Session Item

Aim and introduction

Margin recipes to deal with treatment uncertainties are commonly used in radiotherapy. The common margin recipes are based on the classic work of van Herk et al. and build upon a theoretical framework, assuming an idealized dose distribution, perturbed under random and systematic normally distributed errors.

Modern radiotherapy aims to increase delivery precision by different means, including online motion management and online plan adaptation. Not only does this shrink the width of the underlying error distribution, it also changes some of the assumptions underlying the margin recipes. The aim of this work is to present a theoretical framework to discuss treatment margins for these developments.

Methods

Motion management for intrafraction drift effectively introduces a cut-off in the error distribution, changing it to a truncated Gaussian. Assuming a cut-off at c   we study the dependency of the margin on the cut-off parameter for motion modelled as a discrete ‘jump’ and a continuous drift. We study the relative change in the margin for different number of fractions and underlying error distributions.

In online adaptive radiotherapy a new plan is generated every fraction based on the delineations of that fraction. In such a workflow, the margin does not need to be constant over the whole course of treatment. Using adaptive margins, the dose already delivered to the target can be taken into account allowing for margin reductions when treatment progresses smoothly and increased margins when unexpected motion during delivery has occurred. We introduce a methodology to calculate adaptive margins based on the accumulated dose to the CTV and study its benefit.

Results

We found that the relative change due to motion management in the margin m_c/m is equal to 0.3c, independently of the standard deviation, number of fractions and for both motion patterns (see figure 1A).

Online adaptation of the margins allows for the margins to be on average over the treatment down to 65% of the corresponding constant margins. Most of the benefit can be ascribed to the elimination of the effective systematic error over the course of treatment and therefore the benefit is larger when the number of fractions increases (see figure 1B)

Conclusion

Based on the results presented, the required treatment margins can be determined when motion management strategies or online adaptation are applied. Our analysis can be used to study the potential benefit of different strategies for different treatment sites. This allows clinicians to choose the most appropriate strategy for margin reduction for modern radiotherapy.