Geometrical tracking accuracy and appropriate PTV margins for robotic radiosurgery of liver lesions by SBRT

<b>Purpose:</b> To assess the geometrical accuracy and estimate adequate PTV margins for liver treatments using the Synchrony respiratory tracking system. <b>Material and methods</b>: Treatment log files are analyzed for 72 liver patients to assess tracking accuracy. The tracking error is calculated as the quadratic sum of the correlation, the predictor and the beam positioning errors. Treatment target rotations and rigid body errors reported by the system are also evaluated. The impact of uncorrected rotations is assessed by rotating the planned dose distribution and reassessing target coverage. Total PTV margins are estimated by summing in quadrature tracking errors and rigid body errors. Relationships are explored between tracking errors, model linearity and motion amplitudes of internal and external markers. <b>Results</b>: Margins of 3, 2, 2 mm in SUP-INF, LT-RT and ANT-POST directions, respectively, are sufficient to account for tracking and beam positioning errors for 95% of patients. If rigid body error is also considered, margins increase to 4 mm isotropic. Rotations could not be corrected for 92% of patients due to imperfect fiducial implantation and limitations in the magnitude of corrections that the system can apply. Uncorrected rotations would lead to average estimated dose reductions of 2.7% ± 5.8% of the prescribed dose for D99 of GTVs (5 mm PTV expansion) in which the target was well covered in the original plan (28 of 31 GTVs). 80% of tracking models exhibit near linear correlation between internal and external marker motions with small tracking errors (&lt;2.2 mm). <b>Conclusions</b>: Isotropic PTV margins considering tracking errors and target rigid body errors could be used for liver SBRT treatments if rotational corrections can be calculated accurately so that systematic rotational offsets can be avoided. The linearity of the internal and external breathing motions might be useful for other types of treatment modalities for liver cancer.<b>研究目的：</b> 评估使用Synchrony呼吸追踪系统（Synchrony respiratory tracking system）开展肝脏放疗时的几何精度，并估算适宜的计划靶区（Planning Target Volume, PTV）外放边界。<b>材料与方法：</b> 本研究对72例肝脏肿瘤患者的治疗日志文件进行分析，以评估追踪精度。追踪误差通过相关性误差、预测误差与射束定位误差的平方和计算得出。同时对系统记录的治疗靶区旋转误差与刚体误差进行评估。通过旋转计划剂量分布并重新评估靶区覆盖度，分析未校正旋转带来的影响。总PTV外放边界通过将追踪误差与刚体误差进行平方和求和估算得到。本研究还探究了追踪误差、模型线性度与内外标记物运动幅度之间的关联。<b>研究结果：</b> 在头足（Superior-Inferior, SUP-INF）、左右（Left-Right, LT-RT）与前后（Anterior-Posterior, ANT-POST）方向上，分别采用3mm、2mm、2mm的外放边界，即可覆盖95%患者的追踪与射束定位误差。若同时考虑刚体误差，则各向同性外放边界需增至4mm。由于标记物植入不精准，且系统可施加的校正幅度存在限制，92%的患者无法完成旋转校正。对于采用5mm PTV外放边界的31个大体肿瘤靶区（Gross Tumor Volume, GTV），其中初始计划中靶区覆盖良好的28个靶区，未校正旋转将导致其D99剂量平均较处方剂量降低2.7%±5.8%。80%的追踪模型在内外标记物运动间呈现近线性相关，且追踪误差较小（＜2.2mm）。<b>研究结论：</b> 若可精准计算旋转校正以避免系统性旋转偏移，那么兼顾追踪误差与靶区刚体误差的各向同性PTV外放边界，可应用于肝脏立体定向体部放疗（Stereotactic Body Radiation Therapy, SBRT）。内外呼吸运动的线性相关性，或许可推广应用于肝癌的其他放疗技术中。