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Measurements of in‐air output ratios for a linear accelerator with and without the flattening filter
Author(s) -
Zhu X. R.,
Kang Y.,
Gillin M. T.
Publication year - 2006
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 - 10.1118/1.2349695
Subject(s) - linear particle accelerator , collimator , optics , physics , beam (structure) , dosimetry , ionization chamber , radiation , linearity , backscatter (email) , range (aeronautics) , particle accelerator , nuclear medicine , materials science , ionization , telecommunications , quantum mechanics , computer science , wireless , medicine , ion , composite material
The in‐air output ratio ( S c ) for photon beams from linear accelerators describes the change of in‐air output as a function of the collimator settings. The physical origin of theS cis mainly due to the change in scattered radiation that can reach the point of measurement as the geometry of the head changes. The flattening filter (FF) and primary collimator are the major sources of scattered radiation. The change in amount of backscattered radiation from the collimator into the beam‐monitoring chamber also contributes to the variation of output. In this work, we measured theS cand backscatter factors ( S b ) into the beam‐monitoring chamber for a linear accelerator with and without the FF. We measured theS cwith a Farmer‐type chamber in a miniphantom at the depth of 10 g ∕ cm 2for 6‐ and 18‐MV x‐ray beams from a Varian Clinac 2100EX linear accelerator. TheS bwere measured with a universal pulse counter and a diode array with build‐in counting hardware and software. The head scatter component ( S h ) was then derived from the relationshipS c = S h × S b , whereS bwas the linear fit of measured results. Significant differences were observed forS cwith and without the FF. Within the range of experimental uncertainty, theS bwas similar with and without the FF. The variations inS hdiffered significantly over the range of field sizes of 3 × 3 to 40 × 40cm 2with and without the FF; for the 6‐MV beam, it was 8% vs 3%, and for the 18‐MV beam, 7% vs 1%. By analyzing the contributions of backscatter factor and total in‐air output ratios with and without the FF, we directly gained insight into the contributions of different components to the total variations inS cof a linear accelerator.S c ,S b , andS hare basic and useful dosimetric quantities for delivery of intensity‐modulated radiation therapy using a linear accelerator operating in a mode without the FF.