Ghila A, Fallone B G, Rathee S
Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada.
Department of Medical Physics, Cross Cancer Institute, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada and Departments of Oncology and Physics, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada.
Med Phys. 2016 Nov;43(11):5808. doi: 10.1118/1.4963803.
Magnetic resonance guided teletherapy systems aspire to image the patient concurrently with the radiation delivery. Thus, the radiofrequency (RF) coils used for magnetic resonance imaging, placed on or close to patient skin and in close proximity to the treatment volume, would be irradiated leading to modifications of radiation dose to the skin and in the buildup region. The purpose of this work is to measure and assess these dose modifications due to standard off-the-shelf RF coil materials.
A typical surface coil was approximated as layered sheets of polycarbonate, copper tape, and Teflon to emulate the base, conductor, and cover, respectively. A separate investigation used additional coil materials, such as copper pipe, plastic coil housing, a typical coil padding material, and a thin copper conductor. The materials were placed in the path of a 6 MV photon beam at various distances from polystyrene phantoms in which the surface and buildup doses were measured. The experiments were performed on a clinical Varian linac with no magnetic field and with a 0.21 T electromagnet producing a magnetic field parallel to the beam central axis. The authors repeated similar experiments in the presence of a 0.22 T magnetic field oriented perpendicular to the beam central axis using an earlier linac-MR prototype, with a biplanar permanent magnet. The radiation detectors used for the measurements were two different parallel plate ion chambers and GAFChromic films.
A typical open beam surface dose of 20% (relative to open beam D) was increased to 63% by the coil padding material and to >74% by all other materials when placed in direct contact with the phantom, irrespective of magnetic field presence or orientation. Without a magnetic field, the surface dose decreased as the test materials were moved away from the phantom surface toward the radiation source, reaching between 30% and 40% at 10 cm gap between the phantom and the test materials. In the presence of the transverse magnetic field, the surface dose reduction was more rapid reaching a dose level of 30%-40% with only 3-4 cm gap. In the presence of the parallel magnetic field, as expected, the surface dose did not decrease considerably as the gap between the phantom surface and test materials was increased; the surface dose remained >60% at 10 cm gap for all tested materials except for the thin copper conductor.
As expected, placing coil materials in direct contact with the phantom surface increases the surface dose considerably. The surface dose is reduced by creating a gap between the coil materials and phantom surface. This dose reduction happens more rapidly in the presence of a transverse magnetic field. However, the surface dose stays relatively large irrespective of the gap in the presence of a parallel magnetic field. Thus, the standard, off-the-shelf RF coils should be used with caution in integrated linac-MR systems, especially those using a parallel magnetic field orientation in which case the RF coils will probably need to be reconfigured to create open ports for the radiation beam.
磁共振引导远程治疗系统旨在在放射治疗的同时对患者进行成像。因此，用于磁共振成像的射频（RF）线圈放置在患者皮肤上或靠近皮肤且紧邻治疗区域，会受到辐射，从而导致皮肤和剂量建成区的辐射剂量发生改变。本研究的目的是测量和评估由于使用标准的现成RF线圈材料而导致的这些剂量改变。
将一个典型的表面线圈近似为聚碳酸酯、铜带和特氟龙层状薄片，分别模拟基座、导体和覆盖层。另一项研究使用了其他线圈材料，如铜管、塑料线圈外壳、一种典型的线圈填充材料和细铜导体。将这些材料放置在6 MV光子束的路径中，与聚苯乙烯模体保持不同距离，测量模体表面和剂量建成区的剂量。实验在一台无磁场的临床瓦里安直线加速器上进行，以及在一台配备产生与束流中心轴平行磁场的0.21 T电磁铁的直线加速器上进行。作者还使用早期的直线加速器 - 磁共振原型机（配备双平面永磁体），在垂直于束流中心轴的0.22 T磁场存在的情况下重复了类似实验。用于测量的辐射探测器是两种不同的平行板电离室和GAFChromic胶片。
当与模体直接接触时，典型的开放束表面剂量为20%（相对于开放束剂量D），通过线圈填充材料可增加到63%，通过所有其他材料可增加到>74%，无论磁场是否存在或方向如何。在无磁场的情况下，随着测试材料从模体表面移向辐射源，表面剂量降低，当模体与测试材料之间的间隙为10 cm时，表面剂量降至30%至40%之间。在存在横向磁场的情况下，表面剂量降低更快，仅3 - 4 cm的间隙就能达到30% - 40%的剂量水平。在存在平行磁场的情况下，正如预期的那样，随着模体表面与测试材料之间间隙的增加，表面剂量没有显著降低；对于除细铜导体之外的所有测试材料，在10 cm间隙时表面剂量仍>60%。
正如预期的那样，将线圈材料与模体表面直接接触会显著增加表面剂量。通过在线圈材料与模体表面之间形成间隙可降低表面剂量。在存在横向磁场的情况下，这种剂量降低发生得更快。然而，在存在平行磁场的情况下，无论间隙大小，表面剂量都相对较高。因此在直线加速器 - 磁共振集成系统中，应谨慎使用标准的现成RF线圈，特别是那些采用平行磁场方向的系统，在这种情况下可能需要重新配置RF线圈以形成辐射束的开放端口。
