Monte Carlo study on megavolt x-ray therapy for development of suitable targets for the evaluation of nano particle dose enhancement
MetadataShow full item record
English: INTRODUCTION: Radiation dose enhancement with nanoparticles is a treatment technique involving the irradiation of tumour seeded with high atomic number (high Z) material. This work describes the generation of x-ray beams using a 6 MeV Elekta Precise linac head using low-Z Bremsstrahlung target materials, water and carbon combined with tungsten. The aim of the study was to simulate photon energy spectra appropriate for high-Z nanoparticles dose enhancement in tumour using EGSnrc MC codes. MATERIALS AND METHOD: BEAMnrc Monte Carlo (MC) code successfully modelled the treatment head components of a flattening filter free 6 MV Elekta Precise linear accelerator. Simulations were run using suitable histories to generate high energy x-ray beams of differing quality from electron spectra obtained using 6 MeV electron beam. Water and carbon layers were the primary target which were inserted in the path of the 6 MeV electron pencil beam before it hits the tungsten Bremsstrahlung target to act as moderators that slow down electron before they hit a tungsten layer. The electron spectra obtained just after the primary target was used as the incident beam to the tungsten target which acts as the secondary target to generate x-ray photon beams. Therefore the x-ray beam source target was either water/tungsten or carbon/tungsten combination. Different photon spectra were obtained for investigation in nanoparticles (NPs) based photon therapy. An original linac using a normal tungsten target of 0.3 cm thickness was also simulated to benchmark the results. The photon spectra obtained below X,Y jaws were used as input sources in the DOSXYZnrc MC code to simulate dose distribution in water and a patient CT phantom. The simulations were carried out using source 2 in DOSXYZnrc. A 40 x 40 x 40 cm3 water phantom was simulated at 100 cm SSD using a range of field sizes to characterize the beams. The phantom had voxel size of 0.2 × 0.2 × 0.2 cm3. The photon beams were characterised in terms of percentage depth doses and beam profiles. These x-ray beams were then used to quantify the variation of tumour dose enhancement in a constructed patient CT phantom. The prostate tumour was used as the planning target volume (PTV). The PTV composition was either a tumour only or a tumour volume seeded with atoms of gold nanoparticles with concentration of 7mg/g of tumour. These tumour/NPs model was manually drawn on to the CT dataset from actual CT images of the patient using MCSHOW graphical user interface (GUI). The tumour composition was made part of the patient CT data set using a locally-developed Interactive Data Language (IDL) code that converts the density of the drawn volume into the desired tumour density. The 3DCRT was used as the treatment strategy and 4, 5 and 6 field beams were investigated. With this model, we were able to estimate more accurately the effect of altered beams on NPs radiation dose enhancement. For both simulations using BEAMnrc and DOSXYZnrc the electron cut-off energy (ECUT) and photon cut-off energy (PCUT) was 0.521 MeV and 0.01 MeV respectively. The number of histories was chosen so that the statistical uncertainty along the CAX had an average value of 1% at 0 – 30 cm depth. RESULTS AND CONCLUSION: The results showed that the use of electron moderators in generating x-ray beams for use in NPs seeded tumours can lead to a significant dose enhancement. Photon spectra obtained with water/tungsten or carbon/tungsten Bremsstrahlung targets combinations showed significant changes at various target thickness. There is a significant dependence of dose enhancement factors (DEF) on the mean energy of the x-ray beams as well as the target thickness. DEFs ranging from 0.05% to 7.5% were obtained at various Bremsstrahlung target combinations. Based on the results, carbon is more efficient at moderating the electron beam to generate photon beams for dose enhancement at lower thickness (approximately 1.4 cm) compared to water (approximately 2.5 cm), although water can just be as good at larger thickness. At these thicknesses the mean photon beam energy is approximately 0.4 MV. In summary, the results of this work indicate that the use of photon beams from low-Z Bremsstrahlung targets as electron slowing down medium could enable significant clinical dose enhancement during external beam radiotherapy for NPs seeded tumours. MC techniques showed to be valuable tools for dose calculations in both water and patient CT phantom.