| Cancer is a major cause of morbidity and mortality in people of all ages. Radiation therapy may be used alone or in combination with surgery and chemotherapy to treat primary or metastatic tumor. Nearly 60% to 70% of cancer patients benefit from radiotherapy.At present, the Intensity-Modulated Radiation Therapy (IMRT), Stereotactic Radiosurgery (SRS), Stereotactic Radiotherapy (SRT), Volumetric Modulated Arc Therapy (VMAT) as well as the Flattening-Filter-Free (FFF) beam radiotherapy have been widely used in clinical practice. The multi-leaf collimators provide small and nonstandard fields in these new radiotherapy technologies. The improved dose conformity is achieved through the use of small and nonstandard fields which lead to the opportunity for margin reduction and normal tissue sparing. However, it also brings dosimetric challenges to dose measurement.The accurate measurement of the reference dose and relative dose is essential to radiotherapy. The American Association of Physicists in Medicine (AAPM) TG-51 and International Atomic Energy Agency (IAEA) TRS-398 standard codes of practice (CoP) provide the methodology to perform dosimetry in a standard reference field, usually 10 cm × 10 cm, where the lateral charge particle equilibrium is obtained. However, these standards for absorbed dose to water formalisms were invalid for small and nonstandard fields due to the lateral electron disequilibrium and source occlusion effect. In a joint effort between IAEA and AAPM, a new formalism has been proposed for reference dosimetry in small and nonstandard fields. The proposed formalism introduces a new factor that corrects for the differences between the conventional reference field and the machine-specific reference field. This factor can be determined with Monte Carlo (MC) simulations, but it is necessary to know the accurate geometric and material parameters of the ionization chamber.TrueBeam (Varian Medical Systems, Palo Alto, CA) is a new generation of linear accelerator providing 6 and 10 MV FFF photon beams with the highest intensity mode at 1400 MU/min and 2400 MU/min respectively. Because of the FFF beam provides much higher dose rate and less head scattering, which has been increasingly applied to the SRT and SRS for better delivery efficiency. Due to the single high dose of SRT, accurate measurement and calculation of the dose is not only especially and extremely crucial for the target but also for the surrounding normal tissues. Alternatively, the MC method provides accurate simulation of the machine geometry and particle interactions, which was used to investigate the small field dosimetry of FFF beams in this study.Accurate geometric and material parameters of the Linac head are critical for the MC modelling, yet they have not been made available for TrueBeam except for the first and second generation phase-space files. In this study, we selected appropriate material and geometry of the target and foil for 6 and 10 MV FFF beams of TrueBeam Linac based on the training materials of Varian. The other structures were consistent with Varian iX Linacs that have been released for research before. Based on these physical models, the BEAMnrc and DOSXYZnrc codes were used to simulate the percentage depth doses (PDDs) and the off-axis ratios (OARs) curves for 6 and 10 MV FFF X-ray with field sizes ranging from 4 × 4 to 40 cm × 40 cm. The incident beam energy, radial intensity distribution and angular spread were adjusted respectively to get the optimum parameters for the model, which were used to investigate the characteristics of small fields of less than 4 cm × 4 cm. In addition to the PDDs and OARs curves, the minimum beam radius required to achieve lateral electron disequilibrium and the absorbed dose to water were also calculated.The Full Width Half Maximum (FWHM), mono-energetic energy, and angular spread of theresultant incident Gaussian radial intensity electron distribution were 0.75 mm,6.1 MeV and 0.9° respectively for the nominal 6 MV FFF beam, and were 0.7 mm,10.8 MeV and 0.3° respectively for the 10 MV FFF beam. The differences between the simulated and measured beam qualities were less than 0.5% for both beams.For the depths from 0.1 cm to 30 cm, Gamma criteria of 1 mm/1%(Local dose) can be met by all PDDs for the fields of larger than 1 cm × 1 cm in size. For the field size of 1 cm × 1 cm, the criteria of 1 mm/2% can be fulfilled. The gamma analysis results of OARs for various field sizes at 10 cm depth shown that the agreement was within 1 mm/1% for field sizes of less than 20 cm x 20 cm, within 1 mm/2% for other field sizes of 6 and 10 MV FFF beams respectively. Meanwhile, the MC simulated total scattering factor as well as the absorbed dose to water agreed well with the corresponding measured results of various field sizes (the discrepancies were less than 1%), except for the 1 cm × 1 cm field.Using the proposed model parameters in this study, the MC simulated results agreed well with the measurements hence can be used for further clinical dosimetric studies involving 6 and 10 MV FFF X-ray. This study may provide a theoretical basis and reference data for the development of formalism and measurement device for reference dosimetry in small and nonstandard fields. Although the head model used in this study can approximate the beam data, the actual structural information of the TrueBeam accelerator is necessary to verify the accuracy of these model parameters. Further studies are needed for a complete investigation of the characters of FFF beams especially for the small field sizes. |
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