不同混合容积旋转调强治疗胸段食管癌的剂量学比较

Dosimetric comparison of two different hybird VMAT techniques for thoracic esophageal carcinoma

目的利用容积旋转调强技术（volumetric—modulated arc therapy，VMAT）治疗食管癌，在计划设计时添加前后对穿适形野可以有效减少肺脏低剂量照射体积。本研究比较2种不同的混合VMAT技术应用于胸段食管癌放疗时的剂量学差异，为临床治疗技术的选择提供参考依据。方法收集10例于2015—04-01—2015—05—31在吉林省肿瘤医院放疗科接受放射治疗的中段食管癌患者作为研究对象。对每例患者分别重新设计2种混合VMAT治疗计划。处方剂量为60Gy，单次剂量2Gy，5次／周，6周完成治疗。全弧计划：前后对穿适形野投递一半的处方剂量（1Gy），之后添加1个全弧，利用全弧投递另外一半的处方剂量（1Gy）；部分弧计划：与全弧计划相同，也是利用前后对穿适形野投递一半的处方剂量，但是，另一半的处方剂量，是由181°～240°和120°～180°2个顺时针旋转的小段弧完成剂量投递。2种计划VMAT弧的逆向优化过程采用相同的优化处方。比较2种计划靶区和各个危及器官的剂量学参数及计划的机器跳数（monitor units，MU）和治疗时间。结果2种计划均能很好地满足临床治疗需要。在计划靶体积（planning target volume，PTV）的各项剂量学参数的比较中，除了全弧计划的适形指数（conformity index，CI）明显优于部分弧计划，Z=-2．675，P=0．007；其他剂量指标均差异无统计学意义。在全肺的比较中，全弧计划的V5比部分弧计划高约16．5％，Z=-2．668，P=0．008；肺的平均剂量比部分弧计划多约15．5％，Z=-2．142；P=0．029；同时，全弧计划的V15和V20分别比部分弧计划少约13．8％（Z=-2．054，P=0．026）和13．5％（Z=-1．984，P=0．039）。全弧计划心脏的V40和V50分别比部分弧计划少约25．3％（Z=-2．684，P=0．007）和23．3％（Z=-2．492，P=0．013）；心脏的平均剂量也比部分弧计划少约6．2％，Z=-2．569，P=0．026。全弧计划MU为373．2±20．6，比部分弧计划的324．2±33．7多约15．1％，Z=-2．737，P=0．007；但是全弧计划的治疗时间为（156±20）S，部分弧计划的治疗时间为（161±25）S，差异无统计学意义，z=-0．873，P=0．614。结论在胸段食管癌的放射治疗中，混合VMAT计划采用全弧设计时，可以更好地保护心脏，肺的V20更小；利用部分弧设计时可以使肺的低剂量V5照射体积进一步减少。在临床工作中，要根据患者的具体情况及不同的剂量限定标准选用更为合适的技术手段。

OBJECTIVE When using Volumetric-modulated arc therapy （VMAT） in treatment of esophageal cancer, adding anterior-posterior-posterio-anterior （AP-PA） conformal fields into VMAT can effectively reduce the lung volume received from low dose irradiation. This study compared the dosimetric difference of two different Hybird VMAT techniques for thoracic esophageal carcinoma to provide reference for clinical choice. METHODS Form April 1,2015 to May 31,2015,10 patients with thoracic esophageal carcinoma were given two Hybird VMAT plans, One plan delivered half of the prescription dose（1 Gy） using AP-PA conformal fields; after this, a full arc was added, and the other half of the prescribed dose （1 Gy） was delivered using the full arc（the full arc plan）. The other plan delivered half of the prescription dose using AP-PA conformal fields too; the other half of the prescribed dose was delivered using two partial arcs that Clockwise rotation from 181° to 240° and from 120° to 180° respectively （the partial arc plan）. The total prescription dose of all the plans was 60 Gy （2 Gy/fraction,30 f）. The reverse optimization procedure of VMAT arcs was accomplished in two plans by the same optimization prescription. Target coverage, sparing of OARs, monitor units （MU） and delivery time were compared. RESULTS Both plans could meet target dose specifications and normal tissue constraints. There were no significant differences between two plans in most of the dose parameters for PTV, except the conformity of the full arc plan was significantly better than that of the partial arc plan（Z=- 2. 675, P = 0. 007）. Compared to the partial arc plan, V5 and the mean dose of lung in the full arc plan were increased 16.5% （Z= -2. 668,P= 0. 008） and 15.5% （Z=-2. 142,P=0. 029）, respectively. But V15 and V20 of lung were lower 13.8%（Z=-2. 054,P= 0. 026） and 13.5%（Z=-1. 984, P=0. 039） respectively. At the same time, V40, V50 and the mean dose of heart in the full arc plan were lower 25.3 % （Z= --2. 684,P=0. 007）, 23.3 % （Z= -2. 492,P=0. 013） and 6.2%（Z= -2. 569;P= 0. 026） respectively. The MUs of the full arc plan （373.2±20.6） was increased 15. 1%（Z=-2. 737;P=0. 007） than that of the partial arc plan （324.2± 33.7）, but there were no significant difference between the full arc plan （156 ±20） and the partial arc plan （161±25） in the treatment time（Z=-0. 873,P=0. 614）. CONCLUSIONS In the radiation therapy for thoracic esophageal carcinoma, Hybrid VMAT got better sparing effect on heart, and lung V20 was also smaller when using full arc design, and the volume of normal lung that receives low doses（V5） was further reduced when using partial arcs design.