A study of the portal dose image calculation of patient by Monte Carlo simulation

EPID has been used to determine the patient position during radiation therapy treatment. By comparing the computed Portal Dose Image(PDI) with the EPID's PDI for defining patient setup EPID can be utilized for verifying treatment dose. Energy spectrum of Varian 2100C(6MV) was obtained by the MCNP4B simulation. In order to verify this spectrum PDD(Percentage Depth Dose) and OAR(Off Axis Ratio) in the water phantom were calculat ed by EGS4 code and compared with the measurements. Within 10cm depth PDD agreed with the measurement but the difference between calculation and measurement was increased beyond this depth. OAR was calculated and measured in 1.6cm and 10cm depth. The calculations agreed well with the measurements in 1.6cm depth and in 10cm depth except that in the latter depth the error in penumbra region increased. And the portal dose image(PDI) calculated by EGS4 was compared with PDI measured with EPID and film. Calculated and measured PDIs agreed within 3% each other. Through this study, it can contribute to establish the patient's treatment dose verifying system