Reiner Béatrice, Bownes Peter, Buckley David L, Thwaites David I
Division of Biomedical Imaging, University of Leeds, Leeds, LS2 9JT, UK.
Radio-Onkologie, Kantonsspital Graubünden, CH-7000 Chur, Switzerland.
J Radiosurg SBRT. 2017;4(4):275-287.
Besides conventional single fraction stereotactic radiosurgery (SRS), the newer Gamma Knife® (Elekta Instruments, AB, Sweden) (GK) models, Perfexion™ and Icon™, also allow the option of hypofractionated SRS, to widen the treatment range, reduce toxicity and provide flexibility. With fractionation, set-up uncertainties are introduced to the treatment. The question of what margin is required to cover set-up uncertainties arises. In this work, the characteristics of positional uncertainties and margins required to ensure coverage are evaluated. An alternative strategy on how to deal with set-up errors is suggested.
Five large single metastases have been considered to simulate the dosimetric effects of set-up uncertainties. Since random error would not create a representative average for three or five fractions a "worst case scenario" with maximal displacement in different directions has been evaluated. Displacements of 0.5, 1.0, 2.0, 3.0 and 4.0 mm have been considered. Three fraction treatments were simulated with displacements in X, Y and Z directions (3F) and in ±X and Z directions (3F). For five fractions, the displacements were ±X, ±Y and Z (5F) resulting in the largest possible spread of the dose distribution. Plans were evaluated by considering the volumes of the reference dose and the 50% isodose, as well as the Paddick conformity index (PCI), the gradient index (GI) and coverage. Based on the change in coverage a margin was evaluated that would have prevented underdosage of the target.
Two fractions displaced up to ±2 mm in opposing directions almost cancel each other out. Minimum dose was reduced by 10% (3F), 4% (3F) and 1% (5F). The main contribution to the underdosage was due to the small systematic error (shift) included in the simulated displacements.The margin required to keep the coverage of the target comparable with the original plan would be 0.5 mm for the 3F plan, 0.2 mm for the 3F plan and 0.0 mm for the 5F for a set-up error of 2 mm.
For fractionated GK treatment of large brain metastases prescribing to the 50% isodose minimizes the effect of random error.For conventional single fraction SRS, the effect of positional uncertainty is always systematic. However, for fractionated treatments, positional uncertainty can be separated into random and systematic components, where random displacements in opposing directions at least partially cancel each other out and thus reduce the overall systematic effect (i.e. from the residual systematic uncertainty). Therefore fractionated schemes with more fractions are preferable from the point of view of accounting for positional uncertainties. If the random and systematic uncertainties can be assessed and determined separately by imaging, then the residual systematic value can be corrected for the remaining fractions.
除了传统的单次分割立体定向放射外科治疗（SRS）外，新型伽玛刀（瑞典医科达公司）Perfexion™和Icon™型号还允许进行低分割SRS，以扩大治疗范围、降低毒性并提供灵活性。采用分割治疗时，治疗会引入摆位不确定性。由此产生了需要多大边界来涵盖摆位不确定性的问题。在本研究中，评估了位置不确定性的特征以及确保覆盖所需的边界。并提出了一种处理摆位误差的替代策略。
考虑了五个大的单发转移瘤来模拟摆位不确定性的剂量学效应。由于随机误差不会为三或五次分割产生具有代表性的平均值，因此评估了不同方向上最大位移的“最坏情况”。考虑了0.5、1.0、2.0、3.0和4.0毫米的位移。模拟了在X、Y和Z方向（3F）以及±X和Z方向（3F）上的三次分割治疗。对于五次分割，位移为±X、±Y和Z（5F），导致剂量分布的最大可能扩散。通过考虑参考剂量体积、50%等剂量线体积、帕迪克适形指数（PCI）、梯度指数（GI）和覆盖范围来评估计划。根据覆盖范围的变化评估一个可防止靶区剂量不足的边界。
两次分割在相反方向上位移达±2毫米时几乎相互抵消。最小剂量降低了10%（3F）、4%（3F）和1%（5F）。剂量不足的主要原因是模拟位移中包含的小系统误差（偏移）。对于2毫米的摆位误差，3F计划保持靶区覆盖与原计划相当所需的边界为0.5毫米，3F计划为0.2毫米，5F计划为0.0毫米。
对于大的脑转移瘤的伽玛刀分割治疗，处方至50%等剂量线可使随机误差的影响最小化。对于传统的单次分割SRS，位置不确定性的影响始终是系统性的。然而，对于分割治疗，位置不确定性可分为随机和系统成分，其中相反方向的随机位移至少部分相互抵消，从而降低整体系统效应（即来自残余系统不确定性）。因此，从考虑位置不确定性的角度来看，分割次数更多的分割方案更可取。如果可以通过成像分别评估和确定随机和系统不确定性，那么可以对剩余分割校正残余系统值。
