1. 'Monte Carlo modelling of a novel transmission detector: comparison of simulated and measured VMAT beams'
Authors: D. Johnson1, S.J. Weston1, V.P. Cosgrove1, D.I. Thwaites2
1 Medical Physics, St James Institute of Oncology, and Leeds University, Leeds, UK 2 Institute of Medical Physics, School of Physics, University of Sydney, Australia
Introduction
The DAVID is a multi-wire transmission detector (Figure 1), mounted on the head of the linac with wires aligned with each MLC leaf pair, which has the potential to be used for in vivo dosimetry (IVD) of advanced treatments. The aim of this work was to build a model of the detector using the EGSnrc Monte Carlo software. The model was verified by comparing the DAVID signal measured on the linac with the simulated signal for 5 VMAT deliveries. All VMAT deliveries were made on an Elekta Synergy linac using plans created by Monaco 3.1. The simulated results were created using the EGSnrc Monte Carlo software.
Figure 1
The Monte Carlo Model
The DAVID contains collection wires.  It was shown that these wires do have an effect on the primary (Figure 2) and scatter signal (Figure 3), however  in each case the relationship was multiplicative showing that comparison of normalised results is valid. The Monte Carlo model of the DAVID created in EGSnrc’s dosxyznrc was constructed from two 4mm thick pieces of Perspex separated by a by a 2 mm air gap, ie omitting the wires. The phase space for the VMAT beams, used by dosxyznrc, was created using BEAMnrc in conjunction with the SYNCMLC and SYCJAWS modules [1]. In each case 1.5×109 histories were used and the simulation was distributed across 64 cores on the Leeds MARC1 grid [2].
Figure 2
Figure 3
Figure 2 shows the Monte Carlo signal in the collection volume associated with the opening leaf pair, with and without a collection wire, for various leaf separations.
Figure 3 shows the Monte Carlo signal in the collection volume associated with the closed leaf pair, with and without a collection wire, for various leaf bank separations.
Leaf separation
Leaf separation
Results and conclusion
Using in-house software a fluence map could be extracted from the dosxyznrc phase space file; an example is shown in figure 4. Summing along the length of the collection volumes it was possible to generate a Monte Carlo signal and compare it with the actual signal measured on the linac (Figure 5).
In total, 5 VMAT beams were modelled (table 1). The differences between the measured signal and the simulated signal were consistently small, showing that the simplistic model is sufficient, and the fluence models generated are likely to reflect the integrated fluence generated in the DAVID from a VMAT delivery.
Table 1
signal
Figure 5
MLC pair
[1] J. Lobo and I. A. Popescu. Two new dosxyznrc sources for 4d monte carlo simulations of continuously variable beam configurations, with applications to rapidarc, vmat, tomotherapy and cyberknife. Phys. Med. Biol., 55:4431 – 4443, 2010.
[2] This work was undertaken on MARC1, part of the High Performance Computing facilities at the University of Leeds, UK.
Some of this work was sponsored by PTW
Figure 4