Monte Carlo study on megavolt x-ray therapy for development of suitable targets for the evaluation of nano particle dose enhancement

[摘要] English: INTRODUCTION: Radiation dose enhancement with nanoparticles is a treatment techniqueinvolving the irradiation of tumour seeded with high atomic number (high Z) material. This workdescribes the generation of x-ray beams using a 6 MeV Elekta Precise linac head using low-ZBremsstrahlung target materials, water and carbon combined with tungsten. The aim of thestudy was to simulate photon energy spectra appropriate for high-Z nanoparticles doseenhancement in tumour using EGSnrc MC codes.MATERIALS AND METHOD: BEAMnrc Monte Carlo (MC) code successfully modelled thetreatment 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 differingquality from electron spectra obtained using 6 MeV electron beam. Water and carbon layerswere the primary target which were inserted in the path of the 6 MeV electron pencil beambefore it hits the tungsten Bremsstrahlung target to act as moderators that slow down electronbefore they hit a tungsten layer. The electron spectra obtained just after the primary target wasused as the incident beam to the tungsten target which acts as the secondary target togenerate x-ray photon beams. Therefore the x-ray beam source target was eitherwater/tungsten or carbon/tungsten combination. Different photon spectra were obtained forinvestigation in nanoparticles (NPs) based photon therapy. An original linac using a normaltungsten target of 0.3 cm thickness was also simulated to benchmark the results. The photonspectra obtained below X,Y jaws were used as input sources in the DOSXYZnrc MC code tosimulate dose distribution in water and a patient CT phantom. The simulations were carried outusing 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 tocharacterize the beams. The phantom had voxel size of 0.2 × 0.2 × 0.2 cm3. The photon beamswere characterised in terms of percentage depth doses and beam profiles. These x-ray beamswere then used to quantify the variation of tumour dose enhancement in a constructed patientCT phantom. The prostate tumour was used as the planning target volume (PTV). The PTVcomposition was either a tumour only or a tumour volume seeded with atoms of goldnanoparticles with concentration of 7mg/g of tumour.These tumour/NPs model was manually drawn on to the CT dataset from actual CT images ofthe patient using MCSHOW graphical user interface (GUI). The tumour composition was madepart of the patient CT data set using a locally-developed Interactive Data Language (IDL) codethat converts the density of the drawn volume into the desired tumour density. The 3DCRTwas used as the treatment strategy and 4, 5 and 6 field beams were investigated. With thismodel, we were able to estimate more accurately the effect of altered beams on NPs radiationdose enhancement. For both simulations using BEAMnrc and DOSXYZnrc the electron cut-offenergy (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 anaverage value of 1% at 0 �?30 cm depth.RESULTS AND CONCLUSION: The results showed that the use of electron moderators ingenerating x-ray beams for use in NPs seeded tumours can lead to a significant doseenhancement. Photon spectra obtained with water/tungsten or carbon/tungstenBremsstrahlung targets combinations showed significant changes at various target thickness.There is a significant dependence of dose enhancement factors (DEF) on the mean energy ofthe x-ray beams as well as the target thickness. DEFs ranging from 0.05% to 7.5% were obtainedat various Bremsstrahlung target combinations. Based on the results, carbon is more efficient atmoderating the electron beam to generate photon beams for dose enhancement at lowerthickness (approximately 1.4 cm) compared to water (approximately 2.5 cm), although watercan just be as good at larger thickness. At these thicknesses the mean photon beam energy isapproximately 0.4 MV. In summary, the results of this work indicate that the use of photonbeams from low-Z Bremsstrahlung targets as electron slowing down medium could enablesignificant clinical dose enhancement during external beam radiotherapy for NPs seededtumours. MC techniques showed to be valuable tools for dose calculations in both water andpatient CT phantom.

[发布机构] University of the Free State