Keall P J, Lauve A D, Hagan M P, Siebers J V
Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298, USA.
Med Phys. 2007 Jun;34(6):1944-51. doi: 10.1118/1.2731484.
A strategy is proposed in which intrafraction internal target translation is corrected for by repositioning the multileaf collimator position aperture to conform to the new target pose in the beam projection, and the beam monitor units are adjusted to account for the change in the geometric relationship between the target and the beam. The purpose of this study was to investigate the dosimetric stability of the prostate and critical structures in the presence of internal target translation using the dynamic compensation strategy. Twenty-five previously treated prostate cancer patients were replanned using a four-field conformal technique to deliver 72 Gy to 95% of the planning target volume (PTV). Internal translation was introduced by displacing the prostate PTV (no rotation or deformation was considered). Thirty-six randomly selected isotropic displacements of magnitude 0.5, 1.0, 1.5 and 2.0 cm were sampled for each patient, for a total of 3600 errors. Due to their anatomic relation to the prostate, the rectum and bladder contours were also moved with the same magnitude and direction as the prostate. The dynamic compensation strategy was used to correct each of these errors by conforming the beam apertures to the new target pose and adjusting the monitor units using inverse-square and off-axis factor corrections. The dynamic compensation strategy plans were then compared to the original treatment plans via dose-volume histogram (DVH) analysis. Changes of more than 5% of the prescription dose (3.6 Gy) were deemed clinically significant. Compared to the original treatment plans, the dynamic compensation strategy produced small discrepancies in isodose distributions and DVH analyses for all structures considered apart from the femoral heads. These differences increased with the magnitude of the internal motion. Coverage of the PTV was excellent: D5, D95, and Dmean were not increased or decreased by more than 5% of the prescription dose for any of the 3600 simulated internal motion shifts. Dose increases for adjacent organs at risk were rare. D33 of the rectum and D20 of the bladder were increased by more than 5% of the prescription dose in 9 and 1 instances of the 3600 sampled internal motion shifts, respectively. Dmean of the right femoral head increased by more than 5% of the prescription dose in 651 (18%) internal motion shifts, predominantly due to the projection of the lateral beams through the femoral head for anterior prostate motion. However, D2 was not increased by more than 5% for any of the internal motion shifts. These data demonstrate the robustness of the proposed dynamic compensation strategy for correction of internal motion in conformal prostate radiotherapy, with minimal deviation from the original treatment plans even for errors exceeding those commonly encountered in the clinic. The compensation strategy could be performed automatically with appropriate enhancements to available delivery software.
提出了一种策略，即通过重新定位多叶准直器的位置孔径，使其在射束投影中符合新的靶区位置，来校正分次内的靶区内平移，并调整射束监测单位，以考虑靶区与射束之间几何关系的变化。本研究的目的是使用动态补偿策略研究存在靶区内平移时前列腺及关键结构的剂量学稳定性。对25例既往接受过治疗的前列腺癌患者，采用四野适形技术重新计划，将72 Gy的剂量给予95%的计划靶区体积（PTV）。通过移动前列腺PTV引入内平移（未考虑旋转或变形）。为每位患者随机抽取36个大小为0.5、1.0、1.5和2.0 cm的各向同性位移，共3600个误差。由于直肠和膀胱轮廓与前列腺的解剖关系，它们也与前列腺以相同的大小和方向移动。使用动态补偿策略，通过使射束孔径符合新的靶区位置，并使用反平方和离轴因子校正来调整监测单位，对这些误差中的每一个进行校正。然后通过剂量体积直方图（DVH）分析，将动态补偿策略计划与原始治疗计划进行比较。处方剂量（3.6 Gy）变化超过5%被认为具有临床意义。与原始治疗计划相比，除股骨头外，对于所有考虑的结构，动态补偿策略在等剂量分布和DVH分析中产生的差异较小。这些差异随着内运动幅度的增加而增大。PTV的覆盖情况良好：在3600次模拟的内运动偏移中的任何一次中，D5、D95和Dmean相对于处方剂量的增加或减少均未超过5%。邻近危及器官的剂量增加很少见。在3600次抽样的内运动偏移中，分别有9次和1次情况下，直肠的D33和膀胱的D20相对于处方剂量增加超过5%。在651次（18%）内运动偏移中，右侧股骨头的Dmean相对于处方剂量增加超过5%，主要是由于前位前列腺运动时侧射束穿过股骨头。然而，在任何一次内运动偏移中，D2增加均未超过5%。这些数据证明了所提出的动态补偿策略在适形前列腺放疗中校正内运动的稳健性，即使对于超过临床常见误差的情况，与原始治疗计划的偏差也最小。通过对现有输送软件进行适当增强，补偿策略可自动执行。
