Chapter 6 External Photon Beams: Physical Aspects - IRSN

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86.2 QUANTITIES USED IN DESCRIBING PHOTON BEAMS6.2.5 Dose to small mass of medium in airSteps involved in the calculation of D medIAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.2.5 Slide 4 (15/170)6.2 QUANTITIES USED IN DESCRIBING PHOTON BEAMS6.2.5 Dose to small mass of medium in airDetermination of• kr (med)is a correction factor accounting for the photon beamattenuation in the spherical mass of medium with radius r med justlarge enough to provide electronic equilibrium at point P.• kr ( ) is given by:D medDk r X f k r XmcGyabmed0.876 μ = (med) P med(med)PR airmedkr ( ) = emedμ ab rmed med• For water as the medium kr (med) = 0.985 for cobalt-60 gammarays and equal to 1 for lower photon energies.IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.2.5 Slide 5 (16/170)
96.3 PHOTON SOURCES FOR EXTERNAL BEAM THERAPYPhoton sources with regard to type of photons:• Gamma ray sources• X-ray sourcesPhoton sources with regard to photon energies:• Monoenergetic sources• Heterogeneous sourcesPhoton sources with regard to intensity distribution:• Isotropic• Non-isotropicIAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.3 Slide 1 (17/170)6.3 PHOTON SOURCES FOR EXTERNAL BEAM THERAPYFor a given photon source, a plot of number of photonsper energy interval versus photon energy is referred toas the photon spectrum.All photons in a monoenergetic photon beam have thesame energy . hIAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.3 Slide 2 (18/170)
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Chapter 6 External Photon Beams: Physical Aspects - IRSN

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