Automated Treatment Planning Of Prostate Stereotactic Body Radiotherapy With Focal Boosting On A Fast-Rotating O-Ring Linac: Plan Quality Comparison With C-Arm Linacs

Purpose The integration of auto-segmentation and automated treatment planning methods on a fast-rotating O-ring linac may improve the time efficiency of online adaptive radiotherapy workflows. This study investigates whether automated treatment planning of prostate SBRT with focal boosting on the O-ring linac could generate plans that are of similar quality as those obtained through manual planning on clinical C-arm linacs. Methods For 20 men with prostate cancer, reference treatment plans were generated on a TrueBeam STx C-arm linac with HD120 MLC and a TrueBeam C-arm linac with Millennium 120 MLC using 6 MV flattened dual arc VMAT. Manual planning on the Halcyon fast-rotating O-ring linac was performed using 6 MV FFF dual arc VMAT (HA2-DL10) and triple arc VMAT (HA3-DL10) to investigate the performance of the dual-layer MLC system. Automated planning was performed for triple arc VMAT on the Halcyon linac (ET3-DL10) using the automated planning algorithms of Ethos Treatment Planning. The prescribed dose was 35 Gy to the prostate and 30 Gy to the seminal vesicles in five fractions. The iso-toxic focal boost to the intraprostatic tumor nodule(s) was aimed to receive up to 50 Gy. Plan deliverability was verified using portal image dosimetry measurements. Results Compared to the C-arm linacs, ET3-DL10 shows increased seminal vesicles PTV coverage (D-99%) and reduced high-dose spillage to the bladder (V-37Gy) and urethra (D-0.035cc) but this came at the cost of increased high-dose spillage to the rectum (V-38Gy) and a higher intermediate dose spillage (D2cm). No statistically significant differences were found when benchmarking HA2-DL10 and HA3-DL10 with the C-arm linacs. All plans passed the patient-specific QA tolerance limit. Conclusions Automated planning of prostate SBRT with focal boosting on the fast-rotating O-ring linac is feasible and achieves similar plan quality as those obtained on clinical C-arm linacs using manual planning.