碳纤维体位固定板对椎体转移瘤放射治疗剂量的影响

Dosimetirc effects of immobilization baseplate of carbon fiber for vertebral metastases

目的研究碳纤维体位固定板的透射系数及对椎体转移瘤放射治疗剂量的影响。方法CT定位机下扫描碳纤维体位固定板,计算体位固定板在不同角度下的衰减系数,并与相同条件下的实测体位固定板衰减系数比较,分析放射治疗计划系统的准确性;另选择10例椎体转移瘤患者,其中男性5例,女性5例;年龄50~76岁,中位年龄65岁;病灶均位于双肾之间椎体上。在不勾画体位固定板的CT图像上设计无板计划,无板计划完成后将此计划移植到勾画体位固定板的CT图像上,并进行剂量计算,比较无板计划、有板计划靶区和危及器官(OAR)的剂量学差异。结果体位固定板在不同机架角度下的实际衰减系数为0.966~0.995,放射治疗计划系统计算得到的体位固定板的衰减系数与实际测量得到的衰减系数偏差在0.62%以内;对于椎体转移瘤计划,体位固定板勾画参与剂量计算后,靶区的受照剂量明显降低,平均降低2%左右。对于靶区的适形性指数(CI)、均匀性指数(HI),体位固定板的勾画会导致靶区的CI显著降低,差异有统计学意义(0.849±0.051 vs 0.760±0.037。P<0.05),而对HI影响不大,差异无统计学意义(P>0.05)。对OAR,有板计划的剂量体积(V_(10)、V_(20)、V_(30))、D_(mean)、D_(max)均比无板计划偏低,其中小肠、左肾、右肾的剂量体积减少百分点均在1%以内;脊髓D_(max)减少了0.87 cGy(2.3%),且差异均有统计学意义[(38.85±0.89)cGy vs(37.99±0.77)cGy。P<0.05]。结论碳纤维体位固定板对椎体转移瘤患者的放射治疗剂量有一定的影响,为确保靶区剂量准确性,建议在治疗计划设计时考虑体位固定板,使其参与剂量计算。

Objective To study transmission coefficients of carbon fiber immobilization baseplate and radiation dose of carbon fiber immobilization baseplate for treatment of vertebral metastases.Methods The carbon fiber immobilization baseplate was scanned under CT positioning machine.The attenuation coefficient of immobilization baseplate at different angles were calculated and compared with measured immobilization baseplate under the same conditions.The accuracy of radiotherapy planning system was further analyzed.Ten patients with vertebral metastases were enrolled,which included 5 males and 5 females,aged 50-76 years old with median age of 65 years old;the lesions were located on vertebral body between 2 kidneys.Firstly,plateless plan was designed on CT image without outline of immobilization baseplate.Then,it was transplanted to CT image with outline of immobilization baseplate,and dose calculation was performed to compare dosimetric differences of target area and organs at risk(OAR)between plateless plan and plate plan.Results The actual attenuation coefficient of immobilization baseplate at different rack angles was 0.966-0.995,the deviation of attenuation coefficient calculated with and without radiotherapy planning system was within 0.62%.For plan of vertebral metastases,the exposure dose of target area was significantly reduced about 2%after delineation of immobilization baseplate involved in dose calculation.For conformity index(CI)and homogeneity index(HI)of target area,the delineation of immobilization baseplate led to statistically significantly reduction in CI of target area(0.849±0.051 vs 0.760±0.037.P<0.05),but had no effect on HI(P>0.05).For 0AR,the dose volume(V_(10)、V_(20)、V_(30)),D_(mean)and D_(max)of plate plan were lower than those of plateless plane,and dose volume reduction percentage of small intestine,left kidney and right kidney was within 1%;D_(max) of spinal cord was reduced by 0.87 cGy(2.3%),and the difference was statistically significant[(38.85±0.89)cGy vs(37.99±0.77)cGy.P<0.05].Conclusion It is demonstrated that immobilization baseplate has impact on radiation dose of vertebral metastases.In order to ensure accuracy of target dose,the immobilization baseplate is recommended to consider in design of treatment and to participate in dose calculation.