Measurement of ionization chamber absorbed dose factors in megavoltage photon beams
Comparison of photon beam factors for the NE2571 chamber obtained at the NRC. “Vickers repeat” refers to a series of measurements carried out in 2004 using the same linac and a similar setup as reported by Seuntjens et al. (Ref. 10). The data of McEwen and Ross (Ref. 15) were obtained with the same water calorimeter but used an Elekta Precise linac to produce 6, 10, and 25 MV beams. Also shown are the calculated values of given in the TG-51 protocol.
Chamber stabilization of three PTW ion chambers in the beam. The procedure is that the chamber is placed in the water phantom, voltage is applied, and then the chamber is irradiated. The reading is continuously monitored until a stable value is obtained. The dose rate is approximately . The data presented are typical of the chambers investigated from all the manufactures in that Farmer-type chambers show the smallest effect, scanning chambers take slightly longer, and microchambers can show very large initial offsets and require a long irradiation time to achieve an equilibrium reading.
Ion recombination correction as a function of dose per pulse for Farmer-type chambers. Dose per pulse is given in terms of absorbed dose to water. The gradients of the plots ( vs ) are consistent with the effective electrode separation for each chamber design. The standard uncertainty in each point is estimated to be 0.05%.
Ion recombination correction as a function of dose per pulse for scanning-type chambers. Dose per pulse is given in terms of absorbed dose to water. NE2571 data are included as a reference to enable comparisons with Figs. 3 and 5. PTW31010 chamber data are highlighted with a linear fit as an example of anomalous behavior. The apparently large initial recombination (extrapolation to zero dose per pulse) for this chamber can be clearly seen. Note the two PTW233642 chambers which show similar gradients but quite different intercepts. The larger gradient for the CC08 is due to the chamber design, which is basically spherical rather than cylindrical and results in a larger effective electrode separation (and, therefore, lower electric field strength for the same applied voltage).
Ion recombination correction as a function of dose per pulse for microtype chambers. Dose per pulse is given in terms of absorbed dose to water. NE2571 data are included as a reference to enable comparisons with Figs. 3 and 4. The PTW31014 chamber data are highlighted with a linear fit merely as an example of one of the several chambers of this type that show anomalous behavior. As can be seen, the Exradin chambers (A14 and A16) give an apparent component of initial recombination that is less than unity.
Jaffé plots ( vs ) for the CC01 microchamber obtained when collecting positive (POS) and negative (NEG) charges. The data are normalized to the intercept of POS data. For both polarities, the outer thimble of the chamber is always at ground potential. The difference in the scatter around the straight line fit between POS and NEG was consistent for this chamber for different values of dose per pulse.
Ion recombination correction as a function of dose per pulse for two PTW31010 chambers, obtained by collecting POS and NEG charges. The gradients for both chambers at POS and NEG agree within the estimated uncertainties.
Determination of sleeve effect as a function of beam quality. The data labeled “This work” are those obtained in this investigation for a 1 mm PMMA sleeve. Also shown are data taken from Ross and Shortt (Ref. 38) for a similar 1 mm PMMA sleeve and the calculations of Seuntjens et al. (Ref. 10). The data labeled “Thomas et al.” were obtained at the National Physical Laboratory (U.K.) by Thomas et al. (Ref. 4) with a Wellhofer IC70 ion chamber in a 1.6 mm PMMA sleeve.
Uncertainty budget for determination of absorbed dose calibration coefficients and beam quality conversion factors. Values are given as one relative standard uncertainty . Types A and B refer to the method of evaluation (statistical and nonstatistical). These do not correspond directly to the concepts of “random” and “systematic”.
Long-term stability of thimble-type chambers determined from absorbed dose calibration coefficients obtained at the National Research Council. Standard deviations are for individual chambers recalibrated multiple times and calibration periods cover 4–10 yr. Differences are given for chambers recalibrated once and the period is 18–24 months. The wall material is given to indicate if there is a difference in stability between plastic-walled and graphite-walled chambers.
Classification of thimble chamber for presentation and analysis of results. Categories are defined in the main text.
Ion chamber leakage currents grouped by category and manufacturer. The values presented are typical values for the chamber/extension cable/electrometer system used. The leakage fraction is determined assuming a typical clinical dose rate of around .
Polarity correction at a polarizing voltage of 300 V as a function of chamber manufacturer and category. Correction is to the NRC standard polarity setting, where positive charge is collected. The mean and standard deviation are obtained from the data for all four energies— and 6, 10, and 25 MV—as no significant energy dependence was observed for any chamber type.
Recombination parameters for a selection of chamber types and comparison with the literature (see body text for references). The parameters and refer, respectively, to the components of initial and general recombination. The typical relative standard uncertainties for this study are estimated to be 17% for and 8% for . Only Farmer-type and scanning chambers are listed due to the behavior of microchambers as shown in Fig. 5.
Determination of experimental factors for thimble ionization chambers and comparison with TG-51 calculated values. Chambers are grouped according to the category (Farmer-type, scanning, and micro) and the number of each chamber type involved in the investigation is given. The effect of the 1 mm PMMA waterproofing sleeve (where required) on the comparison is also shown (sleeve correction applied to calculated values).
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