Main Session
Sep
29
PQA 05 - Physics
3063 - Dose Rate and Microdosimetric Effects on Radiochromic Film Response to Synchrotron-Based Proton Irradiation
Presenter(s)
Daniel Liu, BS - Yale University, New Haven, CT
D. Z. Liu1, J. Tang2, J. Mathew3, D. Ellison4, S. Cameplo4, X. Wang5, Y. Zlateva6, T. M. Briere4, J. M. Pollard4, L. Bronk7, P. A. Taylor8, Y. Li6, and F. Guan6; 1Yale University, New Haven, CT, 2University of Texas Austin, Austin, TX, 3University of Houston, Houston, TX, 4MD Anderson Cancer Center, Houston, TX, 5The University of Texas MD Anderson Cancer Center, Houston, TX, 6Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, 7Department of Radiation Physics, Department of Experimental Radiation Oncology, Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, 8Imaging and Radiation Oncology Core, Houston, TX
Purpose/Objective(s):
To characterize dose rate and microdosimetric dependencies of radiochromic film response in synchrotron-based proton beams and identify the threshold for dose rate effects in ultra-high dose rate (UHDR) regimes, establishing dosimetry guidelines for emerging UHDR therapy applications.Materials/Methods:
Radiochromic films were irradiated using 87.2 MeV synchrotron-based proton beams. Films were positioned at beam entrance and Bragg peak locations with doses ranging from 0.5 to 15 Gy. Three dose rate categories were evaluated: moderate (< 5 Gy/s), FLASH (100-1,000 Gy/s), and lightning (our term for =1000 Gy/s). Microdosimetric effects were characterized by comparing film response across different radiation qualities at moderate dose rates: 6 MV photons (dose-mean lineal energy yD = 0.8 keV/µm), proton entrance (yD = 1.8 keV/µm), proton proximal region (yD = 4.3 keV/µm), and proton Bragg peak (yD =10.2 keV/µm). Microdosimetry quantities were calculated using Geant4 Monte Carlo simulations. Films were scanned 48 hours post-irradiation and analyzed using ImageJ. Dose-response curves were fitted using four-parameter Rodbard function.Results:
Film response demonstrated dose rate independence between moderate and FLASH regimes at both entrance and Bragg peak (|?OD| < 2%). However, lightning dose rates (=1,000 Gy/s) produced notable under-response: 4.6% reduction at entrance (1,000 Gy/s) and 11.3% reduction at Bragg peak (4,500 Gy/s) at 10 Gy. Microdosimetric analysis at moderate dose rates revealed systematic quenching with increasing lineal energy: reductions of 4.1%, 7.3%, and 9.8% at proton entrance, proximal, and Bragg peak positions respectively relative to 6 MV photons at 10 Gy. Both temporal (dose rate =1,000 Gy/s) and spatial (high lineal energy) ionization density patterns produced comparable film quenching effects (under response at the same absorbed dose).Conclusion:
We identified 1,000 Gy/s as the critical threshold defining the lightning proton therapy regime where radiochromic film transitions from dose rate independent to dose rate dependent response. The parallel effects of ultra-high dose rate (temporal ionization density) and microdosimetry (spatial ionization density) demonstrate that both factors cause film under-response. These findings have critical implications for dosimetry protocols in UHDR proton therapy, requiring dose rate-specific and lineal energy-specific calibration factors above 1,000 Gy/s for accurate film dosimetry.| Microdosimetry effect, relative to 6 MV photons | yD (keV/µm) | ?OD (%) @ 2 Gy | ?OD (%) @ 5 Gy | ?OD (%) @ 10 Gy |
| Entrance | 1.8 | -7.2 | -5.1 | -4.1 |
| Proximal | 4.3 | -10.7 | -9.4 | -7.3 |
| Bragg peak | 10.2 | -17.3 | -13.2 | -9.8 |
| Dose rate effect at entrance, relative to 1.1 Gy/s | Dose rate (Gy/s) | ?OD (%) @ 2 Gy | ?OD (%) @ 5 Gy | ?OD (%) @ 10 Gy |
| Flash | 150 | 1.9 | 1.7 | 0.9 |
| Lightning | 1,000 | -6.3 | -3.5 | -4.6 |
| Dose rate effect at Bragg peak, relative to 4.6 Gy/s | Dose rate (Gy/s) | ?OD (%) @ 2 Gy | ?OD (%) @ 5 Gy | ?OD (%) @ 10 Gy |
| Flash | 600 | -1.8 | -0.2 | -0.3 |
| Lightning | 4,500 | -18.9 | -14.9 | -11.3 |