- Title
- Reduction of the effect of non-uniform backscatter from an E-type support arm of a Varian a-Si EPID used for dosimetry
- Creator
- Rowshanfarzad, Pejman; Sabet, Mahsheed; O'Connor, Daryl J.; Greer, Peter B.
- Relation
- Physics in Medicine and Biology Vol. 55, Issue 22, p. 6617-6632
- Publisher Link
- http://dx.doi.org/10.1088/0031-9155/55/22/003
- Publisher
- Institute of Physics Publishing
- Resource Type
- journal article
- Date
- 2010
- Description
- Backscatter from the metallic components in the support arm is one of the sources of inaccuracy in dosimetry with Varian amorphous silicon electronic portal imaging devices (a-Si EPIDs). In this study, the non-uniform arm backscatter is blocked by adding lead sheets between the EPID and an E-type support arm. By comparing the EPID responses on and off the arm, with and without lead and considering the extra weight on the imager, 2 mm of lead was determined as the optimum thickness for both 6 and 18 MV beam energies. The arm backscatter at the central axis with the 2 mm lead in place decreased to 0.1% and 0.2% for the largest field size of 30 x 30 cm² using 6 and 18 MV beams, from 2.3% and 1.3% without lead. Changes in the source–to-detector distance (SDD) did not affect the backscatter component more than 1%. The symmetry of the in-plane profiles improved for all field sizes for both beam energies. The addition of lead decreased the contrast-to-noise ratio and resolution by 1.3% and 0.84% for images taken in 6 MV and by 0.5% and 0.38% for those in 18 MV beams. The displacement of the EPID central pixel was measured during a 360° gantry rotation with and without lead which was 1 pixel different. While the backscatter reduces with increasing lead thickness, a 2 mm lead sheet seems sufficient for acceptable dosimetry results without any major degradation to the routine performance of the imager. No increase in patient skin dose was detected.
- Subject
- portal imaging device; radiation therapy; dose response; verification
- Identifier
- http://hdl.handle.net/1959.13/931860
- Identifier
- uon:11188
- Identifier
- ISSN:0031-9155
- Language
- eng
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