Ohtakara Kazuhiro, Suzuki Kojiro
Department of Radiation Oncology, Kainan Hospital Aichi Prefectural Welfare Federation of Agricultural Cooperatives, Yatomi, JPN.
Department of Radiology, Aichi Medical University, Nagakute, JPN.
Cureus. 2024 Dec 24;16(12):e76325. doi: 10.7759/cureus.76325. eCollection 2024 Dec.
Purpose In linac-based stereotactic radiosurgery (SRS) utilizing a multileaf collimator (MLC) for brain metastases (BMs), a volumetric-modulated arc (VMA) technique is indispensable for generating a suitable dose distribution with efficient planning and delivery. However, the optimal calculation grid spacing (GS) and statistical uncertainty (SU) of the Monte Carlo algorithm for VMA optimization have yet to be determined. This planning study aimed to examine the impacts of GS and GU settings on VMA-based SRS planning and to find the optimal combination for templating. Materials and methods Thirty clinical BMs with a gross tumor volume (GTV) of 0.08-48.09 cc (median 9.81 cc) were included. The treatment platform included a 5-mm leaf-width MLC Agility (Elekta AB, Stockholm, Sweden) and a planning system Monaco (Elekta AB). The prescribed dose was uniformly assigned to the GTV , the minimum dose of GTV minus 0.01 cc, i.e., for GTV >0.20 cc or to the GTV for GTV ≤0.20 cc, to minimize the uncovered GTV to the equivalent of a 3 mm diameter lesion. Five combinations of GS and SU per plan were examined for 12 selected GTVs (median 17.41 cc): GS of 2 mm and SU of 3% (G2U3), 2 mm and 2% (G2U2), 2 mm and 1% (G2U1), 1 mm and 2% (G1U2), and 1 mm and 1% (G1U1). Otherwise, the same arc arrangement and optimization method were uniformly used to prioritize the GTV dose conformity and the steepness of the dose gradient outside the GTV without dose constraints inside the GTV boundary. Further comparisons were conducted using 30 GTVs between the two groups with the highest plan quality. Results The G2U3 and G2U2 resulted in the equivalent total calculation time (tCT) and exactly the same plan quality. The overall plan quality was significantly superior in the G1U2 and G1U1 than in the G2U1 and G2U2, although the tCT was significantly longer in the G1U1 and G1U2 than in the G2U1 and G2U2. In the comparison of the G1U2 and G1U1, the concentric lamellarity of dose gradients 2 mm outside and 2-4 mm inside the GTV boundary was significantly superior in the G1U1 than in the G1U2, while there was no significant difference in the other parameters. The tCT tended to be longer in the G1U1 than in the G1U2. Conclusions The initial settings of GS and SU have significant impacts on the plan quality and tCT. The settings with GS of 1 mm and SU of 1% per plan are recommended to create the most suitable dose distribution for single BMs, especially for irregularly shaped and/or large lesions, although the tCT is long. In addition to common evaluation metrics, the coverage values of 2 mm outside and 2-4 mm inside the GTV surface by the , the minimum dose to cover the irradiated isodose volume equivalent to each target volume, are valuable for in-depth plan comparison.
目的 在基于直线加速器的立体定向放射外科治疗（SRS）中，使用多叶准直器（MLC）治疗脑转移瘤（BMs）时，容积调强弧形（VMA）技术对于通过高效的计划制定和实施来生成合适的剂量分布必不可少。然而，用于VMA优化的蒙特卡罗算法的最佳计算网格间距（GS）和统计不确定性（SU）尚未确定。本计划研究旨在探讨GS和GU设置对基于VMA的SRS计划的影响，并找到用于模板的最佳组合。
材料和方法 纳入30例临床BMs，其肿瘤总体积（GTV）为0.08 - 48.09 cc（中位数9.81 cc）。治疗平台包括叶宽为5 mm的MLC Agility（瑞典斯德哥尔摩的医科达公司）和计划系统Monaco（医科达公司）。规定剂量均匀分配至GTV，对于GTV >0.20 cc，为GTV减去0.01 cc的最小剂量，即 ；对于GTV≤0.20 cc，为GTV ，以使未覆盖的GTV最小化至相当于直径3 mm的病变。针对12个选定的GTV（中位数17.41 cc），每个计划检查了GS和SU的五种组合：GS为2 mm且SU为3%（G2U3）、2 mm且2%（G2U2）、2 mm且1%（G2U1）、1 mm且2%（G1U2）以及1 mm且1%（G1U1）。否则，统一使用相同的弧形排列和优化方法，以优先考虑GTV剂量适形性以及GTV边界外剂量梯度的陡峭度，且GTV边界内无剂量约束。使用计划质量最高的两组中的30个GTV进行进一步比较。
结果 G2U3和G2U2产生了等效的总计算时间（tCT）且计划质量完全相同。G1U2和G1U1的总体计划质量明显优于G2U1和G2U2，尽管G1U1和G1U2的tCT明显长于G2U1和G2U2。在G1U2和G1U1的比较中，GTV边界外2 mm和边界内2 - 4 mm处剂量梯度的同心层状性在G1U1中明显优于G1U2，而其他参数无显著差异。G1U1的tCT往往比G1U2更长。
结论 GS和SU的初始设置对计划质量和tCT有显著影响。建议每个计划设置GS为1 mm且SU为1%，以创建最适合单个BM的剂量分布，特别是对于形状不规则和/或较大的病变，尽管tCT较长。除了常用的评估指标外，GTV表面外2 mm和表面内2 - 4 mm处的覆盖值，即覆盖与每个靶体积等效的照射等剂量体积的最小剂量，对于深入的计划比较很有价值。
