chapter 2. dosimetric principles, quantities and units - IRSN

Chapter 2. Dosimetric Principles, Quantities and UnitsAs a high energy photon beam penetrates the medium, collision kerma is maximalat the surface of the irradiated material because photon fluence is greatest at thesurface. Initially, the charged particle fluence, and hence the absorbed dose,increases as a function of depth until the depth of dose maximum z m axis attained.• If there were no photon attenuation or scattering in the medium, but yetproduction of electrons, a hypothetical situation, as illustrated in Fig. 2.3(a),would occur: the build-up region (with β < 1) is followed by a region of completeCPE where D = K col, i.e., β = 1.• In the more realistic situation, however, due to photon attenuation and scatteringin the medium, a region of TCPE occurs (Fig. 2.3(b)) where there exists anessentially constant relation between collision kerma and absorbed dose. Thisrelation is practically constant since, in high-energy photon beams, the averageenergy of the generated electrons and hence their range does not changeappreciably with depth in the medium.FIG. 2.3. Collision kerma and absorbed dose as a function of depth in a medium, irradiatedby a high-energy photon beam.48