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Study of Uncertainty of EBT3 Film as a Dosimeter for Proton Beam Measurements

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V Ugarte

V Ugarte*, N Wang , A Ghebremedhim , B Patyal , Loma Linda Univ Medical Center, Loma Linda, CA


SU-I-GPD-T-139 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To study the uncertainty of EBT3 film as a dosimeter for 126.8MeV proton SRS beam using Vidar scanner and RIT 113 software application. The variation of the film calibrations and dose response affect the accuracy of dose measurements.

Methods: EBT3 films were calibrated using mod wheel #30 and 30mm diameter aperture in solid water phantom. The films were placed at center of modulation (COM) at the gantry isocenter at 8.2cm depth. They were exposed by 126.8 MeV proton beam with dose in the range of 15 cGy to 600 cGy and scanned 48 hours after exposure. Film calibration curves were generated for three scanner–bed positions: left, center, right. Nine pieces of film were placed at depths from 3cm to11cm in a polystyrene phantom. The phantom was aligned at 8.2cm WEQ depth at the gantry isocenter at COM. Films were irradiated simultaneously by a 126.8MeV beam with 500cGy and scanned at the three scanner-bed locations: left, center, right. Dose from each film was obtained by using the three corresponding calibration curves and compared with the delivered dose.

Results: The measured dose based on three calibration curves varied up to 3.0% in the spread out of Bragg Peak (SOBP). The dose measured by film versus the delivered dose in SOBP differed by a maximum of 7.2%. EBT3 film dose response displayed proton energy dependence along the SOBP. The smallest dose discrepancy was achieved at COM with the EBT3 film scanned at/along the center of scanner-bed.

Conclusion: Response of EBT3 film in a proton beam is dependent on position of the film in the SOBT and also on its location in the scanner-bed during scanning. The best agreement between the measured and the delivered dose is when film is at COM and it is scanned/calibrated near center of Vidar scanner-bed.

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