Periodic intrafraction image-guided focal dose-escalated stereotactic body radiotherapy for prostate carcinoma on a standard linear accelerator

Background and purpose: Intrafraction motion degrades the efficacy of radiation therapy. We reported on the implementation of focal dose-escalated stereotactic body radiation therapy (HypoFocal-SBRT) for prostate carcinoma on standard linear accelerator, and assessed whether periodic intrafraction image guidance (PIF-IG) improved the accuracy of treatment. Materials and methods: Twenty prostate cancer patients underwent HypoFocal-SBRT. Treatment planning prioritized plan robustness and efficiency accommodating image guidance as-needed for treatment on standard accelerator. A three-dose level planning target volume (PTV) SBRT regime with focal boost was prescribed: PTV30 (seminal vesicles), PTV35 (prostate), and PTV40-42 (intraprostatic tumor lesion) in 5 fractions. Fiducial marker-based PIF-IG, triggered at pre-defined imaging control point(s) (ICPs), was performed using 1 mm (translations)/1° (rotations) action thresholds for online couch corrections. Applied marker registration values were compared to estimated ground-truth, as well as simulated treatment with no PIF-IG and effect on delivered dose estimated. Results: Interventions occurred at least once in 95% of patients and 83% of fractions. PIF-IG reduced the geometric uncertainty (root-mean-square error) from 1.6 mm, 2.3 mm, and 2.2 mm for the left–right (LR), superior-inferior (SI), and anterior-posterior (AP) directions to 0.6 mm, 0.9 mm, and 0.8 mm, respectively. With PIF-IG, target volumes D98% doses with 5% less than planned were observed in 2% of fractions for PTV30 and PTV35, and 0% for PTV40-42, compared to 16%, 12%, and 6% without PIF-IG. Conclusions: Focal dose-escalated SBRT for prostate cancer can be efficiently and accurately performed on standard accelerator. PIF-IG improved the geometric and delivered dose accuracy.