Monte Carlo simulations of electron beams collimated with a dual electron multileaf collimator: a feasibility study

Abstract

Electron applicators and cut-outs have been used for some time in the delivery of electron beam therapy. A dual electron multileaf collimator (eMLC) consisting of upper and lower eMLCs was designed, and dose distributions of electron beams delivered by it were evaluated by Monte Carlo simulations by use of the EGSnrc Monte Carlo code. The percentage depth dose (PDD), dose profiles, dose gradient falloff (G), depth of maximum dose (R(100)), surface dose, bremsstrahlung background, beam flatness, and penumbra of the dual eMLC were evaluated and compared with those simulated and measured with the standard applicators inserted into the treatment head of the medical linear accelerator (linac). The results showed good agreement in most cases. Specifically, the flatness and penumbra obtained with the dual eMLC were better than those obtained with standard applicators. It is therefore possible to use the proposed dual eMLC in the delivery of electron beam therapy without the need for applicators and cut-outs. This will minimize the inconvenience of placing cut-outs on the patients; also, changes in the required field can be effected without the therapist going into the accelerator room when the dual eMLC is finally automated. The absence of a helium bag between the upper and lower eMLCs did not offer significant changes in the dosimetry parameters compared to the eMLC fitted with the helium bag.

Optimization of dual electron multileaf collimator materials by use of EGSnrc

“…The applicator was completely replaced by the dual eMLC and simulated by the EGSnrc Monte Carlo code. The detailed design of the dual eMLC has been described elsewhere [7], where it was shown that the dose parameters [percentage depth dose (PDD), dose profiles, depth of maximum dose (R 100 ), dose gradient fall-off (G), surface dose (D s ), bremsstrahlung background (D x ), beam flatness and penumbrae] of the simulated dual eMLC, simulated applicator and physical measurement with applicators hanging in its position on the LINAC head, were all in good agreement. However, for completeness, we present a summary of the design.…”

“…However, for completeness, we present a summary of the design. Figure 1 is a block diagram of the dual eMLC as proposed by Inyang and Chamberlain [7] and used in this study.…”

“…Previous simulations in which the information package was used resulted in dose distributions that were comparable to measured values within 2% [7]. The first set of simulations was done at 6, 9, 12, and 15 MeV with a 10 9 10 cm 2 field at 100 cm SSD, as defined by a dual eMLC developed with materials similar to those of the Varian type III applicator scrapers in an effort to investigate the energy at which maximum radiation leakage would occur.…”

“…Recently, Inyang and Chamberlain [7] proposed a dual eMLC which is capable of being used without the applicator and which may deliver irregular fields for electron therapy. The proposed dual eMLC consists of two sets of eMLC (upper and lower) with a helium bag fitted between them.…”

“…This allowed for a direct comparison of the dosimetry parameters obtained with the dual eMLC and those of the applicator [7]. When we had established the feasibility of using the dual eMLC with the same combination of materials as those of Varian type III applicators in the delivery of electron therapy, it became necessary to investigate whether a better beam quality and minimal leakage radiation could be achieved through the use of dual eMLCs made of materials having densities and atomic numbers different from that investigated by Inyang and Chamberlain [7], based on the fact that low-atomic-number materials are useful in minimizing electron scattering and bremsstrahlung production, whereas high-density materials offer better shielding against leakage radiation [8]. In this study, therefore, we investigated the combination of materials that will allow for minimal radiation leakage and beam penumbra.…”