Monte Carlo Simulation Of The Dosimetry Of Small Field

Radiotherapy, is one of the three principal modalities used in the treatment of malignant disease(cancer), and approximately 60% ~ 70% cancer patients need radiotherapy. The principle of radiation therapy is to destroy the DNA in the nucleus by ionizing radiation, make cells lose their ability to proliferate and kill the tumor cells. For this purpose, requires a strong foundation for the application of nuclear technology to support the radiation therapy. Since 1960, the linear accelerator has been developed in recent years, radiation therapy techniques have evolved to include the delivery of small field sizes traditionally considered unique to stereotactic radiosurgery. Small field plays an important role in modern radiotherapy, especially for some advanced radiotherapy technology, for example, stereotatic body radiation therapy(SBRT), intensity modulated radiation therapy(IMRT), and Gamma knife.The dosimetry of small x-ray fields is difficult, but important, in many radiation therapy delivery methods. The accuracy of ion chambers for small field applications, however, is limited due to the relatively large size of the chamber with respect to the field size, leading to partial volume effects, lateral electronic disequilibrium and calibration difficulties. Monte Carlo method is widely used in various fields of science and technology. In recent years, with the rapid development of Monte Carlo method, which in the radiotherapy has played an important role. The factors of Monte Carlo simulation are used in the calibration rule of the linear accelerator. Monte Carlo methods are also based on the calculation of the treatment options related to the accelerator head.In this work, using two procedures, the EGS4 / BEAM Monte Carlo simulation software BEAMnrc and DOSXYznrc, the head and QA on a Varian Truebeam medical linear accelerator with two-dimensional water tank is simulated. Using Beamnrc to establish the accelerator head part, the treatment information of beam as the input source input to the DOSXYZnrc to establish the simulation of two-dimensional water tank program. In conditions of source skin distance of 95 cm, get a water phantom dose distribution of open field size from 4cm×4cm to 40cm×40cm.And then calculated the phantom scatter factor, and compared with the actual measurement dose of water tank to verify the accuracy of the Monte Carlo program. Then use the same program to simulate small field from 0.5cm× 0.5cm to 3cm×3cm, draw the Percentage Depth Dose and profile curve, to explore the small field’s underwater dose distribution. The dose distribution obtained by the simulation of small field’s percentage depth dose and dose profile, may provide a reference for small field’s dose parameters to detection of future treatment of tumor.