1Md. Anwarul Islam,PhD,,1Md. Abdul Mannan Chowdhury, Ph.D, 2Golam Abu Zakaria, Ph.D
1Dept. of Physics, Jahangirnagar University, Savar, Dhaka, Bangladesh
2Anhalt University of Applied Sciences, Koethen, Germany
The goal of radiotherapy is to deliver optimum dose to the target and minimum dose to the critical structures. It requires fast and accurate methods of dose measurement to achieve this goal. The dose distribution to the tumor depends on the correct dose measurement. Experimental dose measurement is very complex, time consuming and needs a lot of attention including very many factors. A few percentage of error is always considered for the experimental measurement. Monte Carlo methods are well known and considered as gold standard for dose calculation in radiotherapy. To calculate the dose in heterogeneous media and in the regions of electronic disequilibrium, the Monte Carlo method is rooted by the fundamental laws of the particle Physics interactions is used.
The purpose of this study was to simulate dose calculation of external photon beam using EGSnrc Monte Carlo (MC) code and compare them with the measured doses in homogenous and inhomogeneous medium. The complete geometry of the Elekta Synergy and Varian Clinac 2300CD linear accelerator (Linac) treatment head were designed using BEAMnrc MC Code and calculate dose by using DOSXYZnrc MC Code which are user codes of the EGSnrc. All the physical and technical parameters were tracked by the manufacturer’s specifications for designing the module. The Elekta Synergy was simulated to compare the MC Simulated data with experimental measurement data in homogeneous medium. Due to unavailability of Elekta data based commercial TPS, a Varian Clinac 2300CD was also designed to compare MC and commercial TPS data.
The MC simulated data of Varian model were compared with the commercial TPS algorithm of AAA and Acuros XB in homogeneous and inhomogeneous media. The water, air, lung and bone have been used as inhomogeneous media. A wide ranges of beam energy and Field with Half Maximum (FWHM) were investigated for 6 MV beam to match the realistic clinical measured data. A number of relative data and a single absolute data have been investigated. The relative data were PDD, Off Axis Beam Profile and Output Factor for the field size of 5 x 5 cm, 10 x 10 cm, 30 x 30 and 5 x 30 cm. The PDD of inhomogeneous media has been calculated with AAA and AXB algorithm in TPS and compared with MC code at 10 x 10 cm.
The absolute data were investigated for a standard field size of 10 x 10 cm. The gamma criteria of 3%/3mm has been used to evaluate the difference between the measured and MC data. Other two criteria of 2%/2 and 1%/1mm also have been used to see the difference in extreme conditions. In homogeneous media, the average gamma pass rate of PDD for 3%/3mm, 2%/2mm and 1%/1mm for all field sizes were 100%, 100 % and 98.75% respectively. The results of Off Axis Beam Profile were 100%, 98.79% and 85.23% respectively for the same gamma criteria and field sizes. The results showed a very good agreement between measured and MC simulated data in homogeneous media. In inhomogeneous water-air media, the gamma pass rate for 3%/3mm, 2%/2mm and 1%/1mm of AAA VS MC and AXB VS MC were 68.1,63.8, 40.4 and 85.1,83.0 72.3 respectively. Here MC data have comparatively good agreement with AXB algorithm in water-air media. In water-lung media the pass rate were 78.7, 72.3, 55.3 and 97.9, 97.9, 89.4 respectively. Here MC data have very good agreement with AXB data in all criteria. In water-bone media, the gamma pass rate were 97.9, 66.0, 19.1 and 97.9, 97.9, 91.5 respectively. In water-bone media gamma pass rate have good agreement at 3%/3mm with MC data where as in other two conditions were not meet. MC data have very good agreement with AXB data in all gamma pass rate criteria.
The results showed MC simulated data have very good agreement with AXB calculated data in all media. In case of air and lung media the AAA data have showed less pass rate. AXB calculation algorithm adopted MC simulation method and our results also showed good agreement with AXB. So the MC module has been correctly designed in this study.
The results showed that the BEAMnrc and DOSXYZnrc codes were an excellent tool for simulating the PDDs and beam profiles in homogeneous and inhomogeneous media. Therefore, the model built in this study used as promising method to calculate the dose distribution at any media.