金属食管支架对放射治疗剂量分布的影响

Impact of esophageal self-expandable metallic stent on radiotherapeutic dosage

目的 测量网状自扩金属食管支架对放射线引起的空腔效应及散射效应对放射治疗剂量分布的影响,为食管癌支架置入术后放射治疗的剂量修正提供依据。方法 应用模拟食管癌网状自扩金属支架置入术后放射治疗的体模,分别应用^(60)Co γ射线和直线加速器的8MV X射线进行照射,使用薄窗电离室、热释光剂量仪分别对不锈钢合金支架及钛镍合金支架空腔的界面及界面下一定深度进行了对比测量,并用治疗计划系统对单纯空腔情况下百分深度剂量的变化进行了模拟计算与测量结果进行对照。结果 射野7cm×15cm ^(60)Co治疗机照射支架前点、后点与无支架均匀水模对应点剂量增加值:不锈钢支架分别为1.8％和3.2％,钛镍合金支架分别为1.7％和2.9％。直线加速器的8MV X射线照射支架前点、后点与无支架均匀水模对应点剂量增加值:不锈钢支架分别为1.5％和2.8％,钛镍合金支架分别为1.4％和0.9％。射线经过支架空腔后形成二次建成效应,剂量增加的峰值达7.6％。结论 网状金属食管支架对放射线的散射效应造成的剂量增加<2％,支架空腔形成的二次建成效应,剂量增加>5％。建议实际放射治疗时须对支架的空腔效应修正计算剂量。

Objective Objective To determine the cavity and scattering effects of implanted esophageal self expandable metallic net- stent on radiation dose distribution, and to define the mathematical correction parameters for post-implantation irradiation dose calculation. Methods An esophageal self-expandable net-stent was set into the heterogeneous lesion imitation in the water phantom and irradiated by 60Co r-ray or 8 MV X-ray. The ionization measurement was performed with the thin window plate parallel chamber. Thermoluminiscent chips (TLD) were set in front of and behind the air cavity formed by the stent net. Dose distributions derived from two different stent materials, stainless steel and Titanium-Nickel alloy, were compared. The percentage depth dose (PDD) from practical measurement was also compared with the results calculated from the treatment planning system. Results For 7 cm x 15 cm field, the PDD increments with 60Co r-ray were 1.8% at the anterior point and 3.2% at the posterior point for stainless steel stent, while they were 1.7% and 2.9% for Titanium-Nickel alloy stent. If 8 MV X- ray was delivered, the increments were 1.5% , 2.8% for the former stent, while 1.4% and 0.9% for the latter. The peak increment of 7.6% PDD was found beyond the stent cavity due to the second build-up effect derived from the net stent. Conclusions The domestric change caused by the scattering effect of esophageal metallic net-stent is less than 2.0% , and the second build-up caused by the cavity effect gives a PDD increment over 5.0%. These results suggest that correction of dose distribution is required for radiotherapy of patients with stents.