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On the p(dis) correction factor for cylindrical chambers
Stockholm University, Faculty of Science, Department of Physics.
2010 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 55, no 5, l9-L16 p.Article in journal (Refereed) Published
Abstract [en]

The authors of a recent paper (Wang and Rogers 2009 Phys. Med. Biol. 54 1609) have used the Monte Carlo method to simulate the 'classical' experiment made more than 30 years ago by Johansson et al (1978 National and International Standardization of Radiation Dosimetry (Atlanta 1977) vol 2 (Vienna: IAEA) pp 243-70) on the displacement (or replacement) perturbation correction factor p(dis) for cylindrical chambers in Co-60 and high-energy photon beams. They conclude that an 'unreasonable normalization at dmax' of the ionization chambers response led to incorrect results, and for the IAEA TRS-398 Code of Practice, which uses ratios of those results, 'the difference in the correction factors can lead to a beam calibration deviation of more than 0.5% for Farmer-like chambers'. The present work critically examines and questions some of the claims and generalized conclusions of the paper. It is demonstrated that for real, commercial Farmer-like chambers, the possible deviations in absorbed dose would be much smaller (typically 0.13%) than those stated by Wang and Rogers, making the impact of their proposed values negligible on practical high-energy photon dosimetry. Differences of the order of 0.4% would only appear at the upper extreme of the energies potentially available for clinical use (around 25 MV) and, because lower energies are more frequently used, the number of radiotherapy photon beams for which the deviations would be larger than say 0.2% is extremely small. This work also raises concerns on the proposed value of p(dis) for Farmer chambers at the reference quality of Co-60 in relation to their impact on electron beam dosimetry, both for direct dose determination using these chambers and for the cross-calibration of plane-parallel chambers. The proposed increase of about 1% in p(dis) (compared with TRS-398) would lower the k(Q) factors and therefore D-w in electron beams by the same amount. This would yield a severe discrepancy with the current good agreement between electron dosimetry based on an electron cross-calibrated plane-parallel chamber (against a Farmer) or on a directly Co-60 calibrated plane-parallel chamber, which is not likely to be in error by 1%. It is suggested that the influence of the Co-60 source spectrum used in the simulations may not be negligible for calculations aimed at an uncertainty level of 0.1%.

Place, publisher, year, edition, pages
2010. Vol. 55, no 5, l9-L16 p.
National Category
Physical Sciences
URN: urn:nbn:se:su:diva-49125DOI: 10.1088/0031-9155/55/5/L01ISI: 000274554100019OAI: diva2:376402
authorCount :1Available from: 2010-12-10 Created: 2010-12-10 Last updated: 2010-12-10Bibliographically approved

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