  Dose volume histograms are a useful tool in state-of-the-art radiotherapy
planning, and it is essential to be aware of their limitations. Dose
distributions computed by treatment planning systems are affected by several
sources of uncertainty such as algorithm limitations, measurement uncertainty
in the data used to model the beam and residual differences between measured
and computed dose, once the model is optimized. In order to take into account
the effect of uncertainty, a probabilistic approach is proposed and a new kind
of histogram, a dose-expected volume histogram, is introduced. The expected
value of the volume in the region of interest receiving an absorbed dose equal
or greater than a certain value is found using the probability distribution of
the dose at each point. A rectangular probability distribution is assumed for
this point dose, and a relationship is given for practical computations. This
method is applied to a set of dose volume histograms for different regions of
interest for 6 brain patients, 8 lung patients, 8 pelvis patients and 6
prostate patients planned for IMRT. These results show how dose computation
uncertainty has effects on PTV coverage and, to a lesser extent, in dose to
organs at risk. This method allows to quantify these effects.
