Superficial and orthovoltage x-ray beam dosimetry.
Output of superficial and orthovoltage x-ray units may be measured with cylindrical or end-window parallel-plate ionization chambers. The air-kerma calibration factors for these chambers are usually determined free in air, and the x-ray machine output is stated as the air-kerma rate free in air, which, when multiplied with the appropriate backscatter factor, gives the air-kerma rate on the surface of a phantom or patient. For end-window chambers, especially when they are used for measurements of small fields or low x-ray energies, the air-kerma calibration factors may also be determined with the chamber embedded in a tissue-equivalent phantom. This results in field size dependent air-kerma in-air calibration factors but obviates the requirement for knowledge of back-scatter factors when determining the air-kerma rate on the surface of a phantom. Since there still is considerable uncertainty in tabulated backscatter factors as a function of field size and x-ray beam energy, the output measurement technique which determines the air-kerma rate on phantom surface with a phantom-embedded end-window ionization chamber offers a clear advantage over the in-air calibration method.
Department of Medical Physics, Montreal General Hospital, Québec, Canada. epodgorsak@medphys.mgh.mcgill.ca
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D000388:Air; D002138:Calibration; D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation / Q000379:methods; D011879:Radiotherapy Dosage / Q000592:standards; D011882:Radiotherapy, High-Energy / Q000592:standards*; D012542:Scattering, Radiation; D014964:X-Ray Therapy / Q000592:standards*; D014965:X-Rays