射野回避区域的设置对于宫颈癌容积旋转调强计划剂量学的影响

Dosimetric effect of avoidance sector in cervical volumetric modulated arc therapy plans

目的:研究宫颈癌根治性容积旋转调强(volumetric modulated arc therapy,VMAT)放疗计划中设置射野回避区域(avoidance sector,AS)对计划剂量分布的影响,探究通过设置AS来保护小肠的可行性。方法:选取52例宫颈癌根治性放疗的患者进行研究。设计两组计划,第一组为包含两个360°完整旋转野的VMAT计划,简称FA组。第二组计划在第一组的基础上两个旋转野均增加315°~45°回避区域,简称I90组。统计两组计划的剂量学参数并进行分析。结果:FA组和I90组计划的小肠平均剂量分别为D_(meanINT_FA)和D_(meanINT_I90)。将52例患者的全集按照D_(meanINT_FA)大小分为三个子集L(D_(meanINT_FA)≤26 Gy)、M(26 Gy<D_(meanINT_FA)≤27.5 Gy)和H(D_(meanINT_FA)>27.5 Gy)。对子集L内患者D_(meanINT_FA)>D_(meanINT_I90),但对子集H内患者D_(meanINT_FA)<D_(meanINT_I90),差异均有统计学意义(P<0.05)。Pearson相关性分析显示两组计划小肠平均剂量的差值△D meanINT=D_(meanINT_FA)-D_(meanINT_I90)随两组D meanINT增大而显著减小(与D_(meanINT_FA):相关系数r=-0.700;与D_(meanINT_I90):r=-0.830,P=0),并且与小肠在计划靶区(planning target volume,PTV)内的相对体积以及小肠在315°~45°扇形范围内且在PTV外的体积的绝对、相对值,均有显著相关性(r分别为-0.789、0.808和0.792,P=0)。患者全集与三个子集在靶区、盆腔骨髓、股骨头的剂量参数对比结果方面保持一致。靶区95%处方剂量覆盖率、适形度指数比较的结果为FA组显著高于I90组,均匀性指数比较的结果为FA组显著低于I90组;盆腔骨髓和股骨头的体积剂量V_(10 Gy)、V_(20 Gy)比较的结果为FA组显著低于I90组。结论:设置AS仅在小肠平均剂量本身较低时能使之进一步降低,且对靶区剂量产生负面影响,造成盆腔骨髓和股骨头受照剂量增加。故在宫颈癌根治性放疗计划中推荐采用完整旋转野照射。

Objective:To study the effect of avoidance sector(AS)on dose distribution in volumetric modulated arc therapy(VMAT)plans for cervical cancer,and explore the feasibility of promoting small intestine sparing by setting AS.Methods:52 patients with cervical cancer who had received definitive radiotherapy were chosen for study.Two groups of VMAT plans were designed.The first group of plans contained two full arc fields of 360°,referred to as Group FA.The second group of plans was generated with an AS of 315°~45°for both arcs on the basis of the first group,which was referred to as Group I90.Dosimetric parameters of the two groups were compared and analyzed.Results:The mean dose of small intestine of Group FA and I90 were labeled as D_(meanINT_FA) and D_(meanINT_I90),respectively.The whole aggregate of 52 patients was divided into three subaggregates according to D_(meanINT_FA),namely L(D_(meanINT_FA)≤26 Gy),M(26 Gy<D_(meanINT_FA)≤27.5 Gy)and H(D_(meanINT_FA)>27.5 Gy).For patients of subaggregate L,D_(meanINT_FA)>D_(meanINT_I90).Yet for patients of subaggregate H,D_(meanINT_FA)<D_(meanINT_I90).The difference was statistically significant(P<0.05).Pearson correlation analysis showed that the difference of D meanINT between two groups(△D_(meanINT)=D_(meanINT_FA)-D_(meanINT_I90))decreased significantly with the increase of D meanINT of both the two groups(with D_(meanINT_FA) the correlation coefficient r=-0.700 and with D_(meanINT_I90) r=-0.830,P=0).Besides,△D meanINT had significant correlation with the relative value of small intestine volume that was within planning target volume(PTV)and both the absolute and the relative value of small intestine volume that was within the 315°~45°sector while outside PTV(r=-0.789,0.808 and 0.792,respectively,P=0).Comparison results of dose parameters of the target volume,pelvic bone marrow and femoral heads were the same for the whole aggregate and the three subaggregates.The dose that 95%of target volumes reaches and conformity index in Group FA were significantly higher than those in Group I90,and homogeneity index in Group FA was significantly lower than that in Group I90.The volume dose V_(10 Gy) and V_(20 Gy) of pelvic bone marrow and femoral heads in Group FA was significantly lower than that in Group I90.Conclusion:The setting of AS can further reduce the dose received by small intestine only when the mean dose of small intestine is low,and has a negative impact on the dose distribution of target volumes,resulting in increased dose of pelvic bone marrow and femoral heads.Therefore,full-arc fields are recommended incervical definitive VMAT plans.