State Key Laboratory of Oncology in Southern China, Department of Radiation Oncology, Cancer Center, Sun Yat-sen University, Guangzhou 510060, PR China.
Radiat Oncol. 2013 Apr 2;8:78. doi: 10.1186/1748-717X-8-78.
This prospective study was conducted to evaluate inter- and intra-fraction errors in nasopharyngeal carcinoma (NPC) patients undergoing volumetric modulated arc therapy (VMAT) using cone-beam computed tomography (CBCT) and to thus obtain planning target volume (PTV) margins to effectively guide treatment in the future.
Fifteen NPC patients scheduled to undergo VMAT were prospectively enrolled in the study. For each patient, three CBCT scans were obtained; one after daily conventional positioning, one after online correction with 2 mm tolerance and one after 1 week of VMAT delivery. The scans were registered to the planning CT to determine the inter- and intra-fraction errors. Patient positioning errors were analyzed for time trends over the course of radiotherapy. PTV margins were calculated from the systematic (Σ) and random (σ) errors.
The average absolute values of the pre-correction, post-correction and intra-fraction errors (in order) were 1.1, 0.6 and 0.4 mm in the medial-lateral (ML) direction, 1.2, 0.7 and 0.5 mm in the superior-inferior (SI) direction and 1.1, 0.7 and 0.5 mm in the anterior-posterior (AP) direction. The corresponding Σ were 1.0-1.4 mm, 0.4-0.5 mm and 0.2-0.4 mm, while the corresponding σ were 0.7-0.8 mm, 0.6-0.7 mm and 0.5-0.6 mm. With time, gradual increases in both the inter- and intra-fraction three-dimensional displacements were observed (P = 0.019 and P = 0.044, respectively). The total PTV margins accounting for pre-correction and intra-fraction errors were 3.4-4.1 mm and those accounting for post-correction and intra-fraction errors were 1.7-2.2 mm.
CBCT is an effective modality to evaluate and improve the accuracy of VMAT in NPC patients. Inter- and intra-fraction three-dimensional displacements increased as a function of time during the course of radiotherapy. In our institution, we recommend a PTV margin of 5 mm for NPC patients undergoing VMAT without CBCT and 3 mm for those treated with rigorous daily CBCT scans.
本前瞻性研究旨在通过锥形束 CT（CBCT）评估接受容积调强弧形治疗（VMAT）的鼻咽癌（NPC）患者的分次内和分次间误差，从而获得计划靶区（PTV）边界，以便在未来有效指导治疗。
前瞻性纳入 15 例接受 VMAT 治疗的 NPC 患者。对每位患者进行 3 次 CBCT 扫描：一次在每日常规定位后，一次在 2mm 容差在线校正后，一次在 VMAT 治疗 1 周后。将扫描结果与计划 CT 进行配准，以确定分次内和分次间误差。分析患者在放射治疗过程中的定位误差随时间的变化趋势。从系统误差（Σ）和随机误差（σ）计算 PTV 边界。
（1）在 ML 方向上，校正前、校正后和分次内误差的平均绝对值（依次）分别为 1.1、0.6 和 0.4mm；在 SI 方向上，分别为 1.2、0.7 和 0.5mm；在 AP 方向上，分别为 1.1、0.7 和 0.5mm。相应的Σ分别为 1.0-1.4mm、0.4-0.5mm 和 0.2-0.4mm，而相应的σ分别为 0.7-0.8mm、0.6-0.7mm 和 0.5-0.6mm。（2）随着时间的推移，观察到分次内和分次间三维位移均逐渐增加（P=0.019 和 P=0.044）。考虑校正前和分次内误差的总 PTV 边界为 3.4-4.1mm，考虑校正后和分次内误差的总 PTV 边界为 1.7-2.2mm。
CBCT 是评估和提高 NPC 患者 VMAT 精度的有效方法。在放射治疗过程中，分次内和分次间三维位移随时间增加。在本机构中，我们建议对不接受 CBCT 的 NPC 患者进行 VMAT 治疗时，PTV 边界为 5mm，对接受严格每日 CBCT 扫描的患者，PTV 边界为 3mm。
