ESTRO 2024 - Abstract Book

S3507

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

For 19 OARs at least 7 centers had Dmax data. On average, the relative interquartile range (IQR) for reRT Dmax EQD2 was 21%, which mainly reflects the uncertainty in these values due to limited (confidence in) clinical data, but also include variations in Dmax definitions, PRV margins, alpha/beta, and treatment techniques. Differences varied from 8% (spinal cord) to 34% (heart). Average median PRV margins used were 1.1mm (range 0 to 3.5mm). Comparing reRT constraints with first-course constraints yielded on average a 37% difference, which includes not only a recovery-factor but also a possible reRT-factor; even without recovery, constraints can deliberately be increased considering changes in patient status (e.g. allowing more toxicity risk) and/or in applied RT-techniques. Relative differences changed from 16% (optic pathway) to 69% (bladder). Since recovery (range 20-50%) and minimum recovery interval (6-36m) were available for <50% of OARs, they were not reported further.

Conclusion:

In the participating centers, on average 7% of treatments are re-irradiations and this number is expected to grow. The average 21% inter-center variation (IQR) in reRT Dmax constraints found in this study indicates that dose constraints are a major uncertainty in re-irradiations. Although using Dmax alone is questionable for several