Evaluation of Multileaf Collimator Effect on the Build-Up Dose Region Using Different Ionization Chambers

Abstract

In the present work, we studied the effect of a multileaf collimator (MLC) on the build-up region. Ionization measurements (PDI) were performed in a water phantom for a 6 MV photon beam emerging from a Varian 2100C/D accelerator with a 120 – MILLENIUM MLC. We used two different detectors: a PTW 31002 cylindrical ionization chamber (IC) and a Roos parallel-plate chamber. Several MLC-JAWS configurations were investigated and the contribution of MLC for the build-up region was determined by comparing PDIs measured for MLC defined field sizes (from 2 x 2 to 10 x 10 cm²) and the corresponding PDIs measured for open fields. Our results were also compared with previous Monte Carlo (MC) simulated depth dose curves for the same experimental set-up [1].The Roos chamber measured dose values agree with MC values within 2% for all MLC openings and depths greater than 2 mm. We observed differences of about 4% between the values measured with cylindrical IC and previous calculated MC values. A negligible effect (less than 1%) of the MLC on the build-up dose was observed for all MLC defined field sizes. Our measurements can not predict the MC calculated surface dose values.