Hippocampus-sparing whole-brain radiotherapy: Dosimetric comparison between non-coplanar and coplanar planning

Background: To compare non-coplanar and coplanar volumetric-modulated arc therapy (VMAT) for hippocampal avoidance during whole-brain radiotherapy (HA-WBRT) using the Elekta Synergy and Pinnacle treatment planning system (TPS) according to the suggested criteria of the radiation therapy oncology group (RTOG) 0933 trial. Methods: Nine patients who underwent WBRT were selected for this retrospective study. The hippocampus was contoured, and the hippocampal avoidance regions were created using a 5-mm volumetric expansion around the hippocampus for each patient. Non-coplanar and coplanar VMAT plans were generated for each patient. All treatment plans were generated for a prescribed dose (PD) of 30 Gy in 10 fractions. Results: The average volumes of the hippocampus and hippocampal avoidance region were 2.8±0.38 and 27±2.48 cm³, respectively. For coplanar and non-coplanar VMAT plans, the average D_100% of the hippocampus was 8.60 Gy (range, 8.30-8.80 Gy) and 8.56 Gy (range, 8.30-8.90 Gy), respectively, and the average D_max of the hippocampus was 15.29 Gy (range, 14.35-15.92 Gy) and 14.99 Gy (range, 13.80-15.83 Gy), respectively. The non-coplanar VMAT plans showed a significantly lower average D_max of the lens (4.23 Gy) than did the coplanar VMAT plans (4.77 Gy). The average gamma passing rate for non-coplanar and coplanar VMAT quality assurance (QA) with criteria of 3%/3 mm were 95.4%±2.6% and 95.6%±1.6%, indicating good agreement between the calculated plan dose and the measured dose. Conclusions: We showed that the suggested criteria of the RTOG 0933 trial for the hippocampal dose can be achieved in both coplanar and non-planar VMAT plans. We performed VMAT QA for each treatment plan to verify the clinical feasibility.