Walsh & Hoyt: General Considerations

The most common form of radiation in current use is the photon (x-ray or gamma-ray). The prevailing dosage unit is the gray (Gy), defined as 1 joule (J) of energy absorbed by 1 kg of tissue; 1 Gy is equal to 100 cGy or 100 rads. Radiation directly damages DNA and also generates free radicals, which alter DNA. The cell survival curve consists of an initial linear portion of the cells killed by a single exposure. This is called the alpha effect and is nonrepairable. With multiple radiation treatments, the curve becomes exponential. This is the beta effect, and it may be repairable if there is sufficient time between successive exposures to allow cells to recover. The alpha effect predominates in acutely reacting tissues and tumors that are rapidly dividing; the beta effect is predominant in slowly dividing tissues, such as those in the CNS.