Department of Radiation Oncology, Beaumont Health, Troy, MI, USA.
Department of Radiation Oncology, Henry Ford Health System, 2799 West Grand Blvd, Detroit, MI, USA.
Radiat Oncol. 2021 Jun 26;16(1):117. doi: 10.1186/s13014-021-01844-z.
The 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.
A 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-cm to 19.92 × 19.92-cm) 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.
No 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%).
MR 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.
本研究的目的是全面评估 Gafchromic EBT3 和 EBT-XD 薄膜在 0.35 T 磁共振引导放疗中的剂量学质量保证的适用性。
利用 0.35 T 磁场强度，通过研究磁场中膜曝光时间的影响（使用两种定时序列和未暴露于磁共振的膜）、磁场对膜结晶结构的影响以及膜相对于磁场中的孔的方向的影响，评估磁场对 EBT3 和 EBT-XD Gafchromic 膜的影响。使用扫描电子显微镜 (SEM) 对单体晶体的方向进行定性评估，与未辐照的膜进行比较。此外，通过一系列开放场照射（0.83×0.83-cm 至 19.92×19.92-cm）和患者特定质量保证测量来评估剂量学影响。开放场与计划剂量和独立剂量计进行比较。薄膜剂量计应用于 20 例常规和 20 例立体定向体部放疗 (SBRT) 患者特定质量保证病例。
在任何评估的 SEM 图像中均未观察到晶体取向的视觉变化，也未观察到 EBT3 和 EBT-XD 膜在磁场内和磁场外照射之间的任何光学密度差异。在小场尺寸下，与使用独立剂量计和预测治疗计划系统值测量的相同点相比，薄膜沿剂量分布测量的平均差异分别为 1.23%和 1.56%。对于大场尺寸，平均差异分别为 1.91%和 1.21%。在开放场测试中，3%/3 mm 和 3%/1 mm 的平均伽马通过率分别为 99.8%和 97.2%。常规 QA 病例中 3%/3 mm 伽马通过率的中位数（四分位距）为 98.4%（96.3 至 99.2%），SBRT QA 病例中为 95.8%（95.0 至 97.3%）。
0.35 T 的磁共振照射对 EBT3 和 EBT-XD Gafchromic 薄膜几乎没有影响。剂量学薄膜结果与计划剂量、离子室和二极管测量值相当。
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