Principles of radiation therapy:

The basic principle of radiation therapy is to maximize damage to the tumor while minimizing damage to the normal tissue.Factors that determine how much radiation required are:

1.    The type of radiation
2.    The environment of the cell (for example its blood and oxygen supplies 
3.    The type of cell.
4.    The nucleus of the cell is more sensitive of radiation.

Some types  of radiation are more effective in killing cells or  have a higher relative biological effects(RBE)

RBE: The ratio of the number of grays of 250kVp  x- rays needed to produce a given biological effect to the number of grays of the test radiation needed to the same effect .

Lethal dose for 50%(LD50 ): the quantity of radiation that will kill half of the organisms in a population (cells ,mice, people ect)
 LD30: Theamount of radiation that will kill 50% of the organisms in 30 days ,its value for humans about 4.5 Gy.