A stochastic patient movement model and a dosage delivery plan are used to determine probabilities of dosages received by a target volume during a radiation treatment. The stochastic patient movement model is created by identifying possible sequences of patient positions during the treatment with probabilities specified for each. The dosage delivery plan specifies dosage levels and radiation locations over time. Accumulated dosages are calculated for each sequence of patient positions and the dosage delivery plan. The specified probability for each sequence is then correlated to the accumulated dosage for that sequence.
