Abstract
BackgroundThe purpose of this study is to comprehensively evaluate the suitability of Gafchromic EBT3 and EBT-XD film for dosimetric quality assurance in 0.35 T MR-guided radiotherapy.MethodsA 0.35 T magnetic field strength was utilized to evaluate magnetic field effects on EBT3 and EBT-XD Gafchromic films by studying the effect of film exposure time within the magnetic field using two timing sequences and film not exposed to MR, the effect of magnetic field exposure on the crystalline structure of the film, and the effect of orientation of the film with respect to the bore within the magnetic field. The orientation of the monomer crystal was qualitatively evaluated using scanning electron microscopy (SEM) compared to unirradiated film. Additionally, dosimetric impact was evaluated through measurements of a series of open field irradiations (0.83 × 0.83-cm2 to 19.92 × 19.92-cm2) and patient specific quality assurance measurements. Open fields were compared to planned dose and an independent dosimeter. Film dosimetry was applied to twenty conventional and twenty stereotactic body radiotherapy (SBRT) patient specific quality assurance cases.ResultsNo visual changes in crystal orientation were observed in any evaluated SEM images nor were any optical density differences observed between films irradiated inside or outside the magnetic field for both EBT3 and EBT-XD film. At small field sizes, the average difference along dose profiles measured in film compared to the same points measured using an independent dosimeter and to predicted treatment planning system values was 1.23% and 1.56%, respectively. For large field sizes, the average differences were 1.91% and 1.21%, respectively. In open field tests, the average gamma pass rates were 99.8% and 97.2%, for 3%/3 mm and 3%/1 mm, respectively. The median (interquartile range) 3%/3 mm gamma pass rates in conventional QA cases were 98.4% (96.3 to 99.2%), and 3%/1 mm in SBRT QA cases were 95.8% (95.0 to 97.3%).ConclusionsMR exposure at 0.35 T had negligible effects on EBT3 and EBT-XD Gafchromic film. Dosimetric film results were comparable to planned dose, ion chamber and diode measurements.
Highlights
Image guided radiation therapy (IGRT) utilizes ontable imaging of the patient for inter- and intra-treatment patient verification, which has enabled greaterXhaferllari et al Radiat Oncol (2021) 16:117[2]
Gafchromic EBT3 and EBT-XD film were introduced as an improvement over EBT2 film by utilizing a symmetric configuration that leads to improved film handling and to the elimination of film side orientation dependence
Magnetic field exposure time effects on Gafchromic film NetOD-to-Dose calibration curves were generated from the grid points measured on the Gafchromic EBT3 and XD films irradiated using the two exposure timing sequences
Summary
Image guided radiation therapy (IGRT) utilizes ontable imaging of the patient for inter- and intra-treatment patient verification, which has enabled greaterXhaferllari et al Radiat Oncol (2021) 16:117[2]. The active layer is composed of polymer crystals which undergo a topochemical polymerization reaction after exposure to ionizing radiation. This process causes a coloration of the film from the absorption of energy. Recent studies have demonstrated significant effects of the magnetic field on the performance of Gafchromic EBT2 film in both 0.35 T and 1.5 T environments that have resulted in under responses of up to 15% [8, 9]. Gafchromic EBT3 and EBT-XD film were introduced as an improvement over EBT2 film by utilizing a symmetric configuration that leads to improved film handling and to the elimination of film side orientation dependence Both EBT3 and EBT-XD films have demonstrated excellent relative and absolute dosimetric accuracy [10, 11]. The purpose of this study is to comprehensively evaluate the suitability of Gafchromic EBT3 and EBTXD film for dosimetric quality assurance in 0.35 T MR-guided radiotherapy
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