鼻咽癌调强放疗射野与下颈野衔接的研究

Research on the Match between the Fields of IMRT and the Anterior Field for Lower Neck in Nasopharyngeal Carcinoma

目的探讨鼻咽癌调强放疗射野与下颈野的最佳衔接方法。方法采用两种方法设下颈野,保证下颈部淋巴引流区[nl(r)-ptv]每次得到180 cGy的照射。方法A:只设1个常规下颈野T,6MVX线,SSD=100 cm,中心用挡铅保护脊髓,保证下颈部淋巴nl(r)-ptv的单次剂量D95≥180 cGy;方法B:先设一与方法A相同的下颈野T1,拷贝T1野生成T2野,缩小T2野上界,单次输出量(MU)与方法A相同,按需要分配给T1野和T2野。比较两种设野方法与上颈部的调强治疗计划衔接处照射剂量的区别。结果方法A nl(r)-ptv的最大剂量分别达到处方剂量的198%和193%,存在明显的高剂量区,方法B最大剂量分别只有处方剂量的122%和125%,降低了高剂量区的范围。结论由于调强放疗射野的不均匀性,造成方法A存在明显的高剂量区,方法B利用正向调强的原理,在不增加工作量和操作难度的前提下很好地解决了鼻咽癌调强放疗射野与下颈野衔接的问题。

Objective To achieve a perfect transverse plane match between the fields of IMRT and anterior field for lower neck in patients with nasopharyngeal carcinoma（NPC）. Methods Two techniques was used for the design of the anterior field for lower neck. Dose prescription was 180 cGy for the region of lymph drain of lower neck[ nl （r） -ptv ]. The technique A：A single anterior field （T） was used with a center block to protect cord. The technique B：Two anterior fields T1 and T2 was used. T1 was the same as T, and T2 was copied from T1 with some shrinkage in the superior margin. The appropriate weight for T1 and T2 was used so as the MU of the two techniques was the same. Results The high dose of nl（r） -ptv was 198% and 193% for the technique A, 122% and 125% for the technique B ,respectively. The high dose area of technique B was significantly smaller tha.n that of technique A. Conclusion Dose distribution in technique A is inhomogeneous. Better transverse plane match could be＇ achieved by technique B in the treatment of patients with nasopharyngeal carcinoma when IMRT is used.