Rads, Gray (Gy) The gray (symbol: gy) is the si unit of absorbed radiation dose of ionizing radiation (for example, x-rays), and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter (usually human tissue). 1 rad = 0.01 gy.
Voltage. Linear accelerators have energies measured in megavoltage equivelant. 6, 10 or 18 ere examples. Physicists have the properities of a machine programmed in the treatment planning computer and the strength picked will allow us to see how the radiation doses are absorbed. We prescribe a total dosage in centigray to reflect how much we want absorbed in the tissue. Devices can measure it as given.