Background and Aims

To exploit the high conformality of IMPT plans for FLASH, new approaches are required, because the multi-energy-layer deliveries reduce the dose rate substantially. The use of 3D range modulators (3DRMs), which combines the dose-advantages of IMPT with the dose-rate-advantages of transmission proton plans, is a promising modality, referred to as FLASH IMPT.

Our aim was to create a complete workflow for FLASH IMPT treatment planning and dose rate modelling using modified clinical tools and for generating corresponding 3DRM designs, which was validated experimentally. Here, we focus on treatment planning.

Methods

A FLASH-specific research version of Eclipse (FLEX TPS) was modified to optimize dose distributions for 3DRMs. A FLASH IMPT plan was created to cover a complex target shape (field size: 84x84mm²) and a corresponding 3DRM design was saved as a ready-to-print STL file. The dose distribution for each beamlet was combined with the delivery timing model to estimate the 3D PBS dose rate¹ for a target dose of 15Gy. The spot MU list and the 3DRM design were used to simulate a Monte Carlo (MC) dose distribution. In addition, the RM was 3D-printed and the resulting measured 3D dose distribution was compared with the simulation. The full workflow is depicted in Figure 1a).

Results

The target geometry and beam arrangement are shown in Figure 1a) (top). Figure 1b) shows a good agreement between the target dose distributions modelled by Eclipse and by MC. The modelled dose rate distribution is shown in Figure 2a), illustrating an average dose rate of 45Gy/s (50% iso-dose minus target). The differences between the measured and the MC dose for the example slice, shown in Figure 2b), stay within +-3%.

Conclusions

We successfully created a FLASH IMPT treatment plan and modelled the dose rate for a complex target shape, showing good agreement with the corresponding measurement.

¹https://doi.org/10.1002/mp.14456