Confidence intervals in dose volume histogram computation

Abstract

Dose volume histograms (DVHs) are used in radiation therapy plan optimization and evaluation. Irradiation strategies are decided at the planning step, and an assessment of the reliability of computed dose distributions and DVHs is needed to ensure that decisions are made based on reliable information. This work describes a method used to assign confidence intervals to DVHs, caused by the uncertainty associated with dose computation. A simple mathematical model has been developed, based on several alternative models for point dose uncertainty (rectangular, triangular, and Gaussian probability distributions). The result is an alpha-DVH, to be computed for two a values (0.25/0.75 or 0.10/0.90) to obtain a confidence interval. This method has been applied to several DVH curves. alpha-DVHs are dependent on the point uncertainty probability distribution, on the standard uncertainty, and on the choice of confidence level. Confidence intervals can be assigned either to the volume encompassed by an isodose level or to the dose corresponding to a given volume. alpha-DVHs provide bounds for DVH values while dose expected volume histograms can be used as central estimates for DVH when uncertainty is taken into account. They are particularly useful when an optimization approach is necessary.