Chapter 6 External Photon Beams: Physical Aspects - IRSN

296.7 CENTRAL AXIS DEPTH DOSES IN WATER: SSD SETUP6.7.1 Percentage depth doseThe dose at point Q in the patient consists of two components:primary component and scatter component.• The primary component is expressed as:PDD pri = 100 D priQ= 100 f + z maxpriDP f + z2e μ eff (zz max )μ effis the effective linear attenuation coefficient for the primarybeam in the phantom material (for example, μ efffor a cobalt-60beam in water is 0.0657 cm -1 ).IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.7.1 Slide 3 (57/170)6.7 CENTRAL AXIS DEPTH DOSES IN WATER: SSD SETUP6.7.1 Percentage depth doseThe dose at point Q in the patient consists of two components:primary component and scatter component.• The scatter component at point Q reflects the relative contributionof the scattered radiation to the dose at point Q. It depends in acomplicated fashion on various parameters such as depth, fieldsize and source-skin distance.• Contrary to the primary component in which the photoncontribution to the dose at point Q arrives directly from the source,the scatter dose is delivered by photons produced throughCompton scattering in the patient, machine collimator, flatteningfilter or air.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.7.1 Slide 4 (58/170)