Evaluation of prototype of improved electron collimation system for Elekta linear accelerators

PURPOSE: This 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. METHODS: Prototype 10 × 10 and 20 × 20‐cm(2) 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 cm(2)) 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. RESULTS: Prototype 10 × 10 and 20 × 20‐cm(2) 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 cm(2)) 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. CONCLUSIONS: Our 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.