Program Information
Pre-Planning Dry Run for Lung Stereotactic Body Radiation Therapy
P Patel*, K Higgins, A Dhabaan, B Ghavidel, M McDonald, J Roper, Winship Cancer Institute of Emory University, Atlanta, Georgia
Presentations
TH-CD-205-7 (Thursday, August 3, 2017) 10:00 AM - 12:00 PM Room: 205
Purpose: Compared with coplanar (CP) beam arrangements, non-coplanar (NCP) plans may improve dose distributions in stereotactic body radiotherapy (SBRT), although treatment times and patient-machine collisions may be challenges. We evaluated a method to determine a collision-free zone for NCP volumetric modulated arcs and the impact of NCP plans on dosimetry and treatment time.
Methods: Following CT simulation, pre-planning “dry runs” were performed for 11 lung SBRT patients. To determine a collision-free zone, patients were immobilized and aligned to isocenter, and the gantry and couch were rotated about isocenter, noting the position of collisions plus a 6 cm buffer. NCP plans used up to 6 arcs spaced throughout the collision-free zone. Comparative CP plans used 3 half circular arcs. The median prescription was 50 Gy in 5 fractions (range 50-54 in 3-6 fractions). Dose metrics were compared and evaluated for statistically significant differences using the Wilcoxon signed rank test. Treatment delivery time was recorded.
Results: NCP and CP plans had equivalent CI. Compared to CP plans, NCP plans achieved a lower median R50 (4.5 vs 4.2), D2cm (43.8% vs 36.8%), and D4cm (32.9% vs 24.2%); all p<0.01. NCP plans achieved an average reduction in the maximum doses to the heart and spinal cord of 1.6 Gy (p<0.05). NCP plans achieved lower lung V20 (1.7% vs 1.4%) and V10 (4.4% vs 4.0%) but higher V5 (9.0 vs 9.7%) and mean lung dose (1.7 vs 1.8 Gy); all p<0 .01, except V5 p=0.17. Couch rotations added an average of 2.5 minutes to the delivery time of NCP plans. No cases required replanning due to collisions.
Conclusion: Preplanning assessment of a collision-free zone facilitated the development of NCP SBRT plans, which resulted in more compact dose distributions with reduced low-dose spillage with the tradeoff of modest increases in mean lung dose and delivery time.
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