鼻咽癌IMRT中气腔对靶区及OAR剂量影响

目的 明确IMRT中气腔效应对鼻咽癌患者原发肿瘤及OAR受照射剂量影响。 方法 选择鼻咽癌患者9例,放疗前及放疗第25次分别接受CT定位扫描。在放疗前CT图像上勾画靶区及OAR,制定计划plan1。复制plan1,将此放疗前CT图像与放疗第25次CT图像融合。在放疗前CT图像上勾画靶区退缩后形成的空腔并将空腔的密度强制设为0,并将放疗前勾画的靶区减去空腔体积形成新靶区,此CT图像命名为CT_Air。使用plan1计划的设野及计划参数,在CT_Air上计算剂量分布,形成放疗计划plan2。假设plan1和plan2分别被全程使用,配对t检验比较有气腔和无气腔情况下原发肿瘤及OAR受量。

Dosimetric effects of air cavity on target volume and organs at risk during intensity-modulated radiation therapy for nasopharyngeal carcinoma

Objective To evaluate the dosimetric effects of air cavity on primary tumor and organs at risk (OARs) during intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma patients.Methods Nine patients with nasopharyngeal carcinoma had CT simulation before treatment and on the 25 fraction of radiotherapy.Radiotherapy plan1 was first created by delineating the target volumes and OARs on the first CT image, which was then copied and merged with the second CT image.Air cavity that was formed following tumor volume regression was delineated on the first CT image, and the density of air cavity was set to zero.A new gross target volume (GTV) was formed by subtracting the air cavity from the original GTV, and the new CT image was named CTAir.Plan2 was then created by calculating the dose distribution on CTAir using the same portals and parameters as Plan1.Assuming that Plan1 and Plan2 were both used throughout the course of radiotherapy, the dosimetric parameters of nasopharyngeal tumor and OARs in the absence and presence of air cavity were then compared using the paired t-test.Results Dmean, D95, D90, D10, and D5 of tumor were significantly higher in plan2 than in plan1(P=0.000, 0.001, 0.001, 0.001, and 0.005, respectively), with a<0.5 Gy increase in dose.A dose build-up effect was observed within the tumor region posterior to the air cavity, where the highest dose was 1.4 cm posterior to the air cavity, resulting in a dose difference of 0.36 Gy.In addition, D2cc and D1cc of the brain stem were significantly higher in plan2 than in plan1(P=0.036 and 0.044, respectively).Dmax of the optical chiasm, left optical nerve, and right optical nerve were also increased in the presence of air cavity (P=0.438, 0.434, and 0.477, respectively), but the change in dose was<0.12 Gy.Conclusions Air cavity induces a small but negligible increase in the tumor and OARs dose in patients with nasopharyngeal carcinoma during IMRT.However, closer monitoring should be conducted for patients with OARs that is close to or has surpassed tolerance prior to radiotherapy.