Room: Stars at Night Ballroom 1
Purpose: The Precision TPS (Accuray Inc, Sunnyvale, USA) for Cyberknife treatment planning was recently extended with the VOLO optimizer. The aim of this study was to compare this optimizer with the Sequential optimizer currently used in the clinic. These optimizers are highly different in optimization approach.
Methods: VOLO plans were generated for 24 prostate SBRT patients, previously treated with an MLC-based plan created with the Sequential optimizer. All patients received 4x9.5Gy using a heterogeneous PTV dose distribution mimicking brachytherapy. VOLO plans were generated with the same clinical aims as used for generating the Sequential plans. All plans were normalized to the same PTV coverage, i.e. 95% of PTV receiving at least 38Gy (as clinically requested). VOLO and Sequential plans were then compared regarding doses to OARs (rectum, bladder and urethra), dose bath, MU, number of beams, planning and estimated treatment time. Two-sided Wilcoxon signed-rank tests were used for statistical analyses.
Results: With equal PTV coverage, VOLO was equal or superior to Sequential for all plan parameters. Rectum D1cc, V60GyEq and Dmean were reduced by 4.9%, 31.3%, and 36.3% respectively. Bladder D1cc, V60GyEq and Dmean showed reductions of 4.9%, 30.3% and 25.1% respectively. Urethra D5% was 4.9% lower using VOLO. All p-values were <0.001. The dose bath was similar with both optimization methods. VOLO plans had significant (p<0.001) reductions in MU (average reduction: 22338 Â± 10314; maximum: 66%), number of beams (from 71 to 48), with comparable numbers of MLC segments. The average treatment time was reduced from 34min to 26min (maximum 12min). Average planning time with VOLO was 62min compared to 8h with Sequential optimization.
Conclusion: The new commercial optimizer for CyberKnife plan generation showed very large improvements in plan quality, planning and treatment time. More inter-comparison studies for optimizers are needed to avoid sub-optimal treatment with high-end treatment units.
Funding Support, Disclosures, and Conflict of Interest: This work is in part funded by a research grant from Accuray Inc (Sunnyvale, USA). Erasmus MC Cancer Institute also has research collaborations with Elekta AB (Stockholm, Sweden).