Chapter 6 External Photon Beams: Physical Aspects - IRSN

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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)
536.9 OFF-AXIS RATIOS AND BEAM PROFILES In the central region, the off-axis points of the beam profile are affected:• For cobalt-60 beams,by the inverse squarelaw dose fall-off andthe increased phantomthickness as theoff-axis distanceincreases.• For linacs, by the energyof electrons strikingthe target, by the atomicnumber of the target,and the atomic numberand shape of the flatteningfilter.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.9 Slide 8 (105/170)6.9 OFF-AXIS RATIOS AND BEAM PROFILES The total penumbra is referred to as the physical penumbraand consists of three components:• Geometric penumbraresults from the finitesource size.• Scatter penumbraresults from in-patientphoton scatteroriginating inthe open field.• Transmission penumbraresults from beamtransmitted throughthe collimation device.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.9 Slide 9 (106/170)
Page 2 and 3: 26.1 INTRODUCTIONRadiotherapy (also
Page 4 and 5: 46.2 QUANTITIES USED IN DESCRIBING
Page 10 and 11: 106.3 PHOTON SOURCES FOR EXTERNAL B
Page 12 and 13: 126.4 INVERSE SQUARE LAW We assume
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Page 81 and 82: 816.14 DELIVERY OF DOSE WITH A SING
Page 83 and 84: 836.15 EXAMPLE OF DOSE CALCULATION
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Chapter 6 External Photon Beams: Physical Aspects - IRSN

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