James Joshua, Swanson Christine, Lynch Bart, Wang Brian, Dunlap Neal E
Department of Radiation Oncology, University of Louisville, Louisville, Kentucky.
Baptist Health Louisville, Louisville, Kentucky.
Pract Radiat Oncol. 2015 Jul-Aug;5(4):e337-43. doi: 10.1016/j.prro.2014.11.001. Epub 2014 Nov 6.
The use of fiducial markers or direct tumor visualization allows for tumor tracking and ultimately smaller planning target volume (PTV) margins in the treatment of lung tumors, yet many patients are either not amenable to fiducial marker placement or their tumors are unable to be visualized on orthogonal-axis x-ray images. Spine tracking is an alternative method for tumor localization but is limited by the assumption that the location of the lung tumor relative to the spine is constant. The purpose of this study was to quantify the additional PTV margin needed when spine tracking is used to ensure the internal target volume (ITV) receives the prescription dose during treatment.
Daily cone beam computed tomography images, registered based on tumor position, from 63 patients with lung cancer treated with stereotactic body radiation therapy were collected and analyzed. Rigid registrations were reperformed so that the position of the spine on the cone beam computed tomography image was aligned to its position on the planning computed tomography. Shifts from the treatment position to the new position were recorded, and per-patient mean shifts and standard deviations were calculated, as well as group systematic and random standard deviations. These data were used with van Herk's margin recipe to determine the additional margin required to adequately treat the patient population if spine tracking were used instead of direct daily tumor imaging. A retrospective dosimetric analysis was also performed on 6 patients with lung cancer previously treated by CyberKnife using spine tracking to determine the potential decrease in target coverage attributable to insufficient margin on the ITV. This analysis was performed by shifting the PTV volume relative to the CyberKnife treatment geometry to simulate a setup error caused by tracking the spine as opposed to the tumor.
The additional margins calculated by van Herk's margin recipe to adequately cover the ITV with the 95% isodose surface for 90% of the entire patient population in the vertical, longitudinal, and lateral directions were 6.4, 6.0, and 4.5 mm, respectively. The retrospective analysis showed a decrease in PTV coverage from 95.6% to 93.1% and an increase in new conformity index by 2.7% when the average shift data were used to simulate setup error. When the maximum shift data were used to simulate the worst possible outcome, PTV coverage decreased to 73.4% and the new conformity index increased by 26.8%.
Standard margins of 5 mm on the ITV for patients with lung cancer being treated with stereotactic body radiation therapy are insufficient and may result in geographic misses of the tumor when spine tracking is used to locate the position of the tumor in the lung. Therefore, we recommend the addition of 5-mm margins in all directions for a total of 10 mm to take into account the change in position of the tumor relative to the spine from the time of simulation to treatment.
使用基准标记或直接肿瘤可视化可实现肿瘤追踪，并最终在肺部肿瘤治疗中缩小计划靶体积（PTV）边界，但许多患者要么不适合放置基准标记，要么其肿瘤在正交轴X线图像上无法可视化。脊柱追踪是肿瘤定位的另一种方法，但受肺部肿瘤相对于脊柱的位置恒定这一假设的限制。本研究的目的是量化使用脊柱追踪时为确保内部靶体积（ITV）在治疗期间接受处方剂量所需的额外PTV边界。
收集并分析了63例接受立体定向体部放射治疗的肺癌患者基于肿瘤位置配准的每日锥束计算机断层扫描图像。重新进行刚性配准，使锥束计算机断层扫描图像上脊柱的位置与计划计算机断层扫描上的位置对齐。记录从治疗位置到新位置的位移，计算每位患者的平均位移和标准差，以及组系统和随机标准差。这些数据与范赫克边界公式一起用于确定如果使用脊柱追踪而非每日直接肿瘤成像来充分治疗患者群体所需的额外边界。还对6例先前接受射波刀脊柱追踪治疗的肺癌患者进行了回顾性剂量分析，以确定由于ITV边界不足导致的靶区覆盖潜在减少。该分析通过相对于射波刀治疗几何结构移动PTV体积来模拟因追踪脊柱而非肿瘤导致的摆位误差。
范赫克边界公式计算得出，为使90%的患者群体的ITV在垂直、纵向和横向方向上被95%等剂量面充分覆盖，额外边界分别为6.4、6.0和4.5mm。回顾性分析显示，当使用平均位移数据模拟摆位误差时，PTV覆盖率从95.6%降至93.1%，新适形指数增加2.7%。当使用最大位移数据模拟最坏可能结果时，PTV覆盖率降至73.4%，新适形指数增加26.8%。
对于接受立体定向体部放射治疗的肺癌患者，ITV上5mm的标准边界不足，当使用脊柱追踪来定位肺部肿瘤位置时，可能导致肿瘤的几何遗漏。因此，我们建议在各个方向上增加5mm边界，总共10mm，以考虑从模拟到治疗期间肿瘤相对于脊柱的位置变化。
