Multi-leaf Collimation of Electron Beams with Monte Carlo Modelling and Dose Calculation

Tumours and other superficial targets are often treated with electron beams in radiotherapy. The build-up dose and the therapeutic range of electrons are controlled by selecting the electron energy and by using a water-equivalent bolus placed on the skin. Currently, the shaping of clinical electron beams is often done via low melting point alloy block inserts individually made for each patient. However, leaf based collimation with an electron multi-leaf collimator (eMLC) might represent a more practical option. In the present study, the possibility of using a new eMLC as an add-on device to an existing linear accelerator was investigated. The main focus was on the dosimetric properties and feasibility of modelling the eMLC beam with a parametrized Monte Carlo (MC) beam model. Secondly, the effect of eMLC leaf geometry on build-up dose of narrow beams was studied with the aim of replacing a conventional bolus with eMLC. In this work, the Voxel Monte Carlo++ (VMC++) algorithm was demonstrated to be very accurate in the dose calculation of the eMLC beam. Compared against measurements in water, the agreement was within 2% or 2 mm in 88% of the calculated voxels. The maximum dose from eMLC beams was only slightly higher in the target volume when compared with the current insert-based collimation in the chest wall irradiation of breast cancer. Narrow eMLC beams with a field size 1×10 cm2 have a markedly higher build-up dose compared to a large field meaning that it was possible to replace a bolus by the combination of several narrow eMLC fields. Depending on the eMLC leaf end shape, the surface dose at 0.5 mm depth can be up to 90% of the dose maximum with abutting narrow beams which is approximately 25% higher than the surface dose with an open field and is comparable to electron beams used with a with bolus material. The add-on type eMLC results in only marginal differences compared with the present technique for electron beam collimation with block inserts. The novel beam model together with VMC++ enables patient dose calculations and may be developed for clinical use. National Library of Medicine Classification: QT 36, WN 105, WN 250, WN 250.5.R2, WN 665 INSPEC Thesaurus: radiation therapy; electron beams; beam handling equipment; collimators; linear accelerators; dosimetry; modelling; Monte Carlo methods Yleinen suomalainen asiasanasto: sädehoito; ionisoiva säteily; säteilyannokset; hiukkaskiihdyttimet; mallintaminen; Monte Carlo -menetelmät