SU‐E‐T‐198: Dosimetric Effect of Multi‐Leaf Collimator Transmission On Intensity‐Modulated Radiation Therapy

Abstract

Purpose: To investigate the dosimetric effect of multi‐leaf collimator (MLC) transmission on Intensity‐modulated radiation therapy. Methods: MLC transmission through the leaves and rounded ends were measured with ion chamber for Varian Trilogy linear accelerator with the energy of 6 MV and 10 MV. The intraleaf and interleaf transmission were also measured with the electronic portal imaging device of aS1000. 10 cancer patients treated with intensity‐modulated radiation therapy were used to evaluate the MLC parameters modeling in the Eclipse treatment planning system. Results: The average transmissions of Millennium MLC were 1.6% and 1.8% for 6 MV and 10 MV X‐rays. The transmission increased with the field size and depth of measurement. The transmissions at off axis position were a little lower than those on the central axis. The intraleaf transmissions measured with aS1000 were 0.8%–1.2% and 1.1%–1.6%, the interleaf transmission were 1.3%–1.9% and 1.6%–2.5% for 6 MV and 10 MV X‐rays respectively. Modeling with the measured MLC parameters, the Eclipse treatment planning system could calculate the dose distribution accurately. The gamma index pass rate at 3%3 mm was above 95%, except for two patients with nasopharyngeal cancer with the pass rate of 93.6% and 94.5%. Conclusion: The transmissions through the MLC leaves and the leaf ends contribute to the dose throughout the target significantly for IMRT. MLC transmission varied with the field size, depth of measurement and off axis position. This work was supported within the grant project: National Natural Science Foundation of China (No. 81071237).