MO‐D‐T‐6E‐04: On the Dose‐Volume Constraints Based On Radiobiological Considerations

Purpose: To apply radiobiological principles in the estimation of dose‐volume constraints used in treatment planning. To evaluate NTCP distributions affiliated with certain multiple dose‐volume constraints. To estimate the probability PmD‐V that a dose‐volume histogram resulting in a given NTCP level also satisfies certain multiple dose‐volume constraints. Method and Materials: The reverse NTCP mapping method 1 is used here to obtain physical dose‐volume constraints based on radiobiological indices. A procedure for random integral DVH sampling from the space of monotonously decreasing functions is developed. DVHs are randomly simulated and the ones producing an NTCP ε {5 ± 0.5%} are selected. An average DVH is produced from the selected DVHs. We propose that any point from the averaged DVH may serve as a physical dose‐volume constraint. A Monte‐Carlo method is used to estimate the probability PmD‐V for a number of these constraints. Results: Dose volume constraints for 16 organs selected based on the availability of parameter estimates for the Lyman and the Critical Volume NTCP models 2,3 are obtained. The Emami 4 constraints lay on the “upper boundary” of the DVH sub‐space defined by the condition NTCP=5±0.5%. The calculated probabilities PmD‐V are very low, indicating that the physical optimization uses a much smaller subspace of the possible solutions than the biological or the physico‐biological optimization. Conclusion: New dose‐volume constraints based on radiobiological considerations are proposed. DVHs passing through a combination of constraints 1 are outside the range of the DVHs producing NTCP=5±0.5%. The physical RT optimization is more restricted in its choice of solutions than the biological one.