BalajiSubramanian Sitaraman, Balaji Karunakaran, Thirunavukarasu Moorthi, Premkumar Sumana
Department of Radiation Oncology, Gleneagles Global Hospitals, Chennai, India.
Department of Radiation Oncology, Gleneagles Global Hospitals, Chennai, India; School of Advanced Sciences, VIT University, Vellore, India.
Phys Med. 2017 Jul;39:80-87. doi: 10.1016/j.ejmp.2017.05.073. Epub 2017 Jun 12.
Studies using split field IMRT to spare dysphagia/aspiration related structures (DARS) have raised concern regarding dose uncertainty at matchline. This study explores the utility of hybrid VMAT in sparing the DARS and assesses matchline dose uncertainty in postoperative oral cavity cancer patients and compares it with VMAT.
METHODS & MATERIALS: Ten postoperative oral cavity cancer patients were planned with h-VMAT and VMAT using the same planning CT dataset. PTV and DARS were contoured using standard delineation guidelines. In h-VMAT 80% of the neck dose was planned using AP/PA technique and then VMAT optimization was done for the total PTV by keeping the corresponding AP/PA plan as the base dose. Planning goal for PTV was V≥95% and for DARS, adequate sparing. Plans and dose volume histograms were analyzed using dosimetric indices. Absolute point and portal dose measurements were done for h-VMAT plans to verify dose at the matchline.
Coverage in both the techniques was comparable. Significant differences were observed in mean doses to DARS (Larynx: 24.36±2.51 versus 16.88±2.41Gy; p<0.0006, Pharyngeal constrictors: 25.16±2.41 versus 21.2±2.1Gy; p<0.005, Esophageal inlet: 18.71±2 versus 12.06±0.79Gy; p<0.0002) favoring h-VMAT. Total MU in both the techniques was comparable. Average percentage variations in point dose measurements in h-VMAT done at +3.5 and -3.5 positions were (1.47±1.48 and 2.28±1.35%) respectively. Average gamma agreement for portal dose measured was 97.07%.
h-VMAT achieves better sparing of DARS with no matchline dose uncertainty. Since these patients have swallowing dysfunction post-operatively, attempts should be made to spare these critical structures as much as possible.
使用调强适形放疗（IMRT）的分野技术来保护吞咽困难/误吸相关结构（DARS）的研究引发了对射野衔接处剂量不确定性的关注。本研究探讨混合容积调强弧形放疗（h-VMAT）在保护DARS方面的效用，并评估口腔癌术后患者射野衔接处的剂量不确定性，并将其与容积调强弧形放疗（VMAT）进行比较。
使用相同的计划CT数据集，对10例口腔癌术后患者进行h-VMAT和VMAT计划。按照标准勾画指南勾画计划靶区（PTV）和DARS。在h-VMAT中，80%的颈部剂量采用前后对穿野技术进行计划，然后以相应的前后对穿野计划作为基础剂量，对整个PTV进行VMAT优化。PTV的计划目标是V≥95%，对于DARS，目标是充分保护。使用剂量学指标分析计划和剂量体积直方图。对h-VMAT计划进行绝对点剂量和射野剂量测量，以验证射野衔接处的剂量。
两种技术的靶区覆盖情况相当。观察到DARS的平均剂量存在显著差异（喉：24.36±2.51与16.88±2.41Gy；p<0.0006，咽缩肌：25.16±2.41与21.2±2.1Gy；p<0.005，食管入口：18.71±2与12.06±0.79Gy；p<0.0002），h-VMAT更具优势。两种技术的总机器跳数相当。在h-VMAT中，在+3.5和-3.5位置进行的点剂量测量的平均百分比变化分别为（1.47±1.48和2.28±1.35%）。测量的射野剂量的平均伽马通过率为97.07%。
h-VMAT能更好地保护DARS，且射野衔接处无剂量不确定性。由于这些患者术后存在吞咽功能障碍，应尽可能保护这些关键结构。
