Chapter 6 External Photon Beams: Physical Aspects - IRSN

166.5 PENETRATION OF PHOTON BEAMS INTO PATIENT6.5.3 Depth of dose maximumDepth of dose maximum z max depends upon:• Photon beam energy (main effect)• Field size (secondary effect)For a given field size:• z max increases with photonbeam energy.• For 5x5 cm 2 fields, thenominal values of z max are:Energy 100 kV p 350 kV p Co-60 4 MV 6 MV 10 MV 18 MVz max (cm) 0 0 0.5 1.0 1.5 2.5 3.5IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.5.3 Slide 1 (31/170)6.5 PENETRATION OF PHOTON BEAMS INTO PATIENT6.5.3 Depth of dose maximum z maxAt a given beam energy:• For fields smaller than 5x5 cm 2 ,z max increases with increasingfield size because of in-phantomscatter.• For field 5x5 cm 2 , z max reachesits nominal value.• For fields larger than 5x5 cm 2 ,z max decreases with increasingfield size because of collimatorand flattening filter scatter.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.5.3 Slide 2 (32/170)