射波刀追踪治疗中组织密度不均匀对吸收剂量影响的评估

Effect of density heterogeneity on absorbed dose with CyberKnife Synchrony Respiratory Tracking System

目的 应用射波刀的Synchrony同步呼吸追踪系统对呼吸运动模体进行追踪照射并测量模体实际吸收剂量,研究射线入射时组织密度不均匀对肿瘤GTV吸收剂量的影响并加以评估。方法 将模体分为9组,分别模拟不同厚度（0、20、50 mm）等效肋骨和不同振幅（0、10、15 mm）肿瘤运动。将9组模体分别在4DCT的不同时相上移植静态CT计划,保持射野方向和跳数不变,计算4D累积剂量。静态和4D计划均采用Ray-tracing 和 Monte Carlo两种算法计算,并实测得到9组模体GTV的吸收剂量。结果 随着模拟肋骨厚度的增加和运动振幅的增大,测得的GTV剂量逐渐减小,GTV计划剂量和实测剂量的偏差变大。结论 组织密度不均匀对GTV的吸收剂量有影响。在组织密度较小和肿瘤运动振幅较小时,静态CT计划和4DCT计划均可评估出GTV吸收剂量;而对于组织密度较大或肿瘤运动振幅较大时,Monte Carlo算法的4DCT计划可较好地评估出GTV吸收剂量（偏差均〈3%）。

Objective To measure the actual absorbed dose of the target in the QUASAR Respiratory Motion Phantom using the CyberKnife Synchrony Respiratory Tracking System, and to evaluatethe effect of density heterogeneity on the absorbed dose of tumor gross target volume （GTV）. Methods Nine groups were obtained by making different patterns of QUASAR phantom：rib thickness of 0, 20, and 50 mm, and motion amplitudes of 0, 10, and 15 mm. The nine groups were treated with static computed tomography （CT） in different time phases of four-dimensional CT （4DCT） plan, with the same beam and number of monitor units, and the 4D accumulated dose was calculated. The doses of static and 4D plans were calculated using Ray-tracing and Monte Carlo algorithms, and the absorbed doses of GTV in the nine groups were measured at the same time. Results There were a decrease in calculated absorbed dose of GTV and an increase in deviation between the planned and actual dose, with the increases in simulated rib thickness and motion amplitude. Conclusions The density heterogeneity has an impact on the absorbed dose of GTV. Both static CT and 4DCT plan can evaluate the absorbed dose of GTV in case of small rib thickness and motion amplitude, and 4DCT plan with Monte Carlo algorithm may be the optimal method for evaluation of the absorbed dose of GTV in case of large rib thickness and motion amplitude （deviation〈3%） .