Oborn B M, Ge Y, Hardcastle N, Metcalfe P E, Keall P J
Illawarra Cancer Care Centre (ICCC), Wollongong, NSW 2500, Australia and Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, Australia.
Sydney Medical School, University of Sydney, NSW 2006, Australia.
Med Phys. 2016 Jan;43(1):368. doi: 10.1118/1.4938580.
To report on significant dose enhancement effects caused by magnetic fields aligned parallel to 6 MV photon beam radiotherapy of small lung tumors. Findings are applicable to future inline MRI-guided radiotherapy systems.
A total of eight clinical lung tumor cases were recalculated using Monte Carlo methods, and external magnetic fields of 0.5, 1.0, and 3 T were included to observe the impact on dose to the planning target volume (PTV) and gross tumor volume (GTV). Three plans were 6 MV 3D-CRT plans while 6 were 6 MV IMRT. The GTV's ranged from 0.8 to 16 cm(3), while the PTV's ranged from 1 to 59 cm(3). In addition, the dose changes in a 30 cm diameter cylindrical water phantom were investigated for small beams. The central 20 cm of this phantom contained either water or lung density insert.
For single beams, an inline magnetic field of 1 T has a small impact in lung dose distributions by reducing the lateral scatter of secondary electrons, resulting in a small dose increase along the beam. Superposition of multiple small beams leads to significant dose enhancements. Clinically, this process occurs in the lung tissue typically surrounding the GTV, resulting in increases to the D98% (PTV). Two isolated tumors with very small PTVs (3 and 6 cm(3)) showed increases in D98% of 23% and 22%. Larger PTVs of 13, 26, and 59 cm(3) had increases of 9%, 6%, and 4%, describing a natural fall-off in enhancement with increasing PTV size. However, three PTVs bounded to the lung wall showed no significant increase, due to lack of dose enhancement in the denser PTV volume. In general, at 0.5 T, the GTV mean dose enhancement is around 60% lower than that at 1 T, while at 3 T, it is 5%-60% higher than 1 T.
Monte Carlo methods have described significant and predictable dose enhancement effects in small lung tumor plans for 6 MV radiotherapy when an external inline magnetic field is included. Results of this study indicate that future clinical inline MRI-guided radiotherapy systems will be able to deliver a dosimetrically superior treatment to small (PTV < 15 cm(3)), isolated lung tumors over non-MRI-Linac systems. This increased efficacy coincides with the reimbursement in the United States of lung CT screening and the likely rapid growth in the number of patients with small lung tumors to be treated with radiotherapy.
报告平行于6兆伏光子束放疗小肺肿瘤时磁场引起的显著剂量增强效应。研究结果适用于未来的在线磁共振成像引导放疗系统。
使用蒙特卡罗方法对总共8例临床肺肿瘤病例进行重新计算，并纳入0.5、1.0和3特斯拉的外部磁场，以观察对计划靶体积（PTV）和大体肿瘤体积（GTV）剂量的影响。3个计划为6兆伏三维适形放疗计划，6个为6兆伏调强放疗计划。GTV范围为0.8至16立方厘米，PTV范围为1至59立方厘米。此外，还研究了小射束在直径30厘米的圆柱形水体模中的剂量变化。该体模中心20厘米包含水或肺密度填充物。
对于单射束而言，1特斯拉在线磁场对肺剂量分布影响较小，通过减少二次电子横向散射导致沿射束方向剂量小幅增加。多个小射束叠加会导致显著的剂量增强。临床上，此过程发生在通常围绕GTV的肺组织中，导致D98%（PTV）增加。两个PTV非常小（3和6立方厘米）的孤立肿瘤，D98%分别增加了23%和22%。PTV较大的13、26和59立方厘米分别增加了9%、6%和4%，表明随着PTV尺寸增加增强效应自然下降。然而，三个与肺壁相邻的PTV未显示显著增加，原因是密度较高的PTV体积中缺乏剂量增强。一般来说，在0.5特斯拉时，GTV平均剂量增强比1特斯拉时低约60%；在3特斯拉时，则比1特斯拉时高5% - 60%。
蒙特卡罗方法已描述了在包含外部在线磁场的6兆伏放疗小肺肿瘤计划中显著且可预测的剂量增强效应。本研究结果表明，未来临床在线磁共振成像引导放疗系统相较于非磁共振直线加速器系统，能够为小（PTV < 15立方厘米）的孤立肺肿瘤提供剂量学上更优的治疗。这种疗效提升与美国肺部CT筛查的报销政策相契合，且可能接受放疗的小肺肿瘤患者数量将迅速增长。
