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J Korean Soc Ther Radiol Oncol > Volume 28(1); 2010 > Article
The Journal of the Korean Society for Therapeutic Radiology and Oncology 2010;28(1): 50-56. doi: https://doi.org/10.3857/jkstro.2010.28.1.50
External Auditing on Absorbed Dose Using a Solid Water Phantom for Domestic Radiotherapy Facilities
Chang Heon Choi, Jung In Kim, Jong Min Park, Yang Kyun Park, Kun Woo Cho, Woon Kap Cho, Chun Il Lim, Sung Joon Ye
1Department of Radiation Applying Life Science, Seoul National University Graduate School, Seoul, Korea. sye@snu.ac.kr
2Institute of Radiation Medicine, Medical Research Center, Seoul National University College of Medicine, Seoul, Korea.
3Radiation Research, Korean Institute of Nuclear Safety, Seoul, Korea.
4Korea Food and Drug Administration, Seoul, Korea.
5Department of Radiation Oncology, Seoul National University College of Medicine, Seoul, Korea.
We report the results of an external audit on the absorbed dose of radiotherapy beams independently performed by third parties. For this effort, we developed a method to measure the absorbed dose to water in an easy and convenient setup of solid water phantom.
In 2008, 12 radiotherapy centers voluntarily participated in the external auditing program and 47 beams of X-ray and electron were independently calibrated by the third party's American Association of Physicists in Medicine (AAPM) task group (TG)-51 protocol. Even though the AAPM TG-51 protocol recommended the use of water, water as a phantom has a few disadvantages, especially in a busy clinic. Instead, we used solid water phantom due to its reproducibility and convenience in terms of setup and transport. Dose conversion factors between solid water and water were determined for photon and electron beams of various energies by using a scaling method and experimental measurements.
Most of the beams (74%) were within +/-2% of the deviation from the third party's protocol. However, two of 20 X-ray beams and three of 27 electron beams were out of the tolerance (+/-3%), including two beams with a >10% deviation. X-ray beams of higher than 6 MV had no conversion factors, while a 6 MV absorbed dose to a solid water phantom was 0.4% less than the dose to water. The electron dose conversion factors between the solid water phantom and water were determined: The higher the electron energy, the less is the conversion factor. The total uncertainty of the TG-51 protocol measurement using a solid water phantom was determined to be +/-1.5%.
The developed method was successfully applied for the external auditing program, which could be evolved into a credential program of multi-institutional clinical trials. This dosimetry saved time for measuring doses as well as decreased the uncertainty of measurement possibly resulting from the reference setup in water.
Key Words: Quality assurance, External auditing, Solid water phantom, Dosimetry
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