Chapter 6 External Photon Beams: Physical Aspects - IRSN

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786.13 RELATIVE DOSE MEASUREMENTS WITH IONIZATIONCHAMBERSDoses and doserates at referencepoints in a phantomfor megavoltagephoton beams aremeasured withrelatively large volume(0.6 cm 3 ) cylindricalionization chambers inorder to obtain areasonable signal andgood signal to noiseratio.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.13 Slide 3 (155/170)6.13 RELATIVE DOSE MEASUREMENTS WITH IONIZATIONCHAMBERSRelative dosedistributions forphoton beamsbeyond z max areusually measuredwith small volume(0.1 cm 3 ) ionizationchambers in order toobtain good spatialresolution.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.13 Slide 4 (156/170)
796.13 RELATIVE DOSE MEASUREMENTS WITH IONIZATIONCHAMBERSSurface doses and doses in the buildup region forphoton beams are usually measured with parallel-plateionization chambers incorporating:• Thin polarizing electrode window for measuring surface dose.• Small electrode separation ( 1 mm) for better spatial resolution.The measured depth dose curves in the buildup regiondepend on the chamber polarity and this dependence iscalled the polarity effect of ionization chambers.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.13 Slide 5 (157/170)6.13 RELATIVE DOSE MEASUREMENTS WITH IONIZATIONCHAMBERSIn the buildup region of megavoltage photon beams,positive parallel-plate chamber polarity produces alarger signal than the negative polarity (polarity effect).The difference in signalsis most pronounced onthe phantom surface andthen diminishes withdepth until it disappearscompletely at depths ofz max and beyond.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.13 Slide 6 (158/170)
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26.1 INTRODUCTIONRadiotherapy (also
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46.2 QUANTITIES USED IN DESCRIBING
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106.3 PHOTON SOURCES FOR EXTERNAL B
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126.4 INVERSE SQUARE LAW We assume
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146.5 PENETRATION OF PHOTON BEAMS I
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166.5 PENETRATION OF PHOTON BEAMS I
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186.6 RADIATION TREATMENT PARAMETER
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Chapter 6 External Photon Beams: Physical Aspects - IRSN

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