图像引导对宫颈癌放疗中直肠、膀胱照射剂量的影响

Effect of image guidance on doses to the rectum and bladder in radiotherapy for cervical cancer

目的分析宫颈癌根治性外照射图像引导与否对直肠和膀胱受照剂量的影响，探讨IGRT技术合理应用的模式。方法选取2012—2016年于陆军总院行HT的宫颈癌患者20例。每次治疗前均进行MVCT扫描，应用MVCT图像在HT的自适应模块上进行剂量重建，得到当次的受量，并模拟出该次无图像引导下的受量；将各单次剂量分布和对应的融合CT图像传输至形变软件MIM6.0中进行剂量叠加，得到总照射剂量。对比图像引导与否对直肠及膀胱受量和体积的影响。结果无图像引导的Plan-2的直肠和膀胱受量均高于图像引导下的Plan-1，其中直肠Dmax、V50及膀胱V50均不同（P＝0.040、0.000、0.047）；分次间初次治疗的Dmax和V50及治疗第13～21次的直肠V50与Plan-1比差异有统计学意义（P＝0.047、0.037，P＝0.009、0.017、0.028），首次及21～23次放疗的膀胱Vmax、V50与Plan-1比接近有统计学意义（P＝0.061、0.053，P＝0.072、0.058）。结论 图像引导可以降低直肠和膀胱的受照剂量及体积，尤其是直肠从图像引导获益更大；建议外照射半量左右（13次左右），肿瘤退缩明显时段，重新定位修改治疗计划；对于难以实现全程图像引导的情况下，进行选择性的图像引导，也可以达到有效地降低直肠和膀胱损伤发生的效果。

Objective To analyze the effect of image guidance on the doses to the rectum and bladder in radical external beam radiotherapy for cervical cancer, and to investigate the reasonable application mode of image-guided radiotherapy （IGRT） in the treatment of cervical cancer.Methods A total of 20 patients with cervical cancer who underwent helical tomotherapy （HT） in PLA Army General Hospital from 2012 to 2016 were enrolled in this study. A megavoltage computed tomography （MVCT） scan was performed before each treatment. The obtained MVCT images were used for dose reconstruction in the adaptive module of HT to obtain the actual dose （Plan-1） and the non-image-guided dose was simulated （Plan-2）. Each single dose distribution and the corresponding fused CT image were sent to the software MIM 6.0 to obtain the total radiation dose by dose superposition. The radiation doses and volumes of the rectum and bladder were compared between the two therapeutic plans. Results The radiation doses to the rectum and bladder in Plan-2 were significantly higher than those in Plan-1. There were significant differences in Dmax and V50 of the rectum and V50 of the bladder between Plan-1 and Plan-2（P=0.040; P=0.000; P=0.047）. Compared with Plan-1, there were statistical differences in inter-fractional Dmax and V50 during the initial treatment （P=0.047, 0.037）, and V50 of the rectum within the 13th to 21st radiotherapy, respectively （P=0.009, 0.017, 0.028）. Besides, differences regarding Vmax and V50 in the initial treatment and the 21st to 23rd radiotherapy were close to the statistical significance when compared to those in Plan-1, respectively （P=0.061, 0.053; P=0.072, 0.058）. Conclusions IGRT can reduce the radiation doses and volumes of the rectum and bladder, especially the rectum. The therapeutic plan should be rescheduled when tumor retraction is evident at half of the total radiation dose （around 13th fraction） in external beam radiotherapy. If it is difficult to achieve image guidance in each treatment, selective image guidance could be performed to effectively reduce the injuries of the rectum and bladder.