Ravichandran R, Binukumar J P, Sivakumar S S, Krishnamurthy K, Davis C A
Medical Physics Unit, Department of Radiotherapy, National Oncology Center, Royal Hospital, Muscat, Sultanate of Oman.
J Med Phys. 2009 Jan;34(1):31-6. doi: 10.4103/0971-6203.48718.
The objective of the present study is to establish radiation standards for absorbed doses, for clinical high energy linear accelerator beams. In the nonavailability of a cobalt-60 beam for arriving at Nd, water values for thimble chambers, we investigated the efficacy of perspex mounted extrapolation chamber (EC) used earlier for low energy x-rays and beta dosimetry. Extrapolation chamber with facility for achieving variable electrode separations 10.5mm to 0.5mm using micrometer screw was used for calibrations. Photon beams 6 MV and 15 MV and electron beams 6 MeV and 15 MeV from Varian Clinac linacs were calibrated. Absorbed Dose estimates to Perspex were converted into dose to solid water for comparison with FC 65 ionisation chamber measurements in water. Measurements made during the period December 2006 to June 2008 are considered for evaluation. Uncorrected ionization readings of EC for all the radiation beams over the entire period were within 2% showing the consistency of measurements. Absorbed doses estimated by EC were in good agreement with in-water calibrations within 2% for photons and electron beams. The present results suggest that extrapolation chambers can be considered as an independent measuring system for absorbed dose in addition to Farmer type ion chambers. In the absence of standard beam quality (Co-60 radiations as reference Quality for Nd,water) the possibility of keeping EC as Primary Standards for absorbed dose calibrations in high energy radiation beams from linacs should be explored. As there are neither Standard Laboratories nor SSDL available in our country, we look forward to keep EC as Local Standard for hospital chamber calibrations. We are also participating in the IAEA mailed TLD intercomparison programme for quality audit of existing status of radiation dosimetry in high energy linac beams. The performance of EC has to be confirmed with cobalt-60 beams by a separate study, as linacs are susceptible for minor variations in dose output on different days.
本研究的目的是为临床高能直线加速器束流建立吸收剂量的辐射标准。由于没有钴 - 60束流来确定指形电离室的水模体值（Nd, water），我们研究了先前用于低能X射线和β剂量测定的有机玻璃安装的外推电离室（EC）的有效性。使用带有微米级螺丝可实现10.5毫米至0.5毫米可变电极间距的外推电离室进行校准。对瓦里安Clinac直线加速器产生的6兆伏和15兆伏光子束以及6兆电子伏和15兆电子伏电子束进行了校准。将有机玻璃的吸收剂量估计值转换为固体水的剂量，以便与水中FC 65电离室测量值进行比较。考虑对2006年12月至2008年6月期间进行的测量进行评估。在整个期间，所有辐射束的外推电离室未经校正的电离读数在2%以内，表明测量具有一致性。外推电离室估计的吸收剂量与光子束和电子束在水中的校准值在2%以内高度一致。目前的结果表明，除了 Farmer 型电离室外，外推电离室可被视为吸收剂量的独立测量系统。在没有标准束流质量（以钴 - 60辐射作为Nd, water的参考质量）的情况下，应探索将外推电离室作为直线加速器高能辐射束吸收剂量校准的主要标准的可能性。由于我国既没有标准实验室也没有二级标准剂量实验室，我们期待将外推电离室作为医院电离室校准的本地标准。我们还参与了国际原子能机构邮寄的热释光剂量计比对计划，以对高能直线加速器束流中辐射剂量测定的现有状况进行质量审核。由于直线加速器在不同日期的剂量输出容易出现微小变化，因此必须通过单独的研究用钴 - 60束流来确认外推电离室的性能。
