Analytical probabilistic modeling of dose-volume histograms.

Radiotherapy is sensitive to executional and preparational uncertainties that propagate to uncertainty in dose and plan quality indicators like dose-volume histograms (DVHs). Current approaches to quantify and mitigate such uncertainties rely on explicitly computed error scenarios and are thus subject to statistical uncertainty and limitations regarding the underlying uncertainty model. Here we present an alternative, analytical method to approximate moments of the probability distribution of DVH-points and evaluate its accuracy on patient data. We use analytical probabilistic modeling (APM) to derive those moments for individual DVH-points based on the probability distribution over dose. Further we use the computed moments to parameterize distinct probability distributions over DVH-points (here normal or beta distributions) to compute percentiles/$\alpha$-DVHs. The model is then evaluated on three patient cases in 30- and single-fraction scenarios by assuming the dose to follow a multivariate normal distribution obtained from APM. The results are compared to a sampling benchmark. The evaluation of the new probabilistic model against the sampling benchmark proves its correctness under perfect assumptions as well as good agreement in realistic conditions. Ca. 90% of all computed expected DVH-points and their standard deviations agree within 1% volume with their empirical counterpart from sampling, for both fractionated and single fraction treatments. $\alpha$-DVHs achieve better agreement with empirical percentiles when assuming a beta instead of a normal distribution: While in both cases probabilities show large local deviations (up to $\pm$0.2), the respective $\alpha$-DVH only showed small deviations (up to $\pm$5% volume for a normal, and up to 2% for a beta distribution). A previously published model by different authors yielded substantially deviating $\alpha$-DVHs.