Room: Exhibit Hall | Forum 3
Purpose: A laying-down technique for Total Skin Irradiation was recently implemented at our clinic for patients who are too frail for the standing position. The aims of this study were to validate the dose measurements with Monte Carlo simulation.
Methods: Monte Carlo simulation was performed within the EGSnrc environment, using phase space files provided by the linac vendor. Parameters of Jaws and a customized Cu filter were firstly tuned to match the profiles and outputs measured at SSD=100 cm. Particles traveling through the isocenter plane were scored and used as the secondary source for downstream simulation. The secondary source was further rotated to simulate different gantry angles, and doses were calculated in the water phantoms placed at the treatment SSDs (vertex fields: SSD=195 cm; oblique fields: SSD=305 cm). The following quantities were studied and compared with the measurements: PDDs, profiles at the depth of maximum, and absolute dosimetric output; composite dose distribution on a cylindrical phantom of 30 cm diameter.
Results: Cu filter increases the beam FWHM by 44% but also reduces the output by 60%. Profiles and output factors for both vertex fields and oblique fields agreed within 3% on average. When all fields are combined on the cylindrical phantom, a uniform dose distribution was observed and dmax shifted toward surface region. Surface dose, dmax and R80 of the simulated PDDs matched with the measured data within 2mm. The maximum X-ray contamination at the central axis was (MC: 2.2; Measurement: 2.1) %, and reduced to 0.2% at 40 cm off the central axis. Body-factor and X-ray contamination decrease as phantom sizes increase, while surface dose increases by approximately 2% with every 5 cm increase in diameter.
Conclusion: The Monte Carlo results in general agree well with the measurement, providing secondary support in our commissioning procedure and three-dimensional dose distribution.