ESTRO 2022 - Abstract Book

S242

Abstract book

ESTRO 2022

Conclusion A detector system for direct measurements of the instantaneous dose rate was developed and calibrated in a wide range of dose rates. The detector system was successfully applied in vivo in mouse irradiations, enabling time-resolved dose rate measurements in pre-clinical proton FLASH studies on a sub-spot level.

OC-0282 Improved dose rate by scanning-pattern optimization in FLASH proton therapy of small lung lesions

R. José Santo 1 , S. Habraken 1,2 , S. Breedveld 1 , M. Hoogeman 1,2

1 Erasmus University Medical Center, Radiotherapy, Rotterdam, The Netherlands; 2 HollandPTC, Medical Physics & Informatics, Delft, The Netherlands Purpose or Objective FLASH dose rates >40Gy/s are readily available in proton therapy (PT) with cyclotron-accelerated beams and pencil-beam scanning (PBS). The overal highest is achieved with protons at maximum energy shooting through the patient (transmission beam, no Bragg peak). The local time structure of the PBS, quantified in terms of PBS dose rate (PBS-DR), is critical for FLASH. However, methods to optimize it are lacking. Our aim was to optimize patient-specific scanning patterns in stereotactic FLASH-PT of small lung lesions, maximizing the percentage irradiated with a PBS-DR>40 Gy/s of the OAR voxels irradiated to >8Gy (FLASH coverage). Materials and Methods Plans to 54Gy/3 fractions with 3 equiangular coplanar 244MeV proton shoot-through beams for 20 patients were optimized in in-house developed software. PTV-based planning with a 5mm margin was used with a median PTV of 8.7cc (range: 4.4- 84 cc). Beams avoided serial OARs and, for each direction, the shortest path length from beam entrance to PTV was used. To enable FLASH-enhanced single-beam per fraction delivery, a single-field uniform dose approach was used. Sequential scanning pattern optimization was performed with a Genetic Algorithm to optimize the PBS-DR, run in parallel for 20 independent populations (islands). Mapped crossover, inversion, swap, and shift operators were applied to achieve (local) optimality on each island. Migration between islands was implemented to approach global optimality. The cost function was chosen to maximize the FLASH coverage at >8 Gy, >40Gy/s. The PBS-DR was calculated for a beam current of 40 nA. The optimized scanning patterns were evaluated in terms of the PBS-DR distribution and population PBS-DR-volume histograms, compared to conventional line by line scanning (snake pattern). Results Optimized patterns for various PTVs are shown in figure 1. They have a snowflake-like structure with adjacent pencil beams being irradiated consecutively where possible. For larger PTVs the scanning pattern and the high-PBS-DR volume develop a swirl-like shape.