金标伪影对CyberKnife射野剂量学影响的计算与测量

目的:分析植入的金标在CT扫描时产生的金属伪影对Cyber Knife射野的剂量学影响。方法:用CT模拟机对固体水进行扫描以获取金标不同深度位置的CT图像,在计划系统中,分别制作以电离室收集极为等中心的单射野照射计划,并将相应计划移植到指定金标伪影密度覆盖值的CT图像并执行计划进行测量。结果:金标的植入对等中心剂量的影响在0.5%左右,而伪影对剂量及相同深度离轴点处吸收剂量影响达3%,并随金标植入深度的增加而降低;给伪影指定密度覆盖值后,其对剂量的影响可降低至1%;金标伪影使得金标后方的百分深度剂量(percentage depth dose,PDD)有明显跌落区,伪影密度值的覆盖可以减少对PDD的影响;金标对偏心比(off-center ratio,OCR)的影响较小,但使OCR曲线相对稍加平坦。结论:通过伪影覆盖可有效降低金标伪影对剂量的影响。

Calculation and measurement of effect of gold marker artifact on Cyber Knife field dosimetry

Objective To analyze the influences of the gold marker artifact on Cyber Knife field dosimetry during CT scanning. Methods Solid water was scanned with CT simulator to obtain CT images with the gold marker at different depths.In the planning system, some single field radiation scheme was established with ionization chamber collector as the isocenter, and then the scheme was applied to the CT images with the specified artifact density for measurement. Results The gold marker had influences on the isocenter dose by 0.5%, and the artifact had influences on the dose and the offaxis absorbed dose at the same depth by 3%, which increased with the depth of the gold marker. When gifted with the fixed overlay for the artifact density, the artifact had influences on the dose by 1%. The gold marker artifact resulted in an apparent drop zone for the percentage depth dose（PDD） behind the marker, which could be weakened by the overlay of the artifact density. The marker made OCR curve flattened with few influences on it. Conclusion The overlay for the artifact decreases the effects of the gold marker artifact on the dose.