Radiochromic Film Dosimetry for QA, In-Vivo Dosimetry, and Range Assessment In Pbs Proton Therapy
Abstract
Purpose
To commission and clinically implement a robust radiochromic film dosimetry workflow for PBS proton therapy, providing a practical solution for small-field patient-specific QA, in-vivo dose verification, and WET-based range assessment when dedicated in-vivo and range dosimetry systems are unavailable.
Methods
Six PBS wedge calibration plans were generated, three with a range shifter (RS) and three without (non-RS), each containing three dose levels spanning 0.5–20 Gy. Calibration doses were measured using a MatriXX ion-chamber array at 5 cm depth (non-RS) and 3 cm and 10 cm depths (RS). EBT3 and EBT-XD films were irradiated at the same depths, and 10-point PV-to-dose calibration curves were generated for RS and non-RS conditions. Films were scanned 18–24 hours post-irradiation using ROI analysis. Patient QA was performed in solid water, a dosimetry cube, and a CIRS lung phantom using absolute-gamma analysis. Film was also used for surface dose measurements and SPR assessment using 11 CIRS-062M density plugs benchmarked to water-tank ion-chamber scanning measurements.
Results
Due to LET under-response, film underestimated maximum absolute dose in the open-field pristine Bragg peak by up to 23%; however, R90 for open fields agreed with reference values within 0.8 cm, with discrepancies of 0.14–0.76 mm across energies from 70–220 MeV. Film-derived SPR showed an average absolute difference of ~0.04–0.05 for lung and soft-tissue plugs, with underestimation for high-density bone reaching −0.12 for Bone-1750. Patient-specific QA pass rates ranged from 92.7–100% (3%/3 mm) and 89.7–100% (5%/2 mm). Phantom surface doses (89–154 cGy) differed from TPS by 4.1% on average (maximum 11.5%), while in-vivo surface doses (149–200 cGy) differed by 0.97% on average (maximum 5.5%).
Conclusion
With calibration and awareness of LET response, radiochromic film provides a practical method for patient-specific QA, surface dosimetry, and SPR-based range assessment in PBS proton therapy.