To evaluate the accuracy of dynamic versus static IMRT delivery using portal dosimetry

INTRODUCTION: To evaluate the delivery accuracy of dynamic (DMLC) and static (SMLC) intensity modulated radiation therapy (IMRT) techniques using portal dosimetry (PD) in Varian Eclipse Treatment Planning System. MATERIALS AND METHODS: Seven DMLC IMRT Head and Neck plans were retrospectively generated for the study using SMLC mode at 20, 10 and 5 levels of intensity (SMLC20, SMLC10, SMLC5). Dosimetric verifications performed by PD on a total of 107 fields were evaluated using the gamma index (maximum (γmax), average (γavg), percentage of points with (γ%) ≤ 1). The images were acquired at a source-detector distance of 100 cm at gantry zero degree and also at clinically planned gantry angles. RESULTS: For both modes, measurements are within acceptable criteria. (γ%) ≤ 1 improves by increasing SMLC levels (+3.4 % from SMLC5 to SMLC20, p < 0.001) and using DMLC (+3.9 % and +0.6 % compared to SMLC5 and SMLC20, respectively, p < 0.001). Also (γmax) parameter improves significantly by increasing SMLC levels (+22 % from SMLC5 to SMLC20) and using DMLC (+34 % and +16 % compared to SMLC5 and SMLC20, respectively). The effect of the gantry rotation influences the delivery accuracy by up to -7 % (p < 0.05). The effect of leaves travelling direction was almost negligible (1 %). CONCLUSIONS: A good agreement between calculated and measured fluences was obtained for DMLC and SMLC techniques at higher intensity levels; however, DMLC delivery ensures the best reproduction of computed fluence maps. The gantry rotation influences the delivery accuracy in particular for SMLC modes at lower intensity levels.