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 γ _max, average γ _avg, 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 γ _max 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.