Abstract
PurposeThis study evaluated a new electron collimation system design for Elekta 6–20 MeV beams, which should reduce applicator weights by 25%–30%. Such reductions, as great as 3.9 kg for the largest applicator, should result in considerably easier handling by members of the radiotherapy team.MethodsPrototype 10 × 10 and 20 × 20‐cm2 applicators, used to measure weight, in‐field flatness, and out‐of‐field leakage dose, were constructed according to the previously published design with two minor modifications: (a) rather than tungsten, lead was used for trimmer material; and (b) continuous trimmer outer‐edge bevel was approximated by three steps. Because of lead plate softness, a 0.32‐cm aluminum plate replaced the equivalent lead thickness on the trimmer's downstream surface for structural support. Models of all applicators (6 × 6–25 × 25 cm2) with these modifications were inserted into a Monte Carlo (MC) model for dose calculations using 7, 13, and 20 MeV beams. Planar dose distributions were measured and calculated at 1‐ and 2‐cm water depths to evaluate in‐field beam flatness and out‐of‐field leakage dose.ResultsPrototype 10 × 10 and 20 × 20‐cm2 applicator measurements agreed with calculated weights, in‐field flatness, and out‐of‐field leakage doses for 7, 13, and 20 MeV beams. Also, MC dose calculations showed that all applicators (6 × 6–25 × 25 cm2) and 7, 13, and 20 MeV beams met our stringent in‐field flatness specifications (±3% major axes; ±4% diagonals) and IEC out‐of‐field leakage dose specifications.ConclusionsOur results validated the new electron collimating system design for Elekta 6–20 MeV electron beams, which could serve as basis for a new clinical electron collimating system with significantly reduced applicator weights.
Highlights
All five applicators were modeled with the two minor modifications in a Monte Carlo (MC) code for calculation of in-field flatness and out-of-field leakage doses
The range of a 20 MeV electron beam calculated using the MC 1% threshold method equation is slightly less for lead than tungsten.[6]
Results of this study showed that measurements using prototype 10 9 10 and 20 9 20-cm[2] applicators agreed with calculated weights, in-field flatness, and out-of-field leakage doses for Pitcher et al.’s 5 design of a new Elekta electron collimating system for 7, 13, and 20 MeV beams
Summary
Our cancer clinic has seven Elekta Infinity radiotherapy accelerators (MLCi2 treatment head) with matched, custom electron beams spanning 7–20 MeV (R90 values of 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, and 6.0 Æ 0.1 cm) and slightly modified scattering foils.[1,2] they have exceptional in-field flatness laterally (Æ3% along major axes; Æ4% along diagonals3) and out-of-field leakage doses well below IEC standards[4] for all applicators (6 9 6–25 9 25 cm2), there is opportunity to improve the delivery technology by reducing the weight of the electron applicators, those for larger fields.[5,6] To that end, an optimization procedure utilizing analytical pencil beam and Monte Carlo (MC) calculations was developed to design significantly lighter applicators with similar in-field flatness and out-of-field leakage dose as the current ones. Prototype 10 9 10 and 20 9 20-cm[2] applicators were constructed and used to measure weight, in-field flatness, and out-of-field leakage dose. The two applicators were constructed according to previously published design[5] with two minor modifications: (a) lead was substituted for tungsten for trimmer material, which required a 0.32cm aluminum plate replacing the equivalent lead thickness on the trimmer’s downstream surface for structural support, and (b) trimmer outer-edge bevel was approximated by three steps. All five applicators were modeled with the two minor modifications in a MC code for calculation of in-field flatness and out-of-field leakage doses. Results will show that both measurements and calculations exhibited expected weights, in-field flatness, and out-of-field leakage doses
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