Author: Julien Forthomme, Carolina Llina Fuentes, Valentin Hamaide, Lucian Hotoiu, Rudi Labarbe, Rasmus Nilsson, Arnaud Pin, Francois Vander Stappen, Erik Traneus 👨🔬
Affiliation: RaySearch Laboratories AB, Ion Beam Applications SA 🌍
Purpose: Conformal FLASH achieves ultra-high-dose-rate (UHDR) proton therapy (>40 Gy/sec) by scanning a mono-energetic pencil beam through accessories (range shifter, Conformal Energy Modulator, aperture) for field sizes up to 6x8cm2 in the beam’s eye view (BEV). This study investigates the challenge of optimizing FLASH therapy for larger target volumes that partially overlap with healthy tissues.
Methods: Two planning strategies are proposed for two representative target configurations which were modeled in water phantoms: (1) the Multimodal Combination (MC), where FLASH plans are first optimized on FLASH targets (where the target volume overlaps organs at risk) and then combined with a standard IMPT plan on the rest of the target volume; (2) the Adjoint Field Optimization (AFO) consists of FLASH plans generated side by side over the complete target volume achieving UHDR in the FLASH targets. In both strategies, FLASH plans use a UHDR machine model and the MU/spot range and lateral spot positioning are manually adjusted to maximize the 95% percentile dose-rates while maintaining dose quality.
Results: With a homogeneity index above 93%, AFO FLASH plans successfully achieved UHDR on small sub-volumes (~3.5x8cm2 in BEV), requiring a ~2.5 cm overlapping area without UHDR at the junction. The UHDR part of the MC plan successfully achieved a PBS dose-rates >40 Gy/sec over 98% of the FLASH target volume (up to 220cm3) without any impact from the IMPT contribution. Uniformity and homogeneity indexes are 95% and 96% over the entire target volume.
Conclusion: Large treatment volumes require the definition of Flash targets. The Multimodal Combination secures the UHDR in Flash targets up to 6 cm wide and more than 3 cm apart so that the non-Flash target accommodates the complementary IMPT plan. The AFO technique can address larger or closer Flash targets, with a dose-rate drop at the 2-3 cm junction.