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Optimization of leaf margins for lung stereotactic body radiotherapy using a flattening filter‐free beam
Author(s) -
Wakai Nobuhide,
Sumida Iori,
Otani Yuki,
Suzuki Osamu,
Seo Yuji,
Isohashi Fumiaki,
Yoshioka Yasuo,
Hasegawa Masatoshi,
Ogawa Kazuhiko
Publication year - 2015
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4916683
Subject(s) - stereotactic radiotherapy , flattening , radiation therapy , dosimetry , radiosurgery , beam (structure) , nuclear medicine , medicine , medical physics , radiology , optics , physics , astronomy
Purpose: 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. Methods: 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 D 95. For PTV, the heterogeneity index (HI), conformity index, modified gradient index (GI), defined as the 50% isodose volume divided by target volume, maximum dose ( D max), and mean dose ( D mean) were calculated. Mean lung dose (MLD), V 20 Gy, and V 5 Gy for the lung (defined as the volumes of lung receiving at least 20 and 5 Gy), mean heart dose, and D max to the spinal cord were measured as doses to organs at risk (OARs). Paired t ‐tests were used for statistical analysis. Results: 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. D max 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, D max, 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, V 20 Gy, and V 5 Gy for lung between FFF and FF beams were negligible. The optimal leaf margins for MLD, V 20 Gy, and V 5 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. D max to the spinal cord showed no clear trend for changes in leaf margin. Conclusions: The differences in doses to OARs between FFF and FF beams were negligible. Conformity index, modified GI, MLD, lung V 20 Gy, lung V 5 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.