Experimental determination ofp_Coperturbation factors for plane-parallel chambers

Abstract

For plane-parallel chambers used in electron dosimetry, modern dosimetry protocols recommend a cross-calibration against a calibrated cylindrical chamber. The rationale for this is the unacceptably large (up to 3-4%) chamber-to-chamber variations of the perturbation factors (pwall)Co, which have been reported for plane-parallel chambers of a given type. In some recent publications, it was shown that this is no longer the case for modern plane-parallel chambers. The aims of the present study are to obtain reliable information about the variation of the perturbation factors for modern types of plane-parallel chambers, and-if this variation is found to be acceptably small-to determine type-specific mean values for these perturbation factors which can be used for absorbed dose measurements in electron beams using plane-parallel chambers. In an extensive multi-center study, the individual perturbation factors pCo (which are usually assumed to be equal to (pwall)Co) for a total of 35 plane-parallel chambers of the Roos type, 15 chambers of the Markus type and 12 chambers of the Advanced Markus type were determined. From a total of 188 cross-calibration measurements, variations of the pCo values for different chambers of the same type of at most 1.0%, 0.9% and 0.6% were found for the chambers of the Roos, Markus and Advanced Markus types, respectively. The mean pCo values obtained from all measurements are [Formula: see text] and [Formula: see text]; the relative experimental standard deviation of the individual pCo values is less than 0.24% for all chamber types; the relative standard uncertainty of the mean pCo values is 1.1%.

Beam quality corrections for parallel-plate ion chambers in electron reference dosimetry

“…The theoretical explanation for a wall perturbation correction larger than unity is the electron backscatter deficiency of the wall materials in comparison to water (Hunt et al 1988, Klevenhagen 1991. According to Klevenhagen, the electron backscatter is proportional to the effective atomic number of the scatterer and inversely proportional to the electron energy.…”

“…The discussion about perturbation corrections for parallel-plate chambers in clinical electron beams has a long history. More than 20 years ago, the electron backscatter from different materials and the implication for electron dosimetry was investigated byKlevenhagen et al (1982), Klevenhagen (1990Klevenhagen ( ), (1991 and Hunt et al (1988). From their backscatter measurements using different materials, they concluded that there is an energy dependence under response of parallel-plate chambers in the range of 1%-2% due to a backscatter deficiency of the chambers rear wall in comparison to water.…”

A modified formalism for electron beam reference dosimetry to improve the accuracy of linac output calibration

“…This recommendation, which avoids the need for a calculated (generic) factor, is based on early evidence that minor construction details significantly affect the response of these chambers in cobalt‐60 beams, which would give rise to higher uncertainties in calculations of . More recent publications have demonstrated that for modern parallel‐plate chambers (perhaps because of improved manufacturing techniques) this is no longer as much of an issue. A measured gradient correction factor is required for cylindrical chambers.…”

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

“…[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. Kapsch and Gomola 2 recently showed that chamber-to-chamber variation of k Q factors in photon beams is no longer as significant (less than 0.7% spread in values for ten chambers of each type), at least for two chamber types from one manufacturer.…”