靶区及危及器官剂量学改变与鼻咽癌自适应放疗计划优化时机

Timing of adaptive radiation therapy determined by dosimetric changes in targets and organs at risk for nasopharyngeal carcinoma

目的在调强放疗期间,鼻咽癌患者存在不同程度的靶区及危及器官的解剖学及剂量学改变,通过评估这些解剖学及剂量学改变,研究自适应放疗的最佳干预时机。方法收集2012-08-2013-12广西壮族自治区人民医院入组19例鼻咽癌患者,在调强放疗过程中进行每周CT扫描,通过形变配准方法将计划CT与新CT进行融合,自动勾画靶区及危及器官,评估解剖学改变。将原始放疗计划通过剂量映射传递到新CT图像上,形成合成计划,分别与原始计划对比DVH。结果 GTVnx、GTVnd和腮腺的体积在放疗过程中逐渐缩小。与原始计划相比,各周合成计划中所有靶区的HI差异均无统计学意义。与原计划相比,PTVnx和PTV1的部分剂量学参数显著改善,特别是在第10次放疗后,这些参数包括PTVnx的Dmean、D95、V95和CI,以及PTV1的Dmean、D95和CI;PTVnd的剂量学参数只有D95和V95在某些时间点上有显著改善;对于PTV2,其D95和V95在某些时间点上显著变劣。部分器官如脑干、脊髓、腮腺、声门、右侧眼球和左侧晶体的剂量在某些时点上显著增加。结论鼻咽癌调强放疗过程中发生显著的靶区及危及器官的解剖学和剂量学改变。将PTV2的D95、V95及CI、脑干Dmax、脊髓Dmax、腮腺Dmean、声门喉Dmean、右侧眼球Dmax和左侧晶体Dmax作为预测自适应放疗时机的参数,经过分析建议在鼻咽癌调强放疗至第5次后和第15次后分别进行自适应计划优化。

OBJECTIVE To determine appropriate timing of adaptive radiation therapy （ART） replan by evaluating anatomic and dosimetric changes of target volumes and organs at risk （OARs） during intensity-modulated radiation therapy （IMRT） for nasopharyngeal carcinoma （NPC）. METHODS From August 2012 to December 2013,nineteen NPC patients were recruited. Each patient had repeated diagnostic computed tomography （CT） scans after each five fractions and at treatment completion. Automatic re-contouring the targets and OARs by deformable registration algorithm was conducted through CT-CT fusion. Anatomic changes were assessed by comparing the initial CT and repeated CT. Hybrid plans with re-contouring were generated and the dose-volume histograms （DVH） of the hybrid plan and the original plan were compared. RESULTS Progressive volume reductions in gross target volume for primary disease （GTVnx）, gross target volume for involved lymph nodes （GTVnd） and parotids were observed over time. Comparing with the original plan, some parameters for planning target volumes for primary disease and high-risk clinical target volume （PTVnx and PTV1,respectively） were improved significantly, notably starting from the 10th fraction. These parameters included mean dose （D ）, dose to 95% of the volume （D95）, percentage of the volume receiving 95% of the prescription dose （V95）, and conformity index （CI） for PTVnx, and Dmean, D95, and CI for PTV1. The dosimetric parameters for PTVnd remained the same in general except for D95 and V95 which had significant improvement at specific time points. Dosimetric parameters for some organs such as brain stem, spinal cord, parotid glands, glottis, right eyeball and left lens got significantly worse and significantly increased dose was observed for PTV2 and some OARs respectively, at some time points. CONCLUSIONS There were significant anatomic and dosimetric changes in the targets and OARs. Significant dosimetric changes should be considered as a trigger point at which ART replanning is indicated. D95/V95/CI for PTV2, D for the brain stem, spinal cord, right eyeball and left lens, and Dmean /V30 for the parotids and glottis were taken into account for predicting the need for ART. Two replans at the 5th and 15th fractions were suggested.