ESTRO 2024 - Abstract Book

S3574

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

ESTRO 2024

Conclusion:

In this treatment planning simulation we investigated if MRL boost might be of value in case BT dose planning aims cannot be met. Overall we found that for two patients target dose levels could be improved, for two the target aims were equally achieved, and for two patients the dose levels of the BT plans were better given that OAR hard dose constrains had to be respected. The presented MRL boost plans were simulated on one MRI scan acquired at the first BT. To maintain equipoise, we simulated the BT dose calculations also only on the anatomical situation of the first application. In clinical routine, however, repeated BT applications can help improving dose distributions in case of a challenging anatomy especially due to availability of different types of applicators and the possibility to insert additional interstitial needles. Whereas for MRL, online adaption is required to act on the changing anatomy of the day. For MRL treatments a PTV margin was used to mitigate intrafraction motion, while for BT no PTV margin was used as the applicator is fixed to the patient, hence no relative intrafraction motion between applicator and target is expected. More sophisticated MRL motion monitoring techniques (e.g. drift correction) that are currently developed are anticipated to allow for a PTV margin reduction which will impact achievable target dose levels. Not surprisingly, the underlying dose distributions of BT and MRL plans differ. High dose volumes are larger in the BT plans whereas low dose volumes (i.e. V50Gy V60Gy) are larger in the MRL plans. Although no formal dose constraints are used for those dose levels, it is probable that this impacts tumor control and toxicity. Therefore, caution is required when using MRL boost as an alternative for BT and the outcome and toxicity of MRL boost technique should ideally be monitored (e.g. see MARGARITA).