title: Prediction of back-scatter radiations to a beam monitor chamber of medical linear accelerators by use of the digitized target-current-pulse analysis method.
abstract: In small-field irradiation, the back-scattered radiation (BSR) affects the counts measured with a beam monitor chamber (BMC). In general, the effect of the BSR depends on the opened-jaw size. The effect is significantly large in small-field irradiation. Our purpose in this study was to predict the effect of BSR on LINAC output accurately with an improved target-current-pulse (TCP) technique. The pulse signals were measured with a system consisting of a personal computer and a digitizer. The pulse signals were analyzed with in-house software. The measured parameters were the number of pulses, the change in the waveform and the integrated signal values of the TCPs. The TCPs were measured for various field sizes with four linear accelerators. For comparison, Yu's method in which a universal counter was used was re-examined. The results showed that the variance of the measurements by the new method was reduced to approximately 1/10 of the variance by the previous method. There was no significant variation in the number of pulses due to a change in the field size in the Varian Clinac series. However, a change in the integrated signal value was observed. This tendency was different from the result of other investigations in the past. Our prediction method is able to define the cutoff voltage for the TCP acquired by digitizer. This functionality provides the capability of clearly classifying TCPs into signals and noise. In conclusion, our TCP analysis method can predict the effect of BSR on the BMC even for small-field irradiations.
authors.affiliation: Department of Radiology, Fujita Health University Hospital, 1-98 Dengakugakubo, Kutsukake-cho, Toyoake, Aichi 470-1192, Japan. yusuzuki@fujita-hu.ac.jp
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Suzuki
authors.forename: Naoki
authors.initials: N
authors.lastname: Hayashi
authors.forename: Hideki
authors.initials: H
authors.lastname: Kato
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Fukuma
authors.forename: Yasujiro
authors.initials: Y
authors.lastname: Hirose
authors.forename: Makoto
authors.initials: M
authors.lastname: Kawano
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Nishii
authors.forename: Masaru
authors.initials: M
authors.lastname: Nakamura
authors.forename: Takashi
authors.initials: T
authors.lastname: Mukouyama
mesh_terms: D004864:Equipment and Supplies; D010315:Particle Accelerators; D011874:Radiometry / Q000295:instrumentation*; D012542:Scattering, Radiation