Chapter 6 External Photon Beams: Physical Aspects - IRSN

446.8 CENTRAL AXIS DEPTH DOSES IN WATER: SAD SETUP6.8.2 Relationship between TAR and PDD PDDs at two different SSDs (SSD 1 = f 1 and SSD 2 = f 2 ):Identical field size A Q at depth z in the phantom:PDD( zAf , ,1, h)=PDD( zAf , , , h)2f1+zmaxPSF( A , h) f + z f2+ z 2 1= PSF( A1, h) f2 + zmax2Mayneord factorIAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.8.2 Slide 6 (87/170)6.8 CENTRAL AXIS DEPTH DOSES IN WATER: SAD SETUP6.8.3 Scatter-air ratio SAR TAR(z,A Q , h ) consists of two components:• Primary component TAR(z,0, h ) for zero field size• Scatter component referred to as scatter-air ratio SAR(z,A Q , h)SAR(z,A Q,h) = TAR(z,A Q,h) TAR(z,0,h)The SAR gives the scattercontribution to the dose at pointQ in a water phantom per 1 cGyof dose to a small mass of waterat point Q in air.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.8.3 Slide 1 (88/170)