Monte Carlo Simulation Of The Field Size Influence On The Depth Dose

This research has built a simulated model of a treatment head by analyzing the information of the Varian Clinac 23 EX from the First Affiliated Hospital of Soochow University using the Monte Carlo program BEAMnrc MP.The model was used to calculate the percentage depth dose and off axis ratio at the depth of 10 cm and 20 cm in the field size of 4×4cm2、8×8cm2、12×12cm2、15×15cm2 、 20 × 20cm2 when the source-to-skin distance is 100 cm. The result of the calculation and the measured data was compared and it proved that most of the error placed in the dose drop zone and the simulated model was basically reliable.The model was also used to calculate the percentage depth dose in the field size of4×4cm2、5×5cm2、6×6cm2、8×8cm2、10×10cm2、12×12cm2、15×15cm2、17×17cm2、20×20cm2when the source-to-skin distance is 100 cm. The conclusions are as follows:1. For the 6MV energy X-ray, the dose built area is about 15mm;2. When the beam field size increases, the absorbed dose at the same depth will increase, the total scatter factor will increase fast at the beginning and slow down later;3. When the field size is small, the field size has a significant impact on the scattered rays and when the field is large, the influence of the field size on the scattered rays becomes gradually saturated.