Author: Nebi Demez, Michael Kasper, Noufal Manthala Padannayil, Shyam Pokharel, Suresh Rana, Umesh Rana, Hina Saeed, Nishan Shrestha, Somol Sunny 👨🔬
Affiliation: Florida Atlantic University, Lynn Cancer Institute, Boca Raton Regional Hospital, Baptist Health South Florida 🌍
Purpose: DynamicARC is a novel pencil beam scanning (PBS) proton therapy technique with the potential to provide more conformal dose distributions and reduce treatment time. This study investigates the impact of varying spot spacing on dosimetric outcomes in DynamicARC planning for low-risk prostate cancer.
Methods: Two anonymized low-risk prostate cancer patients were retrospectively studied. Prostate SBRT plans were generated for 3625 cGy(RBE) in 5 fractions using a single partial arc technique. Robust optimization (setup uncertainty (±5 mm) and range uncertainty (±3%)) adhered to pre-defined clinical objectives. Plans were created with varying spot spacings from 0.2 cm to 0.8 cm in 0.1 cm increments, with other parameters unchanged. The Δ value, defined as the difference between maximum and minimum dose across spot spacings, quantified dose variability in dosimetric outcomes for the clinical target volume (CTV) and organs at risk (OARs).
Results: CTV metrics consistently met target coverage clinical goals. CTV_D95% and CTV_D99% achieved at least 3444 and 3589 cGy(RBE), respectively, with minimal variability (Δ: 3–7 cGy(RBE)). CTV_Dmax exceeded the 3878 cGy(RBE) limit in Patient 2, with Δ:23 cGy(RBE). CTV_D95wcs under worst-case-scenario (WCS) in robust evaluation showed target coverage ranging from 3435 to 3463 cGy(RBE) (Δ: 28 cGy(RBE)). OARs exhibited varying sensitivities: rectum D10% ranged from 2076 to 2299 cGy(RBE) (Δ: 223 cGy(RBE)), and urethra Dmax ranged from 3647 to 3674 cGy(RBE) (Δ: 27 cGy(RBE)). The penile bulb Dmax showed the largest variation, with Δ:262–966 cGy(RBE). Bladder and femoral heads were more stable, with Δ values below 100 cGy(RBE).
Conclusion: DynamicARC provided robust CTV dose coverage, with minimal variability under nominal and WCS conditions. Smaller spot spacings offered consistent outcomes, reducing variability in OAR doses, particularly for critical structures like the rectum and penile bulb. Future studies should explore how energy layer spacing variability, in conjunction with spot spacing, affects DynamicARC results.