Premium
Superficial and orthovoltage x‐ray beam dosimetry
Author(s) -
Podgorsak Ervin B.,
Gosselin Micheline,
Evans Michael D. C.
Publication year - 1998
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598309
Subject(s) - dosimetry , nuclear medicine , medical physics , beam (structure) , optics , medicine , physics
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 backscatter 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.