Performance analysis and determination of thep_wallcorrection factor for⁶⁰Co γ-ray beams for Wellhöfer Roos-type plane-parallel chambers

Abstract

The wall perturbation correction factor p(wall) in 60Co for Wellhöfer Roos-type plane-parallel ionization chambers is determined experimentally and compared with the results of a previous study using PTW-Roos chambers (Palm et al 2000 Phys. Med. Biol. 45 971-81). Five ionization chambers of the type Wellhöfer PPC-35 (or its equivalent PPC-40) are used for the analysis. Wall perturbation correction factors are obtained by assuming N(D,air) chamber factors determined by cross-calibration in a high-energy electron and in a 60Co gamma-ray beam to be equal, and by assigning any differences to the wall perturbation factor. The procedure yields a p(wall) value of 1.018 (u(c) = 0.010), which is slightly higher than the value 1.014 (u(c) = 0.010) formerly obtained for the PTW-Roos chambers using the N(D,air) method. The chamber-to-chamber variation in p(wall) for the Wellhöfer-Roos chambers is found to be very small, with a maximum difference of 0.3%. The effect of using new p(cav) values for graphite-walled Farmer-type chambers used in water in electron beams is to decrease p(wall) by approximately 0.5%. The long- and short-term stability of the Roos-type chambers manufactured by Wellhöfer is investigated by measurements at the IAEA Dosimetry Laboratory in Vienna, Austria, and at the Sahlgrenska University Hospital in Göteborg, Sweden. Calibrations made at the IAEA over several months show variations in the N(D,w) calibration factors larger than expected. based on previous experiences with PTW-Roos chambers. Measurements of the short-term stability of the Wellhöfer-Roos chambers show a marked increase in chamber response for the time the chambers are immersed in water, pointing to a possible problem in the chamber design. As a consequence of these findings, Wellhöfer is currently working on a re-design of the chamber to solve the stability problem.

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

“…The cobalt-60 calibration coefficients for plane-parallel chambers obtained in August 2011, eight months following initial calibrations, are systematically lower by 0.6% on average (with 0.3% standard deviation) between calibrations while those for the Farmer chambers show no change within 0.1%. This is consistent with the stability results observed by Palm et al 5 However, calibration coefficients for individual chambers were different by up to 1.2% after the eight month period between measurements. At the time of this recalibration, some chambers were behaving strangely in terms of leakage, as discussed in Sec.…”

“…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%. Bass et al 33 report that for 20 NACP and Roos chambers the average repeatability of calibration coefficients is within 0.1%-0.2% but with some chambers giving repeat results that differ by up to 1%.…”

“…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.…”

Measurements and Monte Carlo simulations for reference dosimetry of external radiation therapy beams

“…Christ et al93 suggested that long-term stability was not an issue for plane-parallel chambers. The standard deviation of calibration coefficients measured by Palm et al94 in cobalt-60 three or four FACTORS ferences of up to 1 % were observed between repeated calibrations. These levels of stability are consistent with the observations of this work.The unexpected sensitivity of leakage currents to levels of relative humidity discussed above suggests that plane-parallel chambers are more sensitive to environmental conditions than cylindrical chambers.…”

“…A cross-calibration procedure against a stable cylindrical chamber would avoid these problems, although this would need to be done each time these chambers are used.3.3 Measured k Q factorsUncertainty budget: The uncertainty budget for measured k Q factors and calibration coefficients for plane-parallel chambers in photon beams is provided in table 3.1, derived using the ISO "Guide to the expression of uncertainty in measurement" 82 recommendations. The use of the indirect method of calibration against a stable secondary standard cylindrical chamber is discussed in chapter 2 and the individual components of uncertainty are discussed in the preceding sections 3.1 and 3.2.Chamber-to-chamber variability of measured k Q factors: Variability of measured k Q factors for plane-parallel chambers of the same type has been investigated by several authors,36,55,86,93,94,96 usually based on measurements in cobalt-60. The main reason that these chambers were not recommended for use in photon beams by reference dosimetry protocols 10,11 was evidence for unacceptable (up to 3.6 %) chamberto-chamber variations of cobalt-60 P wall values for chambers of the same type 36.…”