Units of Radiation Dose and Maximum Permissible Doses

The radiation energy from the radioisotope is absorbed by the living tissue of the patient subjected to the radiation therapy or radiotherapy. The traditional unit of radiation dose is called Rad and one Rad stand for 100 egrs of radiation energy absorbed per gram of the absorber tissue Therapeutic doses are usually measured in hundreds or thousands of Rads. In diagnostic procedures where it is desirable to keep the dose very low, the unit is millirad (mr). One millirad is the thousandths part of a Rad and only a few hundred millirads are permissible dose.

The radiation protection is an important precaution to avoid over exposure of a patient as well as the radiation worker. The radiation protection unit is called dose equivalent and its traditional unit is REM (Rad equivalent man).

• REM = Rad Equivalent Man


• REM = Rad x Relative Badge Effect


• REM = Rad x RBE

RBE depends upon the type of the radiation energy. RBE for x-rays, g-rays and b-particles is an energy up to 3 MeV (3 MeV = 1 Rad). RBE for positrons, neutrons and a-particles is 10 rem (REM). It is evident that positrons, neutrons and a-particles exert lesser side effects as compared to x-rays, g-rays and b-particles. The S.I. unit of radiation protection is Seivert(Sv).

• 1 Sv = 100 Rem


• 1 Sv = 1000 milliSeivert (mSv) = 100 Rem


• 10 mSv = 1 Rem

Maximum permissible dose (MPD) or maximum limit of per year exposure to radioactivity could be computed separately for the patients (general public) and radiation workers. As a rule anybody below the age of 18 years is never permitted to become a radiation worker. The permissible dose can be worked out by the following formula:

D = 5 (N-18); Here D=Dose in Rem, N= Age in years.

A radiation worker who joins the duty after acquiring the age of 18 years could have maximum exposure of 5 x (20-18) = 5 x 2 = 10 Rem only up till the completion of 20 years of age or 5 Rem per year exposure. General public or a patient could be exposed to radiation energy at a MPD of one tenth (¹/₁₀) of the dose for a radiation worker and that comes out to be 0.5 Rem per year (⁵/₁₀ Rem per year). Hands and feet could be exposed at a higher dose if desired but not more than 75 Rem/year or 40 Rem/3 months. Thyroid, Bone and skin could be exposed at 40 Rem/year and other organs at 15 Rem/year. The fetus should not be exposed to a radiation dose of more than 0.5 Rem.