Automated VMAT planning for postoperative adjuvant treatment of advanced gastric cancer

Background: Postoperative/adjuvant radiotherapy of advanced gastric cancer involves a large planning target volume (PTV) with multi-concave shapes which presents a challenge for volumetric modulated arc therapy (VMAT) planning. This study investigates the advantages of automated VMAT planning for this site compared to manual VMAT planning by expert planners. Methods: For 20 gastric cancer patients in the postoperative/adjuvant setting, dual-arc VMAT plans were generated using fully automated multi-criterial treatment planning (autoVMAT), and compared to manually generated VMAT plans (manVMAT). Both automated and manual plans were created to deliver a median dose of 45 Gy to the PTV using identical planning and segmentation parameters. Plans were evaluated by two expert radiation oncologists for clinical acceptability. AutoVMAT and manVMAT plans were also compared based on dose-volume histogram (DVH) and predicted normal tissue complication probability (NTCP) analysis. Results: Both manVMAT and autoVMAT plans were considered clinically acceptable. Target coverage was similar (manVMAT: 96.6±1.6%, autoVMAT: 97.4±1.0%, p=0.085). With autoVMAT, median kidney dose was reduced on average by >25%; (for left kidney from 11.3±2.1 Gy to 8.9±3.5 Gy (p=0.002); for right kidney from 9.2±2.2 Gy to 6.1±1.3 Gy (p< 0.001)). Median dose to the liver was lower as well (18.8±2.3 Gy vs. 17.1±3.6 Gy, p=0.048). In addition, Dmax of the spinal cord was significantly reduced (38.3±3.7 Gy vs. 31.6±2.6 Gy, p< 0.001). Substantial improvements in dose conformity and integral dose were achieved with autoVMAT plans (4.2% and 9.1%, respectively; p< 0.001). Due to the better OAR sparing in the autoVMAT plans compared to manVMAT plans, the predicted NTCPs for the left and right kidney and the liver-PTV were significantly reduced by 11.3%, 12.8%, 7%, respectively (p≤0.001). Delivery time and total number of monitor units were increased in autoVMAT plans (from 168±19 s to 207±26 s, p=0.006) and (from 781±168 MU to 1001±134 MU, p=0.003), respectively. Conclusions: For postoperative/adjuvant radiotherapy of advanced gastric cancer, involving a complex target shape, automated VMAT planning is feasible and can substantially reduce the dose to the kidneys and the liver, without compromising the target dose delivery.