Chapter 6 External Photon Beams: Physical Aspects - IRSN

246.6 RADIATION TREATMENT PARAMETERS6.6.3 Peak scatter factorAt low photon energies, z max is on the phantom surface(z max = 0) and the peak scatter factor is referred to as thebackscatter factor BSF.PSF for field size of zeroarea is equal to 1 forall photon beam energies,i.e., PSF(0 0,h) = 1As the field size increases,PSF first increasesfrom unity linearly as fieldsize increases and thensaturates at very largefields.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.6.3 Slide 4 (47/170)6.6 RADIATION TREATMENT PARAMETERS6.6.3 Peak scatter factorThe interrelationship betweenthe amount of backscatteringand the scattered photonpenetration causes the PSF:• First to increase slowly withbeam energy.• Then to reach a peak aroundHVL of 1 mm of copper.• Finally to decrease with furtherincrease in beam energy.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.6.3 Slide 5 (48/170)