How does radiation work?

Higher energy ionizing radiation is used. This energy damages the genetic instructions (DNA) for tumor cell multiplication by ‘picking up’ electrons and removing them from the atoms and molecules of tumor tissue.  Each time the tumor cell multiplies it has more damage until it can no longer replicate and dies. Although some cells may die immediately, others will die over many years. Normal cells affected by radiation have a better ability to repair themselves than do tumour cells.

The objective of radiation therapy is to target tumor cells but spare as much damage as possible of the surrounding normal pituitary cells. Ionizing stereotactic forms of radiation (stereotactic radiosurgery [SRS]) are used to target pituitary tumours.  This may use proton beams or Intensity Modulated radiotherapy (IMRT) to more precisely focus or ‘conform’ the radiation beam to the 3D shape of the tumor and concentrating damaging energy within the tumour.  As the energy passes through the tumour (exit dose), the energy is lowered, sparing damage to normal tissue.

Radiation doses are delivered in a measurement known as Gray (Gy). This is a measure of the amount of radiation absorbed by the patient’s tissue.  1 Gy=1 Joule of radiation absorbed/kg or organ weight (also = 100 rads, which is an old term). The specific dose needed is calculated based on factors such as tumor size, shape, and location.

Stereotactic radiation may be given in multiple small fractions (fractionated) over several weeks (5-6) or in fewer larger doses as Gamma knife or CyberKnife radiosurgery. The Gamma Knife delivers multiple beams at the same time, whereas the CyberKnife System delivers a single, high-energy photon beam to the tumour.

The type and dose of radiation that is appropriate is determined by a radiation oncologist.