Author: Jack Biondo, Audrey Magness, Anthony E. Mascia, Zhiyan Xiao, Yongbin Zhang 👨🔬
Affiliation: Cincinnati Children's Proton Therapy Center 🌍
Purpose: Proton FLASH-RT has shown promise reducing normal tissue toxicity while maintaining tumor control. However, in vivo experiments critically depend on achieving ultra-high dose rate (UHDR) and uniform dose. This study investigates the impact of varying spot spacing on dose rate and uniformity for small animal in vivo FLASH irradiation research.
Methods: Irradiation was performed using a FLASH-enabled proton pencil beam scanning (PBS) system (250MeV) with a 36x36 mm field at Gantry 0. Four spot spacings (2 mm, 4 mm, 6 mm and 9 mm) were tested for two dose levels (10 Gy and 20 Gy). Dose verification was performed using an Advanced Markus ion chamber. Beam current was adjusted to characterize maximum and minimum dose rates. Field and voxel dose rates were reported using beam on time and a script analyzing delivery log files, respectively. Beam uniformity was assessed through analysis of radiochromic film, with field flatness evaluated for each spacing.
Results: Analysis shows that spot spacing affects both dose rate and field uniformity. For 10Gy, the maximum voxel dose rate was 119.5 Gy/s with 9mm spacing, and remained high at 116Gy/s and 105.7Gy/s for 6mm and 4mm spacing, respectively. However, it dropped drastically to 44.5 Gy/s with 2mm spacing. For 20Gy, the maximum dose rate was 115.4 Gy/s with 9mm spacing, stayed high for 6mm and 4mm, but declined to 89.7 Gy/s with 2mm. Field uniformity analysis showed that 9 mm spacing exhibited the largest variation in flatness. Notably, 6 mm spacing demonstrated the most consistent performance in flatness.
Conclusion: This study evaluates the impact of spot spacing on FLASH proton therapy dose rates and field uniformity. These findings are valuable to optimize beam delivery parameters for future preclinical studies.