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Measurements of backscattered radiation from Therac‐20 collimator and trimmer jaws into beam monitor chamber
Author(s) -
Kubo Hideo,
Lo Koon Kai
Publication year - 1989
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.596380
Subject(s) - collimator , linear particle accelerator , optics , beam (structure) , physics , dosimetry , radiation , photon , backscatter (email) , materials science , collimated light , monte carlo method , nuclear medicine , computer science , mathematics , medicine , telecommunications , wireless , laser , statistics
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.