Relationships between Exposure, Kerma and Absorbed Dose in a Medium exposed to Megavoltage Photons from an External Source

Abstract

The purpose of the paper is to clarify the conditions under which absorbed dose can be determined from a measurement of exposure. It is shown that, for megavoltage photons, the equation D=fX, relating the absorbed dose to the exposure at a point, is valid on the central axis of the field only at the point where the kerma curve, K(z) vs. depth z, crosses the absorbed dose curve, D(z) vs. z. This point is expected to be at a depth slightly less than the depth of peak absorbed dose.At depths where the absorbed dose curve becomes exponential, it is shown that D = {β/(β−μ)}fX, where μ is the effective linear attenuation coefficient for the photons, and β is the effective linear energy absorption coefficient for the secondary electrons. Alternatively, D(z) = f{(F+z−δ)/(F+z)}2 X(z−δ), where D(z) is the absorbed dose at depth z and X(z−δ) is the exposure at depth (z−δ). The equation for δ as a function of μ and β is derived and approximate values of these constants are given for different megavoltage radiations.