Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, 77030, USA.
Elekta Software, Elekta A. B., Maryland Heights, MO, 63043, USA.
Med Phys. 2018 Feb;45(2):884-897. doi: 10.1002/mp.12699. Epub 2017 Dec 21.
The purpose of this study was to acquire beam data for an MR-linac, with and without a 1.5 T magnetic field, by using a variety of commercially available detectors to assess their relative response in the magnetic field. The impact of the magnetic field on the measured dose distribution was also assessed.
An MR-safe 3D scanning water phantom was used to measure output factors, depth dose curves, and off-axis profiles for various depths and for field sizes between 2 × 2 cm and 22 × 22 cm for an Elekta MR-linac beam with the orthogonal 1.5 T magnetic field on or off. An on-board MV portal imaging system was used to ensure that the reproducibility of the detector position, both with and without the magnetic field, was within 0.1 mm. The detectors used included ionization chambers with large, medium, and small sensitive volumes; a diamond detector; a shielded diode; and an unshielded diode.
The offset of the effective point of measurement of the ionization chambers was found to be reduced by at least half for each chamber in the direction parallel with the beam. A lateral shift of similar magnitude was also introduced to the chambers' effective point of measurement toward the average direction of the Lorentz force. A similar lateral shift (but in the opposite direction) was also observed for the diamond and diode detectors. The measured lateral shift in the dose distribution was independent of depth and field size for each detector for fields between 2 × 2 cm and 10 × 10 cm . The shielded diode significantly misrepresented the dose distribution in the lateral direction perpendicular to the magnetic field, making it seem more symmetric. The percentage depth dose was generally found to be lower with the magnetic field than without, but this difference was reduced as field size increased. The depth of maximum dose showed little dependence on field size in the presence of the magnetic field, with values from 1.2 cm to 1.3 cm between the 2 × 2 cm and 22 × 22 cm fields. Output factors measured in the magnetic field at the center of the beam profile produced a larger spread of values between detectors for fields smaller than 10 × 10 cm (with a spread of 2% at 3 × 3 cm ). The spread of values was more consistent when the output factors were measured at the point of peak intensity of the lateral dose distribution instead (except for the shielded diode which differed by up to 2% depending on field size).
The magnetic field of the MR-linac alters the effective point of measurement of ionization chambers, shifting it both downstream and laterally. Shielded diodes produce incorrect and misleading dose profiles. The output factor measured at the point of peak intensity in the lateral dose distribution is more robust than the conventional output factor (measured at central axis). Diodes are not recommended for output factor measurements in the magnetic field.
本研究旨在获取配备和不配备 1.5T 磁场的磁共振直线加速器的射束数据，使用各种市售探测器评估它们在磁场中的相对响应。还评估了磁场对测量剂量分布的影响。
使用 3D 扫描水模体，在配备和不配备正交 1.5T 磁场的 Elekta 磁共振直线加速器束线上，对各种深度和 2×2cm 至 22×22cm 之间的射野大小，测量输出因子、深度剂量曲线和离轴曲线。使用内置的 MV 门控成像系统确保探测器位置的重现性，无论是否存在磁场，误差都在 0.1mm 以内。使用的探测器包括具有大、中、小敏感体积的电离室；钻石探测器；屏蔽二极管；和非屏蔽二极管。
发现每个电离室的有效测量点在与射束平行的方向上至少减小了一半。测量点的横向移动也以洛伦兹力的平均方向向各个室的有效测量点引入了类似大小的横向移动。钻石和二极管探测器也观察到类似的横向移动（但方向相反）。对于 2×2cm 至 10×10cm 之间的磁场，每个探测器的剂量分布的测量横向偏移与深度和射野大小无关。屏蔽二极管在垂直于磁场的横向方向上严重歪曲了剂量分布，使其看起来更加对称。有磁场时，百分深度剂量通常比没有磁场时低，但随着射野尺寸的增加，这种差异减小。在磁场存在的情况下，最大剂量深度几乎不依赖于射野尺寸，在 2×2cm 至 22×22cm 射野之间，值在 1.2cm 至 1.3cm 之间。在射野小于 10×10cm 时，在射束轮廓中心测量的磁场中的输出因子在探测器之间产生了更大的数值分布（在 3×3cm 时分布为 2%）。当在横向剂量分布的峰值强度点测量输出因子时，数值分布更加一致（屏蔽二极管除外，其值取决于射野大小，差异最大可达 2%）。
磁共振直线加速器的磁场改变了电离室的有效测量点，使其向下游和横向移动。屏蔽二极管产生不正确和误导性的剂量分布。在横向剂量分布的峰值强度点测量的输出因子比传统输出因子（在中心轴上测量）更稳健。不建议在磁场中使用二极管进行输出因子测量。
