Evaluation of absorbed dose uncertainty in modulated radiotherapy plans

Abstract

The purpose of this work was to develop and evaluate methods to meet the challenges of quality control (QC) for modulated radiotherapy plans. It was shown that nine of 15 intensity modulated radiotherapy (IMRT) plans, with deliberately introduced dose errors larger than 5% in at least one evaluated dose volume histogram (DVH) metric, were not detected with a QC method which combined Delta4 (ScandiDos) measurements and internationally recommended criteria for evaluation (Paper I). The dose difference between calculation and high spatial resolution measurements, using EBT3 film and electronic portal imaging device (EPID), for 30 static beam apertures of varying size and shape was used as a measure of beam aperture complexity (Paper II). The linear correlation to the beam aperture complexity was evaluated for three aperture-based complexity metrics developed in this study and five other metrics suggested in the literature. The strongest correlation, with a Pearson’s r-value of -0.94, was found for the developed edge area metric (EAM). EAM was further evaluated for 18 static beam openings originating from control points of clinically used volumetric modulated arc therapy (VMAT) plans and for 200 full VMAT plans planned for different treatment sites (Paper III). The results indicated that the EAM must be interpreted differently for different diagnoses. Evaluation of beam aperture shape, modulation variations, measurements, and delivery simulations, as methods for assessment of the dosimetric uncertainty for VMAT plans, showed that the dosimetric uncertainty could differ even though the plans appeared to be equal based solely on dosimetric comparisons of the dose distributions, e.g., DVH metric evaluations (Paper IV). In conclusion, it is possible to decrease dosimetric uncertainties in modulated radiotherapy plans to enable a higher treatment quality. The dosimetric uncertainties can be assessed by different methods but it is important to define the purpose of the method, and to validate that the method fulfils the defined purpose.