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Title: SU-F-T-492: The Impact of Water Temperature On Absolute Dose Calibration

Abstract

Purpose: The Task Group 51 (TG 51) protocol prescribes that dose calibration of photon beams be done by irradiating an ionization chamber in a water tank at pre-defined depths. Methodologies are provided to account for variations in measurement conditions by applying correction factors. However, the protocol does not completely account for the impact of water temperature. It is well established that water temperature will influence the density of air in the ion chamber collecting volume. Water temperature, however, will also influence the size of the collecting volume via thermal expansion of the cavity wall and the density of the water in the tank. In this work the overall effect of water temperature on absolute dosimetry has been investigated. Methods: Dose measurements were made using a Farmer-type ion chamber for 6 and 23 MV photon beams with water temperatures ranging from 10 to 40°C. A reference ion chamber was used to account for fluctuations in beam output between successive measurements. Results: For the same beam output, the dose determined using TG 51 was dependent on the temperature of the water in the tank. A linear regression of the data suggests that the dependence is statistically significant with p-values of the slopemore » equal to 0.003 and 0.01 for 6 and 23 MV beams, respectively. For a 10 degree increase in water phantom temperature, the absolute dose determined with TG 51 increased by 0.27% and 0.31% for 6 and 23 MV beams, respectively. Conclusion: There is a measurable effect of water temperature on absolute dose calibration. To account for this effect, a reference temperature can be defined and a correction factor applied to account for deviations from this reference temperature during beam calibration. Such a factor is expected to be of similar magnitude to most of the existing TG 51 correction factors.« less

Authors:
 [1];  [1];  [2]
  1. State University of New York at Buffalo, Buffalo, NY (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
22649079
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; CALIBRATION; CORRECTIONS; DOSES; IONIZATION CHAMBERS; PHOTON BEAMS; WATER

Citation Formats

Islam, N, Podgorsak, M, and Roswell Park Cancer Institute, Buffalo, NY. SU-F-T-492: The Impact of Water Temperature On Absolute Dose Calibration. United States: N. p., 2016. Web. doi:10.1118/1.4956677.
Islam, N, Podgorsak, M, & Roswell Park Cancer Institute, Buffalo, NY. SU-F-T-492: The Impact of Water Temperature On Absolute Dose Calibration. United States. doi:10.1118/1.4956677.
Islam, N, Podgorsak, M, and Roswell Park Cancer Institute, Buffalo, NY. 2016. "SU-F-T-492: The Impact of Water Temperature On Absolute Dose Calibration". United States. doi:10.1118/1.4956677.
@article{osti_22649079,
title = {SU-F-T-492: The Impact of Water Temperature On Absolute Dose Calibration},
author = {Islam, N and Podgorsak, M and Roswell Park Cancer Institute, Buffalo, NY},
abstractNote = {Purpose: The Task Group 51 (TG 51) protocol prescribes that dose calibration of photon beams be done by irradiating an ionization chamber in a water tank at pre-defined depths. Methodologies are provided to account for variations in measurement conditions by applying correction factors. However, the protocol does not completely account for the impact of water temperature. It is well established that water temperature will influence the density of air in the ion chamber collecting volume. Water temperature, however, will also influence the size of the collecting volume via thermal expansion of the cavity wall and the density of the water in the tank. In this work the overall effect of water temperature on absolute dosimetry has been investigated. Methods: Dose measurements were made using a Farmer-type ion chamber for 6 and 23 MV photon beams with water temperatures ranging from 10 to 40°C. A reference ion chamber was used to account for fluctuations in beam output between successive measurements. Results: For the same beam output, the dose determined using TG 51 was dependent on the temperature of the water in the tank. A linear regression of the data suggests that the dependence is statistically significant with p-values of the slope equal to 0.003 and 0.01 for 6 and 23 MV beams, respectively. For a 10 degree increase in water phantom temperature, the absolute dose determined with TG 51 increased by 0.27% and 0.31% for 6 and 23 MV beams, respectively. Conclusion: There is a measurable effect of water temperature on absolute dose calibration. To account for this effect, a reference temperature can be defined and a correction factor applied to account for deviations from this reference temperature during beam calibration. Such a factor is expected to be of similar magnitude to most of the existing TG 51 correction factors.},
doi = {10.1118/1.4956677},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • The cooling water of nuclear reactors undergoes radiolytic decomposition induced by gamma, fast electron, and neutron radiation in the core. To model the process, recombination reaction rates and radiolytic yields for the water radical fragments need to be measured at high temperature and pressure. Yields for the action of neutron radiation are particularly hard to determine independently because of the beta/gamma field also present in any reactor. In this paper we report the design of an apparatus intended to measure neutron radiolysis yields as a function of temperature and pressure. A new methodology for separation of neutron and beta/gamma radiolysismore » yields in a mixed radiation field is proposed and demonstrated.« less
  • A new calibration algorithm (GLAaS) to derive absolute dose maps from images acquired with the Varian PV-aS500 electronic portal imager (based on amorphous silicon detectors) has been developed incorporating the dependence of detector response on primary and transmitted radiation and on field size. Detector calibration and algorithm validation were performed at different depths (10.0, 3.8, 1.5, and 0.8 cm) in solid water to investigate various application possibilities. Calibration data were obtained against ion chamber measurements. Validation experiments were performed on intensity-modulated fields and comparison was carried out against calculated dose maps as well as against film measurements. Split fields weremore » acquired independently and PV-aS500 images were summed offline with the new algorithm allowing complex fields to be verified in conditions most closely resembling clinical conditions. Excellent results were obtained for the 3.8, 1.5, and 0.8 depths on a set of 34 modulated fields including both split and nonsplit fields. Applying the gamma index analysis (with distance to agreement and dose thresholds set to 3 mm and 4%, respectively), only 2.3% of the field area showed {gamma}>1 at 1.5 cm depth (8.1%, 3.1%, 2.7% at 10.0, 3.8, and 0.8 and 2.5% with films at 10 cm depth). Tests were also performed to verify GLAaS at gantry angles different from 0 deg. . No statistical differences were obtained for the comparison between split and nonsplit fields and between different gantry angles. Highly significant statistical differences were obtained when comparing independent samples of 240 fields verified either with GLAaS or with film. Fields verified with GLAaS presented a mean area with {gamma}>1 of 2.1{+-}1.3% while for film this value was 3.9{+-}3.4% (p<0.001). Absolute dosimetry proved to be reliable with the PV-aS500 detector with the GLAaS algorithm. The minimal settings at depths of 1.5 or 3.8 cm would allow the use of the detector at any gantry angle without the need for any special fixation tool.« less
  • Purpose: Elekta Leksell Gamma Knife{sup ®} (LGK) is a radiotherapy beam machine whose features are not compliant with the international calibration protocols for radiotherapy. In this scope, the Laboratoire National Henri Becquerel and the Pitié-Salpêtrière Hospital decided to conceive a new LKG dose calibration method and to compare it with the currently used one. Furthermore, the accuracy of the dose delivered by the LGK machine was checked using an “end-to-end” test. This study also aims to compare doses delivered by the two latest software versions of the Gammaplan treatment planning system (TPS). Methods: The dosimetric method chosen is the electronmore » paramagnetic resonance (EPR) of alanine. Dose rate (calibration) verification was done without TPS using a spherical phantom. Absolute calibration was done with factors calculated by Monte Carlo simulation (MCNP-X). For “end-to-end” test, irradiations in an anthropomorphic head phantom, close to real treatment conditions, are done using the TPS in order to verify the delivered dose. Results: The comparison of the currently used calibration method with the new one revealed a deviation of +0.8% between the dose rates measured by ion chamber and EPR/alanine. For simple fields configuration (less than 16 mm diameter), the “end-to-end” tests showed out average deviations of −1.7% and −0.9% between the measured dose and the calculated dose by Gammaplan v9 and v10, respectively. Conclusions: This paper shows there is a good agreement between the new calibration method and the currently used one. There is also a good agreement between the calculated and delivered doses especially for Gammaplan v10.« less
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