放疗中呼吸运动对不同形状射野剂量分布的影响

Effect of respiration on the radiation dose distribution within target volume in radiotherapy

目的利用剂量胶片分析研究放射治疗中呼吸运动对靶区剂量分布的影响。方法应用可内置胶片的QUASAR多功能呼吸运动体模，在运动和静止状态下分别照射正方形、圆形、椭圆形、哑铃形和凹形5种形状的模拟射野。比较其剂量分布的差别。用平板体模在位移为0、0.5、1.0、1.5和2.0cm时分别照射圆形、椭圆形、正方形射野，比较不同靶区位移大小对剂量分布的影响。应用的比较方法包括等剂量线、γ值和NAT比较法。提出面积变化因子Fs（运动状态下面积／静止状态面积或位移不为0时的面积／位移为0时的面积。以Fs90、Fs50、Fs25分别代表90％、50％、25％剂量曲线包围的面积在不同状态下的比值）。结果与静态下相比，水平运动状态下的Fs90减小，Fs25增大。位移越大，它们偏离程度越大。垂直运动状态，正方形和哑铃形射野的Fs有变化，其余变化很小。γ值和NAT比较：各射野的水平运动状态和静止状态比较，Pγ〈60％和PNAT〈75％；正方形、圆形、凹形和哑铃形照射野的垂直运动状态与静态下相比，Pr〈85％；平板体模验证中，Pr和PNAT随着位移的增大而减小。结论呼吸运动对靶区放射治疗的剂量分布的影响表现为沿运动方向高剂量区域内收，低剂量区扩大，这种影响随着肿瘤位移的增大而增大。

Objective To evaluate the influence of respiration on the radiation dose distribution within target volume in radiotherapy with film dosimetry. Methods Radiation of 50 MU was delivered by a square, round, ellipse, dumb bell, or female shaped filed to the films within a moving or motionless Respiration Motion Phantom respectively, the dose distributions for the two motion status were measured and compared. In order to further verify the impact of respiration, a plank phantom was used on different shifting value： 0, 0.5, 1.0, 1.5 and 2.0 cm, respectively. A square, round, or ellipse-shaped filed was used for irradiation and the distributions in different status were measured and compared with film dosimetry. Iso-dose line comparison, NAT（Normalized Agreement Tests） and γ comparison were used for the comparison of dose distributions. Fs can be an index to reflect the variability of the areas that surrounded by iso-dose lines. （ Fs90, Fs50, Fs25 delegates the ratio of the areas that surrounded by 90%, 50%, 25% iso-dose line in different situation respectively）. Results （1） Compared with motionless situation, the Fs90 in horizontal movement situation became small and the Fs25 became large. As the displacement became larger, the Fs90 became larger and the Fs25 became smaller. Fs in vertical movement situation, square and dumb bell fields changed while the others didn＇t have a change. （2）γ and NAT comparison： In the horizontal movement situation, compared with the static phantom, Pr 〈 60% and PNAT〈 75 %. Under vertical movement situation, Pr were less than 85 % for the square, round, dumb bell and female shaped fileds. In the plank phantom verification, Pr and PNAT became smaller as the movement became larger. Conclusions The respiration can impact on the dose distribution within the target volume in radiotherapy, leading to a smaller area of higher dose level and an expanded area of lower dose level. The influence will become more significant with larger movement of the target.