Integrating a MRI scanner with a 6 MV radiotherapy accelerator: impact of the surface orientation on the entrance and exit dose due to the transverse magnetic field
Integrating a MRI scanner with a 6 MV radiotherapy accelerator: impact of the surface orientation on the entrance and exit dose due to the transverse magnetic field
复制标题
DOI：
10.1088/0031-9155/52/4/005
复制
发表时间：
2007-02-21
影响因子：
3.5
通讯作者：
Lagendijk, J. J. W.
中科院分区：
文献类型：
--
作者：
Raaijmakers, A. J. E.;Raaymakers, B. W.;Lagendijk, J. J. W.
在UMC Utrecht，我们与Elekta和Philips Research Hamburg合作，正在开发具有集成MRI功能的放射治疗加速器。将在存在1.5 T横向磁场的情况下输送辐射剂量。虽然光子束不受磁场的影响，但实际的剂量沉积是由次级电子级联完成的，这些电子受到洛伦兹力的影响。磁场导致累积距离减小：由于碰撞之间的电子轨迹是弯曲的，因此组织中的入射深度减小。此外，在组织空气界面处，由于所谓的电子返回效应（ERE）而发生剂量增加：离开组织的电子将在空气中描述圆形路径并重新进入组织，从而产生局部剂量增加。在本文中，一个1.5 T的磁场的建立距离和剂量增加的影响，由于ERE将作为一个函数的表面和入射光束之间的角度进行调查。Monte Carlo模拟表明，在1.5 T磁场的存在下，表面剂量，建立距离和出口剂量依赖于更严重的表面取向比在没有磁场的情况下。这是由于在1.5 T的存在下的不对称点扩散核和重新进入电子的方向行为造成的。对几何体模的模拟表明，使用相对的射束可以避免在空气腔处的ERE剂量增加，并且当空气组织边界不垂直于射束时。对于患者解剖结构中更一般的情况，可能会出现更多的问题。未来的工作将解决在磁场中结合调强放射治疗的可能性和限制。
At the UMC Utrecht, in collaboration with Elekta and Philips Research Hamburg, we are developing a radiotherapy accelerator with integrated MRI functionality. The radiation dose will be delivered in the presence of a lateral 1.5 T field. Although the photon beam is not affected by the magnetic field, the actual dose deposition is done by a cascade of secondary electrons and these electrons are affected by the Lorentz force. The magnetic field causes a reduced build-up distance: because the trajectory of the electrons between collisions is curved, the entrance depth in tissue decreases. Also, at tissue air interfaces an increased dose occurs due to the so-called electron return effect (ERE): electrons leaving tissue will describe a circular path in air and re-enter the tissue yielding a local dose increase. In this paper the impact of a 1.5 T magnetic field on both the build-up distance and the dose increase due to the ERE will be investigated as a function of the angle between the surface and the incident beam. Monte Carlo simulations demonstrate that in the presence of a 1.5 T magnetic field, the surface dose, the build-up distance and the exit dose depend more heavily on the surface orientation than in the case without magnetic field. This is caused by the asymmetrical pointspread kernel in the presence of 1.5 T and the directional behaviour of the re-entering electrons. Simulations on geometrical phantoms show that ERE dose increase at air cavities can be avoided using opposing beams, also when the air-tissue boundary is not perpendicular to the beam. For the more general case in patient anatomies, more problems may arise. Future work will address the possibilities and limitations of opposing beams in combination with IMRT in a magnetic field.