Sec. 16.1] INTERNAL DOSIMETRY 405 



Tissue dose obtained by calculation is most conveniently stated in terms of 

 one of the units of energy absorbed (ergs, mev, rep, or gram roentgens) since 

 calculations are based directly on the energy released by disintegration of a 

 radioactive isotope or on the energy flux (ergs or mev per square centimeter 

 per second) of beams of neutrons, protons, or other radiations. Conversion 

 of energy absorbed to ionization units, and also to roentgens in the case of 

 gamma radiation, again requires that the value of W be known for the 

 medium. 



A distinction, sometimes overlooked, should be made in the terms dose, 

 dosage rate, total dose, integral dose, and the various combinations of these 

 terms that are frequently used in connection with radioactive isotopes. The 

 term dose refers to roentgens (for gamma rays only), to energy absorbed per 

 gram of tissue, or to ionization produced per gram of tissue at a specified 

 point. These quantities usually vary throughout the volume of an organ 

 or animal in some complicated manner depending upon the source and type 

 of radiation. Since dose is a point function it can often be represented, as 

 in x-ray dosimetry, by a family of isodose surfaces, each surface containing 

 all points in the medium at which the dose has a particular constant value. 

 A statement of dose should, in general, always be qualified by a precise 

 description of the point at which the dose was measured or computed. It 

 must be assumed otherwise that the dose was uniform throughout the organ 

 or entire animal involved. The interval of time in which a specified dose 

 was given is not involved explicitly since all dose units are independent of 

 time. 



Time is introduced explicitly only in the term dosage rate, which, depending 

 upon the units chosen, is the roentgens exposure, energy absorbed, or ioniza- 

 tion produced per unit time at the point under consideration. For radio- 

 active isotopes the dosage rate usually is not constant but depends in some 

 more or less complicated way on metabolic activity and radioactive decay. 

 It is then convenient to speak of total dose, which is the dose received at a 

 point in the tissue during the whole interval of time the isotope remains in 

 the tissue, or during complete decay of the isotope if the isotope remains 

 fixed in the body. 



The terms above refer only to points within the tissue or other medium, 

 and the dose units are roentgens for gamma rays, rep, energy units, or gram 

 roentgens per gram, or J units for any ionizing radiation. Integral dose 

 alone refers to the total energy absorbed or total ionization produced in an 

 entire organ or in a given mass of tissue. The single unit of integral 

 dose is the gram roentgen, and in particular, the roentgen, rep, energy unit 

 and J cannot be used as a unit of integral dose. 



Combinations of terms, such as total integral dose and integral dosage 

 rate, are also frequently used. 



