背景:艾伯塔省旋转双平面直线加速器-MR具有与光束线平行的0.5T磁场。在开发新的直线加速器-MR系统时,带电粒子与磁场的相互作用需要仔细考虑皮肤剂量和组织界面效应。
目的:使用测量和蒙特卡罗(MC)模拟研究磁场对皮肤剂量的影响。
方法:我们开发了直线加速器-MR的MC模型,我们通过与水箱中的离子室测量结果进行比较来验证。此外,将MC模拟结果与固体水中的辐射变色膜表面剂量测量结果进行比较。使用固体水中的平行板离子室测量表面剂量随场大小的变化。使用具有2毫米厚皮肤层的拟人化计算体模,我们研究了三束排列产生的剂量分布。所有测量和模拟都考虑了磁场开启和关闭情况。
结果:对于20×20cm2的场大小,D0.2cc${D_{0.2cc}}$(对最热连续0.2cc体积的最小剂量)当磁场打开时,简单水模顶部2毫米为72%,与磁场关闭时的34%相比(值归一化为中心轴剂量最大值)。平行板离子室的测量表明,由于磁场引起的表面剂量的相对增加随场大小的增加而减小。对于拟人化的幻影,当磁场打开时,与关闭时相比,D*0.2cc${D_{\\sim0.2cc}$(最热的1×1×1cm3立方体中的最小皮肤剂量)显示出20%-28%的相对增加。在磁场关闭的情况下,皮肤D*0.2cc${D_{\\sim0.2cc}}$为71%,56%,内外切线为21%,前后梁,和五场安排,分别。对于磁场,相应的皮肤D~0.2cc${D_{\\sim0.2cc}}$值为91%,67%,和25%。
结论:使用经过验证的直线加速器-MRMC模型,在各种情况下计算表面剂量。MC计算的皮肤剂量取决于视野大小,倾斜度,和梁的数量。总的来说,由于带电粒子沿磁场线盘旋,平行直线加速器-MR布置导致皮肤剂量增强,这阻碍了远离中心轴的横向运动。尽管如此,考虑到本文提出的结果,治疗计划可以设计为通过减少皮肤剂量,例如,避免倾斜光束和使用大量的领域。
BACKGROUND: The Alberta rotating biplanar linac-MR has a 0.5 T magnetic field parallel to the beamline. When developing a new linac-MR system, interactions of charged particles with the magnetic field necessitate careful consideration of skin dose and tissue interface effects.
OBJECTIVE: To investigate the effect of the magnetic field on skin dose using measurements and Monte Carlo (MC) simulations.
METHODS: We develop an MC model of our linac-MR, which we validate by comparison with ion chamber measurements in a water tank. Additionally, MC simulation results are compared with radiochromic film surface dose measurements on solid water. Variations in surface dose as a function of field size are measured using a parallel plate ion chamber in solid water. Using an anthropomorphic computational phantom with a 2 mm-thick skin layer, we investigate dose distributions resulting from three beam arrangements. Magnetic field on and off scenarios are considered for all measurements and simulations.
RESULTS: For a 20 × 20 cm2 field size, D 0.2 c c ${D_{0.2cc}}$ (the minimum dose to the hottest contiguous 0.2 cc volume) for the top 2 mm of a simple water phantom is 72% when the magnetic field is on, compared to 34% with magnetic field off (values are normalized to the central axis dose maximum). Parallel plate ion chamber measurements demonstrate that the relative increase in surface dose due to the magnetic field decreases with increasing field size. For the anthropomorphic phantom, D ∼ 0.2 c c ${D_{ \\sim 0.2cc}}$ (minimum skin dose in the hottest 1 × 1 × 1 cm3 cube) shows relative increases of 20%-28% when the magnetic field is on compared to when it is off. With magnetic field off, skin D ∼ 0.2 c c ${D_{ \\sim 0.2cc}}$ is 71%, 56%, and 21% for medial-lateral tangents, anterior-posterior beams, and a five-field arrangement, respectively. For magnetic field on, the corresponding skin D ∼ 0.2 c c ${D_{ \\sim 0.2cc}}$ values are 91%, 67%, and 25%.
CONCLUSIONS: Using a validated MC model of our linac-MR, surface doses are calculated in various scenarios. MC-calculated skin dose varies depending on field sizes, obliquity, and the number of beams. In general, the parallel linac-MR arrangement results in skin dose enhancement due to charged particles spiraling along magnetic field lines, which impedes lateral motion away from the central axis. Nonetheless, considering the results presented herein, treatment plans can be designed to minimize skin dose by, for example, avoiding oblique beams and using a larger number of fields.