Furuya Tomohisa, Tanaka Hiroshi, Ruschin Mark, Nihei Keiji, Pinnaduwage Dilini, Ma Lijun, Sahgal Arjun, Karasawa Katsuyuki
Department of Radiology, Tokyo Metropolitan Cancer and Infectious Diseases Center, Komagome Hospital, Tokyo, 113-8677, Japan.
Department of Radiation Oncology, Sunnybrook Odette Cancer Center, University of Toronto, M4N 3M5, Toronto, Ontario, Canada.
J Radiosurg SBRT. 2015;3(4):307-314.
Stereotactic body radiotherapy (SBRT) planning for spinal metastases is a challenging task that involves complex target shapes and steep dose gradients proximal to the spinal cord. The aim of the present study is to investigate dosimetric variability among delivery systems and institutions doing spine SBRT.Materials and Methods: Three institutions (in Japan, Canada, and the USA) participated in this retrospective treatment planning study. Computed tomography (CT) datasets for three patients including fully delineated targets and organs-at-risk (OAR) were distributed to all three institutions for planning. Delivery systems included the Clinac 21EX, Vero4DRT, Synergy S, and CyberKnife. All treatment plans were generated using a prescribed dose of 24 Gy in 2 fractions and met the following objectives: the evaluated planning target volume (PTV, defined as the PTV minus spinal cord) should receive greater than 16.8 Gy in at least 95% of the volume (D > 16.8 Gy) and a maximum dose to the less than 140% of the prescribed dose (D < 33.6 Gy). The maximum dose of planning risk volume (PRV) cord or thecal sac was limited to 0.035 cm receiving less than 17 Gy. Aside from minimum and maximum dose objectives for the PTV, there were no criteria regarding the shape of the PTV dose-volume histogram (DVH). For each completed treatment plan, the following DVH parameters were evaluated for the PTV: D, D, D, D and sigma-index (S-index, standard deviation of the differential DVH).
The PTV and OAR dose volume constraints were satisfied in all treatment plans. For Case 1, the mean PTV D was 25.4 ± 1.5 Gy (range: 23.7 - 27.8 Gy), for Case 2 it was 26.7 ± 2.0 Gy (23.6 - 28.6 Gy), and for Case 3 it was 26.0 ± 1.3 Gy (24.1 - 27.3 Gy). The mean PTV D was 27.3 ± 2.2 Gy (24.4 - 30.2 Gy), 28.9 ± 3.0 Gy (24.5 - 31.4 Gy) and 28.7 ± 2.7 Gy (25.2 - 31.6 Gy) for Cases 1, 2, and 3, respectively. However, there were statistically significant variations in the DVH parameters of PTV between apparatuses (CyberKnife versus non-CyberKnife) and among institutions (between 2 CyberKnife sites or between 2 conventional accelerator sites).
Although all institutions met the minimum prescribed objectives, inter-institutional and inter-apparatus target dose variations were observed. Further study is necessary to determine target dose constraints that may minimize inter-institutional variations and lead to plan standardization.
脊柱转移瘤的立体定向体部放疗（SBRT）计划是一项具有挑战性的任务，涉及复杂的靶区形状以及脊髓附近陡峭的剂量梯度。本研究的目的是调查进行脊柱SBRT的不同放疗设备及机构之间的剂量学差异。
三个机构（分别位于日本、加拿大和美国）参与了这项回顾性治疗计划研究。包含完整勾画靶区和危及器官（OAR）的三名患者的计算机断层扫描（CT）数据集被分发给所有三个机构用于计划制定。放疗设备包括Clinac 21EX、Vero4DRT、Synergy S和射波刀。所有治疗计划均采用24 Gy分2次的处方剂量生成，并满足以下目标：评估的计划靶区体积（PTV，定义为PTV减去脊髓）至少95%的体积应接受大于16.8 Gy的剂量（D>16.8 Gy），且最大剂量应小于处方剂量的140%（D<33.6 Gy）。计划风险体积（PRV）脊髓或硬膜囊接受小于17 Gy的最大剂量被限制在0.035 cm范围内。除了PTV的最小和最大剂量目标外，对于PTV剂量体积直方图（DVH）的形状没有其他标准。对于每个完成的治疗计划，针对PTV评估以下DVH参数：D、D、D、D和西格玛指数（S指数，微分DVH的标准差）。
所有治疗计划均满足PTV和OAR剂量体积限制。对于病例1，PTV的平均D为25.4±1.5 Gy（范围：23.7 - 27.8 Gy）；对于病例2，为26.7±2.0 Gy（23.6 - 28.6 Gy）；对于病例3，为26.0±1.3 Gy（24.1 - 27.3 Gy）。病例1、2和3的PTV平均D分别为27.3±2.2 Gy（24.4 - 30.2 Gy）、28.9±3.0 Gy（24.5 - 31.4 Gy）和28.7±2.7 Gy（25.2 - 31.6 Gy）。然而，在不同设备（射波刀与非射波刀）之间以及不同机构（两个射波刀治疗中心之间或两个传统加速器治疗中心之间），PTV的DVH参数存在统计学上的显著差异。
尽管所有机构均达到了最低处方目标，但观察到了机构间和设备间的靶区剂量差异。有必要进一步研究以确定可能使机构间差异最小化并实现计划标准化的靶区剂量限制。
