Monte Carlo simulation of a multi-leaf collimator design for telecobalt machine using BEAMnrc code

Abstract

This investigation aims to design a practical multi-leaf collimator (MLC) system for the cobalt teletherapy machine and check its radiation properties using the Monte Carlo (MC) method. The cobalt machine was modeled using the BEAMnrc Omega-Beam MC system, which could be freely downloaded from the website of the National Research Council (NRC), Canada. Comparison with standard depth dose data tables and the theoretically modeled beam showed good agreement within 2%. An MLC design with low melting point alloy (LMPA) was tested for leakage properties of leaves. The LMPA leaves with a width of 7 mm and height of 6 cm, with tongue and groove of size 2 mm wide by 4 cm height, produced only 4% extra leakage compared to 10 cm height tungsten leaves. With finite 60Co source size, the interleaf leakage was insignificant. This analysis helped to design a prototype MLC as an accessory mount on a cobalt machine. The complete details of the simulation process and analysis of results are discussed.

Monte Carlo simulation based study of a proposed multileaf collimator for a telecobalt machine

“…20 Most of the MLC designs, available for linacs, use tongue-and-groove arrangements to assemble the leaves in order to reduce the interleaf leakage. The design of the proposed MLC by Singh et al 11 and Ayyangar et al 12 for telecobalt units also uses a similar concept for assembling the leaves. The effect of underdosing in the region of overlap between two adjacent leaves (which is also called as tongue and groove effect) (Ref.…”

“…To optimize the value of h, an arbitrary value of R = 9.5 cm was chosen as suggested by Ayyangar et al 12 The beam profiles for 10 × 10 cm 2 were generated through the Monte Carlo simulation by varying the values of h in the range of 0-7 cm (the leaf height of proposed MLC = 7 cm and hence the maximum value of h = 7 cm). The value of h which produced a radiation field width closer to nominal field width was selected as its optimum value.…”

“…This MLC is made up of a cerrobend material which has limited durability and mechanical limitation in its smooth routine use. Ayyangar et al 12 also proposed a design of a tertiary MLC for telecobalt machines which can define field sizes of up to 20 × 20 cm 2 . In addition, the designs of the MLCs proposed by these authors will add additional weight to the head of the telecobalt unit, which may affect the isocentric accuracy.…”

Monte Carlo study of MLC fields for cobalt therapy machine

“…In a previous publication,[ 1 ] the Monte Carlo (MC) modeling of the cobalt machine using the approach published by Mora et al .,[ 2 ] in 1999 was presented. In that paper,[ 1 ] the cobalt source and the collimator systems were modeled using BEAMnrc software and showed that the central axis PDD matched with published values from British Journal of Radiology Supplement 25 (BJR25). The beam profiles for a 10 × 10 cm 2 field were presented; however, they were not compared with any realistic profile data.…”

“…The model was used to test the MLC leakage using 6-cm tungsten leaves at a source distance of 48 cm. That paper[ 1 ] also presented the use of low melting point alloy (LMPA) and concluded that tungsten is preferable for the construction of the MLC leaves. The methodology of the current MC simulation follows the same approach as previously published.…”