鼻咽癌半束照射的剂量分布

目的 比较鼻咽癌半束和全束照射技术射野衔接处的剂量分布以及脑和肺的受量。方法 应用Varian 600CD直线加速器，在固体水模体中采用胶片黑度法测量鼻咽癌面颈联合野和下颈切线野射野衔接处的剂量分布。应用Helax TMS-3D治疗计划系统，根据实际病例的螺旋CT模拟定位资料，在数字重建图像上设计面颈联合野和下颈切线野，分别计算半束和全束照射技术条件下脑组织和肺的受量，比较两者受照射的剂量体积直方图(DVH)及脑组织受照射的最大剂量、最小剂量、中位剂量、平均剂量、25％受较高照射剂量脑组织的下限剂量(D25％)以及受量超过75％肿瘤剂量的脑体积(V75%)。结果 两种照射技术射野衔接处均无漏照及低剂量区情况，半束照射和全束共线照射分别有约4mm、10mm的剂量重叠区，两野衔接处剂量最高点高出剂量归一点分别为28％和117％。两种照射技术脑及肺受照射的DVH相似，脑受照射的最小剂量、最大剂量、中位剂量、平均剂量、D25％、V75％及破裂孔处剂量均以全束照射略高，但相差幅度均不超过1％。结论 鼻咽癌应用面颈联合野和下颈切线野放射治疗时，与全束照射技术相比，半束照射技术照射野衔接处的剂量重叠区较小，脑和肺的照射体积和剂量没有增加。

Dosimetric study on half-beam irradiation in nasopharyngeal carcinoma

Objective To compare the dose distribution at the junction of abutting fields and dose in the involved brain and lung with half-beam or whole-beam irradiation in the treatment of nasopharyngeal carcinoma. Methods The dose distribution at the junction of the facio-neck field and inferior-neck-supraclavicular field was measured with film dosimetiy on solid-water phantom using Varian 600CD liner accelerator. The facio-neck field and inferior-neck-supraclavicular field were set on the digitally reconstructed radiograph based on spiral CT scanning. The doses of brain and lung derived from both radiation techniques were calculated with Helax TMS-3D Treatment Planning System. The dose-volume-histograms (DVH) of each organ by different irradiation techniques were compared. The following parameters of brain, such as the maximum dose, minimum dose, median dose, mean dose, D25% (the lowest dose delivered to 25% of the hottest brain volume) and V75% (the volume of the brain receiving more than 75 % tumor dose), were also calculated for comparison analysis. Results There was no underdose at the match-plane no matter half- or whole-beam radiation. An overdose region about 4 mm on the match-plane of the portals was found in the half-beam technique and 10 mm in the whole-beam technique. The maximum doses at the portal junction were 28% and 117% over the reference dose, respectively. The DVHs of the brain and lung by two techniques were similar. The minimum dose, maximum dose, median dose, mean dose, D25% V75% and the dose to the foramen lacerum were a little higher in the whole-beam technique than in the half-beam technique, with disparities no more than 1 % . Conclusions When radiotherapy is practised with the facio-neck field and inferior-neck-supraclavicular field by the half-beam technique, it is confirmed that the dose-overlap region at the match-plane is smaller than by the whole-beam technique, without increasing radiation volume and dose in the brain and lung.