Abstract

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Title:: Effects of different backscattering conditions on QABC+ measurements

Author(s):: LONG Liang1; XU Zhongbiao1; HAN Junjie1; L?Fengquan1; HUANG Wei1; LIANG Yu1; GAO Xingwang1; 2; 1. Department of Radiotherapy, Guangdong Provincial Peoples Hospital/Guangdong Academy of Medical Sciences, Guangzhou 510080, China 2. Department of Medical Device Management, Shenzhen Center, Cancer Hospital, Chinese Academy of Medical Sciences, Shenzhen 518116, China

Keywords:: Keywords: QA beamchecker plus backscatter linear accelerator treatment couch quality control

Abstract:: Abstract: Objective To investigate the effects of different backscattering conditions on the measurement results of the QA beamchecker plus (QABC+). Methods QABC+ and a reference phantom composed of PTW thimble ionization chamber and solid water were placed at the fixed positions (positions A and B) on the treatment couch, with the source-to-surface distance of 100 cm. Taking the thickness (0-8 cm, 10 cm, and 15 cm) of solid water under the reference phantom and QABC+ as the variations, the corresponding backscatter factor (Bs) was measured. Results Once the solid water thickness exceeded 5 cm, the Bs measured by the reference ionization chamber tended to be stable and varied within 1%, while the Bs measured by QABC+ was larger and still increased by 3.1% for 5 to 15 cm of solid water. Position A on the couch was equivalent to 5 cm of solid water. Conclusion QABC+ measurements are more affected by backscatter than conventional ionization chambers, and the Bs are amplified. Slight differences in backscattering conditions may cause false alarms from QABC+. It is recommended that before the use of daily QA instrument, the effects of backscattering conditions on its reading should be determined through experimental measurement, and the positioning conditions should be fixed to ensure the stability of scattering conditions and make daily QA instrument work efficiently, stably and reliably.