Chapter 6 External Photon Beams: Physical Aspects - IRSN

526.9 OFF-AXIS RATIOS AND BEAM PROFILES Ideal dose profile:• Central region:constant dose fromtarget centre to edgeof target.• Penumbra: zero width.• Umbra: zero dose. Actual dose profile:• Central region: profile flat in 80% of central portion of the field.• Penumbra is typically defined as the distance between 80% and 20%dose on the beam profile normalized to 100% at the central axis.• Umbra is typically less than 1% of the dose on the central axis.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.9 Slide 6 (103/170)6.9 OFF-AXIS RATIOS AND BEAM PROFILES Geometric or nominal field size is:• Indicated by the optical light field of the treatment machine.• Usually defined as the separation between the 50% dose levelpoints on the beam profile measured at the depth of dosemaximum z max .IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.9 Slide 7 (104/170)