Open AccessImproved electron collimation system design for Elekta linear accelerators
Author(s) -
Pitcher Garrett M.,
Hogstrom Kenneth R.,
Carver Robert L.
Publication year - 2017
Publication title -
journal of applied clinical medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.83
H-Index - 48
ISSN - 1526-9914
DOI - 10.1002/acm2.12155
Subject(s) - isocenter , collimated light , linear particle accelerator , flatness (cosmology) , optics , materials science , monte carlo method , physics , beam (structure) , mathematics , laser , imaging phantom , statistics , cosmology , quantum mechanics
Prototype 10 × 10 and 20 × 20‐cm 2 electron collimators were designed for the Elekta Infinity accelerator ( MLC i2 treatment head), with the goal of reducing the trimmer weight of excessively heavy current applicators while maintaining acceptable beam flatness (±3% major axes, ±4% diagonals) and IEC leakage dose. Prototype applicators were designed initially using tungsten trimmers of constant thickness (1% electron transmission) and cross‐sections with inner and outer edges positioned at 95% and 2% off‐axis ratios ( OAR s), respectively, cast by the upstream collimating component. Despite redefining applicator size at isocenter (not 5 cm upstream) and reducing the energy range from 4–22 to 6–20 MeV, the designed 10 × 10 and 20 × 20‐cm 2 applicator trimmers weighed 6.87 and 10.49 kg, respectively, exceeding that of the current applicators (5.52 and 8.36 kg, respectively). Subsequently, five design modifications using analytical and/or Monte Carlo ( MC ) calculations were applied, reducing trimmer weight while maintaining acceptable in‐field flatness and mean leakage dose. Design Modification 1 beveled the outer trimmer edges, taking advantage of only low‐energy beams scattering primary electrons sufficiently to reach the outer trimmer edge. Design Modification 2 optimized the upper and middle trimmer distances from isocenter for minimal trimmer weights. Design Modification 3 moved inner trimmer edges inward, reducing trimmer weight. Design Modification 4 determined optimal X‐ray jaw positions for each energy. Design Modification 5 adjusted middle and lower trimmer shapes and reduced upper trimmer thickness by 50%. Design Modifications 1→5 reduced trimmer weights from 6.87→5.86→5.52→5.87→5.43→3.73 kg for the 10 × 10‐cm 2 applicator and 10.49→9.04→8.62→7.73→7.35→5.09 kg for the 20 × 20‐cm 2 applicator. MC simulations confirmed these final designs produced acceptable in‐field flatness and met IEC ‐specified leakage dose at 7, 13, and 20 MeV. These results allowed collimation system design for 6 × 6–25 × 25‐cm 2 applicators. Reducing trimmer weights by as much as 4 kg (25 × 25‐cm 2 applicator) should result in easier applicator handling by the radiotherapy team.