The use of plane-parallel chambers in electron dosimetry without any cross-calibration

Abstract

Current dosimetry protocols from AAPM, DIN and IAEA recommend a cross-calibration for plane-parallel chambers against a calibrated thimble chamber for electron dosimetry. The rationale for this is the assumed chamber-to-chamber variation of plane-parallel chambers and the large uncertainty in the wall perturbation factor (p(wall)60Co)pp at 60Co for plane-parallel chambers. We have confirmed the results of other authors that chamber-to-chamber variation of the investigated chambers of types Roos, Markus, Advanced Markus and Farmer is less than 0.3%. Starting with a calibration factor for absorbed dose to water and on the basis of the three dosimetry protocols AAPM TG-51, DIN 6800-2 (slightly modified) and IAEA TRS-398, values for (p(wall)60Co)Roos of 1.024 +/- 0.005, (p(wall)60Co)Markus of 1.016 +/- 0.005 and (p(wall)60Co)Advanced Markus of 1.014 +/- 0.005 have been determined. In future this will permit electron dosimetry with the above-listed plane-parallel chambers having a calibration factor N(D, w)60Co without the necessity for cross-calibration against a thimble chamber.

Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration

“…In addition, other recent papers by Palm et al (32) and Stewart and Seuntjens (33) have verified this number; they present consistent $p_{\text{wall}}$ values for various chamber types. The relative standard uncertainty for this factor is estimated at 0.5% (31) . The new combined uncertainty in the calculated $k_{Q,Q_0}$ for electron beams when the reference quality $Q_0$ is ${}^{60}\mathrm{normalC}\mathrm{o}$ can be estimated at 1.0%.…”

“…In the case of plane‐parallel chambers calibrated in a ${}^{60}\mathrm{normalC}\mathrm{o}$ beam, the large uncertainty associated with the calculated $k_{Q,Q_0}$, 1.7%, comes mainly from the factor $p_{\text{wall}}$, because of variations of up to 3% found between chambers of the same type. In a recent publication, Christ et al (31) concluded that chamber‐to‐chamber variations for various plane‐parallel chambers (Roos, Markus, and Advanced Markus) are less than 0.3%. In addition, other recent papers by Palm et al (32) and Stewart and Seuntjens (33) have verified this number; they present consistent $p_{\text{wall}}$ values for various chamber types.…”

Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams

“…3) related to the absorbed dose calibration factor, albeit for older chambers for which manufacturing processes were potentially variable or changing. Other studies such as that by Christ et al 6 indicate that chamber-to-chamber variability is as low as for Farmer chambers.…”

“…Few authors report on long-term stability of calibration coefficients for plane-parallel chambers in photon or electron beams. Christ et al 6 indicated no problems with long-term stability but without quantification. Palm et al 5 indicate that the standard deviation of calibration coefficients for Roos chambers over a period of several months was 0.6%-0.7%.…”

“…It is even possible to perform analytic calculations with the method used by absorbed dose protocols for reference dosimetry 7,8 with the emergence of new studies that provide P wall corrections. [4][5][6]12,13 McEwen 14 showed that it is feasible to provide measured k Q factors for a large set of ion chambers. Kapsch et al 15 used crosscalibration in electron beams and 60 Co calibrations to show that measured chamber-to-chamber variation of perturbation factors for the Roos, Markus, and Advanced Markus chamber types in cobalt-60 are now less than 1.1% within a single chamber type.…”