Mackie T R, Balog J, Ruchala K, Shepard D, Aldridge S, Fitchard E, Reckwerdt P, Olivera G, McNutt T, Mehta M
University of Wisconsin Medical School, Madison, WI 53706, USA.
Semin Radiat Oncol. 1999 Jan;9(1):108-17. doi: 10.1016/s1053-4296(99)80058-7.
Tomotherapy is delivery of intensity-modulated, rotational radiation therapy using a fan-beam delivery. The NOMOS (Sewickley, PA) Peacock system is an example of sequential (or serial) tomotherapy that uses a fast-moving, actuator-driven multileaf collimator attached to a conventional C-arm gantry to modulate the beam intensity. In helical tomotherapy, the patient is continuously translated through a ring gantry as the fan beam rotates. The beam delivery geometry is similar to that of helical computed tomography (CT) and requires the use of slip rings to transmit power and data. A ring gantry provides a stable and accurate platform to perform tomographic verification using an unmodulated megavoltage beam. Moreover, megavoltage tomograms have adequate tissue contrast and resolution to provide setup verification. Assuming only translational and rotational offset errors, it is also possible to determine the offsets directly from tomographic projections, avoiding the time-consuming image reconstruction operation. The offsets can be used to modify the leaf delivery pattern to match the beam to the patient's anatomy on each day of a course of treatment. If tomographic representations of the patient are generated, this information can also be used to perform dose reconstruction. In this way, the actual dose distribution delivered can be superimposed onto the tomographic representation of the patient obtained at the time of treatment. The results can be compared with the planned isodose on the planning CT. This comparison may be used as an accurate basis for adaptive radiotherapy whereby the optimized delivery is modified before subsequent fractions. The verification afforded tomotherapy allows more precise conformal therapy. It also enables conformal avoidance radiotherapy, the complement to conformal therapy, for cases in which the tumor volume is ill-defined, but the locations of sensitive structures are adequately determined. A clinical tomotherapy unit is under construction at the University of Wisconsin.
断层放疗是使用扇形束进行调强旋转放射治疗。NOMOS（宾夕法尼亚州塞威克利）孔雀系统是序贯（或串行）断层放疗的一个例子，它使用一个快速移动、由 actuator 驱动的多叶准直器，该准直器连接到传统的 C 形臂机架上以调制射束强度。在螺旋断层放疗中，当扇形束旋转时，患者在环形机架中连续平移。射束传输几何结构类似于螺旋计算机断层扫描（CT），并且需要使用滑环来传输功率和数据。环形机架提供了一个稳定且精确的平台，可使用未调制的兆伏级射束进行断层验证。此外，兆伏级断层图像具有足够的组织对比度和分辨率以提供摆位验证。假设仅存在平移和旋转偏移误差，也可以直接从断层投影确定偏移量，从而避免耗时的图像重建操作。这些偏移量可用于修改叶片传输模式，以便在一个疗程的每一天使射束与患者的解剖结构相匹配。如果生成了患者的断层图像表示，此信息也可用于进行剂量重建。通过这种方式，可以将实际交付的剂量分布叠加到治疗时获得的患者断层图像表示上。结果可以与计划 CT 上的计划等剂量线进行比较。这种比较可以用作自适应放疗的准确基础，据此在后续分次治疗前修改优化的射束传输。断层放疗所提供的验证允许进行更精确的适形治疗。对于肿瘤体积定义不明确但敏感结构位置已充分确定的情况，它还能够进行适形回避放疗，这是适形治疗的补充。威斯康星大学正在建造一台临床断层放疗设备。
