Wakai Nobuhide, Sumida Iori, Otani Yuki, Suzuki Osamu, Seo Yuji, Isohashi Fumiaki, Yoshioka Yasuo, Hasegawa Masatoshi, Ogawa Kazuhiko
Department of Radiation Oncology, Osaka University Graduate School of Medicine, Suita, Osaka 565-0871, Japan and Department of Radiation Oncology, Nara Medical University, Kashihara, Nara 634-8522, Japan.
Department of Radiation Oncology, Osaka University Graduate School of Medicine, Suita, Osaka 565-0871, Japan.
Med Phys. 2015 May;42(5):2125-31. doi: 10.1118/1.4916683.
The authors sought to determine the optimal collimator leaf margins which minimize normal tissue dose while achieving high conformity and to evaluate differences between the use of a flattening filter-free (FFF) beam and a flattening-filtered (FF) beam.
Sixteen lung cancer patients scheduled for stereotactic body radiotherapy underwent treatment planning for a 7 MV FFF and a 6 MV FF beams to the planning target volume (PTV) with a range of leaf margins (-3 to 3 mm). Forty grays per four fractions were prescribed as a PTV D95. For PTV, the heterogeneity index (HI), conformity index, modified gradient index (GI), defined as the 50% isodose volume divided by target volume, maximum dose (Dmax), and mean dose (D mean) were calculated. Mean lung dose (MLD), V20 Gy, and V5 Gy for the lung (defined as the volumes of lung receiving at least 20 and 5 Gy), mean heart dose, and Dmax to the spinal cord were measured as doses to organs at risk (OARs). Paired t-tests were used for statistical analysis.
HI was inversely related to changes in leaf margin. Conformity index and modified GI initially decreased as leaf margin width increased. After reaching a minimum, the two values then increased as leaf margin increased ("V" shape). The optimal leaf margins for conformity index and modified GI were -1.1 ± 0.3 mm (mean ± 1 SD) and -0.2 ± 0.9 mm, respectively, for 7 MV FFF compared to -1.0 ± 0.4 and -0.3 ± 0.9 mm, respectively, for 6 MV FF. Dmax and D mean for 7 MV FFF were higher than those for 6 MV FF by 3.6% and 1.7%, respectively. There was a positive correlation between the ratios of HI, Dmax, and D mean for 7 MV FFF to those for 6 MV FF and PTV size (R = 0.767, 0.809, and 0.643, respectively). The differences in MLD, V20 Gy, and V5 Gy for lung between FFF and FF beams were negligible. The optimal leaf margins for MLD, V20 Gy, and V5 Gy for lung were -0.9 ± 0.6, -1.1 ± 0.8, and -2.1 ± 1.2 mm, respectively, for 7 MV FFF compared to -0.9 ± 0.6, -1.1 ± 0.8, and -2.2 ± 1.3 mm, respectively, for 6 MV FF. With the heart inside the radiation field, the mean heart dose showed a V-shaped relationship with leaf margins. The optimal leaf margins were -1.0 ± 0.6 mm for both beams. Dmax to the spinal cord showed no clear trend for changes in leaf margin.
The differences in doses to OARs between FFF and FF beams were negligible. Conformity index, modified GI, MLD, lung V20 Gy, lung V5 Gy, and mean heart dose showed a V-shaped relationship with leaf margins. There were no significant differences in optimal leaf margins to minimize these parameters between both FFF and FF beams. The authors' results suggest that a leaf margin of -1 mm achieves high conformity and minimizes doses to OARs for both FFF and FF beams.
作者试图确定在实现高适形性的同时使正常组织剂量最小化的最佳准直器叶片边缘，并评估无均整器（FFF）束流和有均整器（FF）束流使用之间的差异。
16例计划接受立体定向体部放射治疗的肺癌患者接受了针对计划靶区（PTV）的7 MV FFF束流和6 MV FF束流的治疗计划，叶片边缘范围为-3至3 mm。规定PTV的D95为每四分次40 Gy。对于PTV，计算了异质性指数（HI）、适形指数、修正梯度指数（GI，定义为50%等剂量体积除以靶区体积）、最大剂量（Dmax）和平均剂量（D mean）。测量了肺的平均剂量（MLD）、V20 Gy和V5 Gy（定义为接受至少20 Gy和5 Gy照射的肺体积）、平均心脏剂量以及脊髓的Dmax，作为危及器官（OAR）的剂量。采用配对t检验进行统计分析。
HI与叶片边缘的变化呈负相关。适形指数和修正GI最初随着叶片边缘宽度的增加而降低。达到最小值后，这两个值随后随着叶片边缘的增加而增加（“V”形）。7 MV FFF的适形指数和修正GI的最佳叶片边缘分别为-1.1±0.3 mm（均值±1标准差）和-0.2±0.9 mm，而6 MV FF的分别为-1.0±0.4和-0.3±0.9 mm。7 MV FFF的Dmax和D mean分别比6 MV FF高3.6%和1.7%。7 MV FFF与6 MV FF的HI、Dmax和D mean的比值与PTV大小之间存在正相关（R分别为0.767、0.809和0.643）。FFF束流和FF束流之间肺的MLD、V20 Gy和V5 Gy的差异可忽略不计。7 MV FFF的肺MLD、V20 Gy和V5 Gy的最佳叶片边缘分别为-0.9±0.6、-1.1±0.8和-2.1±1.2 mm，而6 MV FF的分别为-0.9±0.6、-1.1±0.8和-2.2±1.3 mm。当心脏位于辐射野内时，平均心脏剂量与叶片边缘呈V形关系。两种束流的最佳叶片边缘均为-1.0±0.6 mm。脊髓的Dmax在叶片边缘变化方面无明显趋势。
FFF束流和FF束流对OAR的剂量差异可忽略不计。适形指数、修正GI、MLD、肺V20 Gy、肺V5 Gy和平均心脏剂量与叶片边缘呈V形关系。在使这些参数最小化的最佳叶片边缘方面，FFF束流和FF束流之间无显著差异。作者的结果表明，-1 mm的叶片边缘可实现高适形性，并使FFF束流和FF束流对OAR的剂量最小化。
