ABSTRACT
The response of normal and cancerous cells to radiation can be modified/influenced by physical, biological, chemical, and technical factors. The physical factors include treatment time, dose, fractionation (TDF) factors, dose rate, volume of tissue irradiated, temperature of the treated volume, and the type of radiation linear energy transfer (LET) used. The biological factors are related to the cell being irradiated, which includes the sensitivity of tissue, bystander effect, and age. The chemical factors are due to the presence or absence of particular molecules, either naturally occurring in the body or administered, which includes radio sensitizers, radio protectors, and radio mitigators. The radiation dose (absorbed dose) is defined as the amount of energy absorbed per unit mass of the irradiated medium/tissue. The most important of them are collectively referred to as "four Rs" of radiation biology. The 4Rs are Repair, Repopulation, Redistribution, and Reoxygenation.
