Room: AAPM ePoster Library
Purpose: To evaluate the properties and reproducibility of FLASH fields delivered by prototype proton pencil beam scanning (PBS) hardware.
Methods: Two sets of four rectangular 250 MeV spot patterns (fields) were created in a DICOM file. Each field contained 13 spots along the primary scanning axis and 14, 18, 22, or 26 spots along the orthogonal axis. All spots were spaced 5.0 mm apart to generate a homogeneous dose distribution. The primary axis for the first and second sets was along the Y and X direction, respectively. Prototype high dose rate hardware was installed and commissioned in a proton research room. The spot MUs were set such that the biological dose (RBE=1.1) was 8.4 GyRBE at isocenter (5 cm depth in HDPE). Each field was measured with film under 5 cm depth in HDPE. A logfile analysis tool was developed to compute PBS dose rate for many points on the isocenter plane within the 7.6 GyRBE isodose line and reported the 10th percentile value. The PBS dose rate is defined as the dose delivered to a voxel divided by the irradiation time experienced by that voxel (elapsed real time). Repeated irradiations were conducted for each field, ion chamber measurements and logfile analysis results recorded.
Results: Dose homogeneity for all fields at 5 cm depth in HDPE was better than ±2.5%. The size of the 7.6 GyRBE isodose line at 5 cm depth in HDPE was 5.6 cm by 6.1, 8.0, 10.0, and 12.1 cm regardless of orientation. The average dose and dose rate for 10 irradiations of the largest fields was 8.5 GyRBE ±2.06% and 67.4 GyRBE/s ±1.5% respectively.
Conclusion: We successfully implemented a qualification and reproducibility procedure to verify that prototype hardware was able to consistently deliver FLASH fields with dose and dose rate fluctuations of less than 3%.
Funding Support, Disclosures, and Conflict of Interest: All authors are full-time employees of Varian Medical Systems, Inc., Palo Alto, CA.