Dose painting by numbers - do the practical limitations of the technique decrease or increase the probability of controlling tumours?

Abstract

One of the important questions regarding the feasibility of dose-painting-by-numbers approaches for treatment planning concerns the influence of the averaging of the imaging techniques used and the resolution of the planned and achieved dose distributions. This study investigates the impact of these aspects on the probability of controlling dynamic tumours. The effectiveness of dose painting approaches to target tumour hypoxia has been investigated in terms of the predicted tumour control probabilities (TCP) for tumours with dynamic oxygenations. Several levels of resolution for the resistance of the tumour or the planned dose distributions have been investigated. A very fine heterogeneous dose distribution ideally calculated at voxel level for a high target TCP would fail to control a tumour with dynamic oxygenation during the course of fractionated radiotherapy as mismatches between hotspots in the dose distribution and resistant hypoxic foci would lead to a significant loss in TCP. Only adaptive treatment would lead to reasonably high TCP. A coarse resolution for imaging or for dose distributions might compensate micro- scale mismatches in dynamic tumours, but the resulting tumour control could still be below the target levels. These results indicate that there is a complex relationship between the resolution of the dose-painting-by-numbers approaches and the dynamics of tumour oxygenation. Furthermore, the clinical success of hypoxia targeting strategies in the absence of adaptive approaches might be explained by changes in tumour radiation resistance through reoxygenation.