radiation, there may be rapidly progressive ataxia, cranial nerve and focal neuro deficits and nystagmus and takes longer (1-2mths) to recover.

Late delayed damage is the most feared complication and vary depending on the region irradiated. This results from the continuation of oligodendroglial loss and endothelial damage leading to demyelination and necrosis of while matter. Less severe forms of late damage may occur in children as cognitive impairment and is age related. Midline structure like midbrain, brain stem and hypothalamus are particularly vulnerable CT scan of brain of long term survivors of large field radiation show generalized atrophy with wide sulci and large ventricle, but it does not seem to be associated with noticeable deterioration on patients intellectual status. Most are irreversible.

Delayed spinal cord damages include, acute ( over several hours) complete quadriplegia/paraplegia, LMN syndrome (muscle atrophy, areflexia, fasciculations) developing over weeks, and Chronic progressive myelitis.

Additional form of delayed radiation injury, is the development of second malignancy of a different histologic type within the irradiated field, following years after irradiation.

Pathology: Radiation injury is characterized by a shrinking and shriveling of the cortex. In the spinal cord, the cord is thinned out. Histologically there are areas of confluent coagulative necrosis of the white matter, vascular thickening, telangiectasia and vascular proliferation.

Imaging: There are no radiographic changes that are pathognomonic of radionecrosis. Isotope scan may help. However, histologic confirmation is required in most instances.

Clinical applications:

Goals of radiotherapy are curative and palliative. In curative radiotherapy. some degree of risk is accepted for a reasonable probability of permanent eradication of the malignancy. In medulloblastoma, risks of radiation injury to cognition, a diminution of bone growth, or other toxicities may be acceptable for a 60%-80% probability of cure. In palliative radiotherapy, the intent is to ameliorate the symptoms and it would be a short course avoiding radiation induced CNS toxicity. Currently, the whole brain radiotherapy for the treatment of GBM is no longer practiced. Localized field covering 2 - 3cm area around the peritumoral edema is the standard portal up to 45Gy. This is followed by the booster dose to the reduced field with 1.5-2.0cm around the tumor.

Doses of radiation are typically prescribed in cGy (centiGray). The cGy, a unit of absorbed dose in tissue, became the standard dose unit of radiation in 1980 when it replaced the term rad (radiation absorbed dose). Appropriate dose depends on radiation tolerance of the surrounding normal tissue and the data concerning the dose response of the tumor to radiation. The dose fraction limits for brain necrosis are approximately 35 Gy/10 fractions or 60Gy/30 fractions. CNS tissue is late reacting tissue and the reaction is more related to dose per fraction as well as total dose. Hence in radical radiotherapy where patient is expected to survive long, daily dose should not exceed 180-200cGy for adults and 150cGy for children The total dose should not exceed 60-65 Gy.

Next is the appropriate volume for irradiation ( amount of tissue that needs to be encompassed in the radiation beam). This is ascertained by appropriate imaging studies, with a margin, and a sound understanding of the patterns of spread of the tumor. For example, in medulloblastoma which is known to seed to the spinal axis, the volume must encompass the entire craniospinal axis.

Finally, one must choose the right technique. A common technique is parallel opposed lateral beams directed at the brain. Other techniques include parallel opposed lateral fields, and a vertex field, arc treatment, or single beams. In the spinal malignancies, either a single posterior field, parallel opposed anterior and posterior fields, or various angled fields. A computer reconstruction of the radiation dose distribution (computerized dosimetry) is widely employed. This has resulted in better and accurate dose delivery minimizing the dose to surrounding normal brain tissue and resulting in fairly uniform dose distribution to the tumor. Imaging modality like MRI has considerably improved the accuracy of target volume definition.

3 D conformal treatment using multiple collimator and stereo tactic radiation can be considered as the ultimate precision radiotherapy.

No statistically significant benefit is noted with combined radiotherapy and chemotherapy.