Purpose: To explore a comprehensive patient-specific QA (psQA) program for ultra-high dose rate spot scanning proton transmission treatment using a high spatiotemporal resolution strip ionization chamber array (SICA).
Methods: The SICA is newly designed open-air strip-segmented parallel plate ionization chamber, which is capable of measuring scanning spot profile and position through 2mm-spacing-strip electrodes at a 20kHz sampling rate (50 µs per event) and has been characterized to exhibit excellent linearity under FLASH conditions. A SICA-based delivery log was generated for each irradiation, containing the measured position, size, dwell time, and delivered MU for each prescribed spot, and compared with the corresponding quantities in the plan. The measured 2D dose map in water at SICA’s intrinsic build-up depth was compared with the treatment planning system (TPS) prediction. Furthermore, the dose and dose rate distributions were reconstructed based on SICA delivery log on patient CT and compared to the TPS prediction using volume histograms and 3D Gamma analysis.
Results: A proton plan of a single transmission field of 250 MeV for femur bone metastasis was measured in a dedicated research beamline of Varian ProBeam system. The Gamma (3% and 3mm) passing rates for 2D dose map in water and 3D dose distribution in patient CT were 99.6% and 95.9%, respectively. The deviations between SICA deliver log from plan were 1.3% for total delivery time (372ms), 3.0 ± 2.7 % for spot MU, 0.06 ± 0.04 mm and 0.12 ± 0.09mm for X/Y spot position. Dose and dose rate volume histogram showed the minimum difference.
Conclusion: This work is the first time to describe a comprehensive psQA program based on SICA to verify the time structure and corresponding dose rate of FLASH delivery, along with quality assurance of data transfer integrity, delivery accuracy and the dose calculation accuracy.
Not Applicable / None Entered.