Absorbed dose beam quality conversion factors ( factors) were obtained for 27 different types of ionization chamber. The aim was to obtain objective evidence on the performance of a wide range of chambers currently available, and potentially used for reference dosimetry, and to investigate the accuracy of the calculation algorithm used in the TG-51 protocol.Methods:
Measurements were made using the irradiator and Elekta Preciselinac facilities at the National Research Council of Canada. The objective was to characterize the chambers over the range of energies applicable to TG-51 and determine whether each chamber met the requirements of a reference-class instrument. Chamber settling, leakage current, ion recombination and polarity, and waterproofing sleeve effects were investigated, and absorbed dosecalibration coefficients were obtained for and 6, 10, and 25 MV photon beams. Only thimble-type chambers were considered in this investigation and were classified into three groups: (i) Reference chambers (“standard” Farmer-type chambers and their derivatives traditionally used for beam output calibration); (ii) scanning chambers (typically volume chambers used for beam commissioning with 3-D scanning phantoms); and (iii) microchambers (very small volume ion chambers used for small field dosimetry).Results:
As might be expected, thimble chambers showed the most predictable performance and experimental factors were obtained with a relative uncertainty of 0.1%. The performance of scanning and microchambers was somewhat variable. Some chambers showed very good behavior but others showed anomalous polarity and recombination corrections that are not fully explained at present. For the well-behaved chambers, agreement between measured and calculated factors was within 0.4%; for some chambers, differences of more than 1% were seen that may be related to the recombination/polarity results. Use of such chambers could result in significant errors in the determination of reference dose in the clinic.Conclusions:
Based on the experimental evidence obtained here, specification for a reference-class ionization chamber could be developed that would minimize the error in using a dosimetry protocol with calculated beam quality conversion factors. The experimental data obtained here for a wide range of thimble chambers can be used when choosing suitable detectors for reference dosimetry and are intended to be used in the upcoming update/addendum to the AAPM TG-51 dosimetry protocol.
The author gratefully acknowledges the essential contributions of Brad Downton in carrying out the calibrations and Carl Ross for assistance in operation of the water calorimeter. The author would also like to thank the directors of the AAPM ADCLs—Geoff Ibbott, Tom Slowey, and Larry DeWerd for supplying calibration data on chamber types. David Rogers of Carleton University generously provided the TG-51-equivalent calculations for the chambers investigated here. The ion chamber manufacturers were very helpful in supplying chambers for this investigation—IBA (Igor Gomola), PTW (Mark Szczepanski), and Standard Imaging (Eric DeWerd). The members of the AAPM Working Group on Update to TG-51 have provided much useful guidance during this project.
II. MATERIALS AND METHODS
II.A. NRC primary standard water calorimeter
II.B. Chamber types
II.C. Chamber calibration procedure
II.D. Chamber stabilization
II.E. Ion recombination and polarity
III. RESULTS AND DISCUSSION
III.A. Chamber classification
III.B. Chamber stabilization
III.C. Ion chamberleakage currents
III.D. Polarity correction
III.E. Ion recombination
III.E.1. Linearity of recombination response
III.E.2. Ion recombination as a function of dose per pulse
III.E.3. Comparison with previous data
III.E.4. Ion recombination at different polarities
III.F. Sleeve effects
III.G. Long-term stability
III.H. Determination of factors
III.I. Specification of a reference-class ion chamber
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