Initial Beam Characteristics calculations of 6 MV FF Beam from Elekta Versa-H Dusing TOPAS MC simulation code

Abstract

The main aim of PhD. research work is to study the Small field dosimetry in Radiotherapy (RT) dosimetry using the experimental measured data Monte Carlo (MC) simulated data.Small fields are very important in RT[1,2],with the development of stereotactic body radiation therapy (SBRT)began in the early 1990s.SBRT uses advanced technology to deliver dose to small deep-seated tumors in the lung, liver, spine, pancreas, kidney and prostate. Ultra high radiation doses per fraction (6 to 30 Gy) are delivered with in five or fewer fractions. Various autors discussed about the challeages during small beam dosimetry[3–5],the challeages are related to :

• There is a loss of lateral charged particle equilibrium (LCPE) on the beam axis.
• There is partial occlusion of the primary photon source by the collimating devices on the beam axis.
• The size of the detector is similar or large compared to the beam dimensions.

All three of these conditions result in overlap between the field penumbrae and the detector volume , so the dosimetry of these type of filed sizes is always a challenging taks , to deal with these type of condations specific dosimetry equpiments are required. The use of MC method to solve problems in the field of radiotherapy for treatment planning and dosimetry has increased almost exponentially since 1970. Various authors have used MC simulation in field of medical physics dosimetry to deal with these types of problems[6]. One of the aim is to simulate the RT machines in using Tool for particlesimulation (TOPAS) MC simulation code. But the MC simulation is very complicated and time consuming process, in this research work, we are trying to overcome the complexity using TOPAS MC simulation, which is a Geometry And Tracking (GEANT4)based MC tool kit but the way of simulation is easy as compare to other codes. So via validating this code for small beams will overcome the difficulties and increase the confidence of new users to use this code in RT dosimetry.There are many publications on the simulation of different RT machines but there are limited studies are available, so the objective of this study was to investigate the initial dosimetric parameters of a 6 MV photon beam with flattening filter (6MV FF) Elekta Versa-HD linear accelerator (LA) at Homi Bhabha Cancer Hospital (HBCH) using TOolforPArticleSimulation TOPASMC simulation code. TOPAS 3.6 simulated data was compared to experimental measured data to conform the accuracy of modelled Linac in TOPAS code. The reference data for Versa-HD Elekta Linac were acquired, using the 3D scanner water phantom system and the SNC125c chamber, dosimetric distribution experiments for 6 MV FF photon beam with field sizes 5x5, 10x10, and 20x20 cm² were performed. The data was collected at a source-to-skin distance of 90 cm, with a depth of 10 cm depth.The Linac modeling was based on the manufacturer's specifications that might be obtained from the machine vendorThe various component of Linac like, target, primary collimator, flattening filter, mirror assembly and head shielding components were modelled in TOPAS using geometry class, like the combination of different geometric components (cylinder, cone, polygon, box). However, the geometry of the multi-leaf collimator (MLC) and Y-jaws collimator is quite complex. As a result, both components were created as 3D stereolithography models (STL). This work may leads to new development in small field dosimetry moreover The clinical small field photon beam dosimetry is not well established unlike the dosimetry of a conventional photon beam. Moreover, recently IAEA TRS-483 has published but this required more attention to implement clinically, so need to be explored. This work will help to understand the small fields along with the dose verification in modern Radiotherapy techniques (SRS, SBRT, and VMAT). Further, work will search the new simple and easily reproducible methods to calculate the small beams dosimetric data, the applications of MC simulation in Medical Physics dosimetry as well as MC simulated and experimental data will may help the community to extend the knowledge in small field dosimetry and may improve the clinical outcomes.The small dosimetry is very important and crucial part of modern RT dosimetry, so the importance of this should be considered. During the dosimetric measurements of small fields.