Dosimetric impact of multileaf collimator position errors during prostatic treatment by dynamic arctherapy

Introduction The quality of a treatment performed by intensity modulation is based on the positioning accuracy of the multileaf collimator leaves (MLC, 120 leaves). The dosimetric impact achieved by the introduction of systematic errors on the leaves positioning has been studied to assess the generated effects. Materials and methods The study was performed on 8 patients treated for prostate localization with 80 Gy prescribed dose in dynamic arc therapy. Original treatment plans clinically approved were exported in DICOM RT to be processed using Matlab ® software. Systematic errors of magnitude between 0.5 mm and 5 mm were introduced by opening the MLC leaves, or by closing and shifting. The 128 revised plans were then introduced into the treatment planning system (TPS). They were recalculated keeping the same optimization constraints than the original plan. Geometric and dosimetric index (SFRO and ICRU used index) were calculated using the Artiview ® software, Aquilab. Results Changes introduced to the original plans are significantly impacting dosimetric treatment planning validation index. Among these index, the near maximum dose D_2%(Gy) received by the target volume (PTV) differs from 12%, 15% and 20% to the original treatment plan for maximum openings, shifts and closures respectively. For the same index and amplitude errors of up to 2 mm, an offset MLC error is less penalizing ( 5%) systematic error. Other dosimetric and geometric index complete the analysis as the Dice similarity coefficient (DSC), which is more sensitive to errors introduction in treatment plans. Conclusion Dosimetric validation is based on calculated and measured doses distributions comparison, thanks to tolerances (generally 3%/3 mm). This study allows us to understand the impact of potential MLC positioning errors on planned treatment and to discuss about the validity of quality assurance tolerance values.