An a priori dose uncertainty model and its clinical applications

The overall dose uncertainty in step-and-shoot intensity-modulated radiation therapy (IMRT) arises from a complex interplay of uncertainties associated with each subfield. To provide accurate prediction of uncertainty distribution during IMRT planning, a novel dose uncertainty model was introduced using statistically quantified uncertainty parameters of IMRT planning and delivery. Clinical applications of the model for IMRT were also developed and their efficacy was investigated.;An analytic form of overall dose uncertainty in IMRT was given by Ioverall + Z · sigmaoverall with a confidence level (Z). The dose uncertainties in IMRT were categorized into space-oriented dose uncertainty (SOU) and non-space-oriented dose uncertainty (NOU). The model further divided the uncertainty sources into planning and delivery. Both SOU and NOU of planning were defined as inherent dose uncertainty (IU; Ioverall). It was assumed to arise from three distinct sources: discrete calculation grid size (Igrid), inaccuracy of calculation algorithm (Ialgo), and possible asymmetric beam delivery (moa). Both SOU and NOU associated with radiation delivery were employed to describe a statistical dose uncertainty (sigmaoverall). Various model parameters for the uncertainty sources were quantified through measurements, accumulated routine quality assurance (QA) data, and peer-reviewed publications.;To examine the applicability of the model to IMRT dose verification, dose uncertainty maps were compared with dose difference distributions between calculation and measurement for 32 clinical IMRT fields and one composite dose distribution. In all QA measurements, most of the dose difference points (more than 96%) were confined within the uncertainty bound of Ioverall + 2 · sigmaoverall as statistically predicted. In addition, a conventional gamma dose verification test was done for all QA measurements. The failed regions of the gamma test remarkably overlaid on regions of high dose uncertainty. It shows that the dose uncertainty map plays an important role as a space-dependent acceptance level of dose verification.;Uncertainty-based IMRT plan evaluation tools such as confidence-weighted dose volume histograms (CW-DVH), confidence-weighted dose distributions (CWDD), and dose uncertainty volume histograms (DUVH) were developed to assess the potential risks of treatment plans. CW-DVH provided an overall inspection of the potential risk of the plans with quantitative evaluation indices. CWDD was an essential method to assess a local risk of plans through slice-by-slice examination. A plot of a cumulative dose uncertainty-volume frequency distribution (DUVH) visually summarizes the uncertainty distribution within a volume of interest. These evaluation tools differentiated candidate treatment plans in terms of uncertainty, which were clinically comparable by the conventional plan evaluation methods.;This study is highly significant in that it provides a framework to minimize the impact of all known uncertainties in the IMRT process and improve the accuracy of dose delivered to patients. The proposed uncertainty model is expected to radically change how dose uncertainty is assessed and controlled. It will significantly contribute to improving the quality and reliability of radiotherapy, resulting in the most accurate dose delivery to patients.