title: Measurements of backscattered radiation from Therac-20 collimator and trimmer jaws into beam monitor chamber.
abstract: The field size dependent photon output is known to be influenced by the existence of backscattered radiation (BSR) generated in the collimator or trimmer jaws of a linear accelerator. This paper describes the results of measurements made to study the existence of such backscatter by simulating the geometry of the treatment head of a Therac-20 linear accelerator. The machine's monitor chamber, flattening filter, and collimator jaws were simulated by another real monitor chamber, a 1-cm thick lead sheet and 2.5-cm thick low-melting-point alloy divergent blocks, respectively. BSR from the simulated collimator jaws (SCJ) was measured with the simulated monitor chamber (SMC) as a function of the openings of the SCJ and as a function of distance between SMC and SCJ. The present results demonstrate the presence of BSR in an 18-MV photon beam from a Therac-20 linear accelerator.
authors.affiliation: Department of Radiation Oncology, University of Rochester Cancer Center, New York 14642.
authors.forename: H
authors.initials: H
authors.lastname: Kubo
authors.forename: K K
authors.initials: KK
authors.lastname: Lo
mesh_terms: D006801:Humans; D010315:Particle Accelerators; D011834:Radiation Monitoring / Q000295:instrumentation; D011874:Radiometry / Q000295:instrumentation; D011882:Radiotherapy, High-Energy; D012542:Scattering, Radiation