Chapter 6 External Photon Beams: Physical Aspects - IRSN

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106.3 PHOTON SOURCES FOR EXTERNAL BEAM THERAPYPhotons in a heterogeneous x-ray beam form a distinctspectrum,• Photons are present in all energy intervals from 0 to amaximum value h maxwhich is equal to the monoenergetickinetic energy of electrons striking the target.• The two spikes in thespectrum representcharacteristic x rays;the continuous spectrumfrom 0 to h max representsbremsstrahlung photons.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.3 Slide 3 (19/170)6.3 PHOTON SOURCES FOR EXTERNAL BEAM THERAPYGamma ray sources are usually isotropic and producemonoenergetic photon beams.X-ray targets are non-isotropic sources and produceheterogeneous photon spectra.• In the superficial and orthovoltage energy region the x-rayemission occurs predominantly at 90 o to the direction of theelectron beam striking the x-ray target.• In the megavoltage energy region the x-ray emission in thetarget occurs predominantly in the direction of the electronbeam striking the target (forward direction).IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.3 Slide 4 (20/170)
116.4 INVERSE SQUARE LAWIn external beam radiotherapy:• Photon sources areoften assumed to bepoint sources.• Beams produced byphoton sources areassumed to be divergent.tan = a /2f a= b /2f bIAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.4 Slide 1 (21/170)6.4 INVERSE SQUARE LAWPhoton source S emitsphotons and produces aphoton fluence Aat adistance f a and a photonfluence at distance f b . BNumber of photons N totcrossing area A is equalto the number of photonscrossing area B.N tot= AA = BB = constIAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.4 Slide 2 (22/170)
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Chapter 6 External Photon Beams: Physical Aspects - IRSN

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