Chapter 6 External Photon Beams: Physical Aspects - IRSN

396.8 CENTRAL AXIS DEPTH DOSES IN WATER: SAD SETUP6.8.1 Tissue-air ratioZero area field is a hypothetical radiation field in which thedose at depth z in phantom is entirely due to primaryphotons, since the volume that can scatter radiation is zero. Zero area TAR(z,A Q , h)is given by a simpleexponential function:TAR(z,0,h) = e μ eff (zz max )For cobalt-60 beam:• μ (Co) eff= 0.0657 cm• TAR(10,0,Co) = 0.5361IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.8.1 Slide 4 (77/170)6.8 CENTRAL AXIS DEPTH DOSES IN WATER: SAD SETUP6.8.1 Tissue-air ratioThe concept of “dose to small mass of medium” is notrecommended for beam energies above cobalt-60.Consequently, the concept of TAR is not used for beamenergies above cobalt-60 gamma rays.TARs are most reliably measured with ionizationchambers; however, the measurements are much morecumbersome than those of PDD, because in TARmeasurement the source-chamber distance must bekept constant.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.8.1 Slide 5 (78/170)