A Gaussian-based planning approach for robust dose-escalated stereotactic body proton therapy

Purpose Proton plans are often very homogenous with minimal hotspots compared to the prescribed dose, which may not be ideal for ablative stereotactic body radiation therapy (SBRT). A treatment planning approach for stereotactic body proton therapy (SBPT) is proposed to match the characteristics of photon SBRT dose distributions, namely a Gaussian-shaped curve with high centralized doses in the gross tumor volume (GTV) and rapid dose falloff in healthy tissues. Methods and Materials The semi-automated technique is derived from a desired maximum dose value, or hotspot. The GTV is divided into concentric shells, and treatment plans are optimized to drive the hotspot to the innermost shell while using the remaining target shells to shape falloff within the GTV. The algorithm is tested in a cohort of 10 patients who previously received SBPT to the liver at our institution. The Gaussian approach is compared to standard homogeneous SBPT planning for both dosimetric and deliverability results. Results Achieved hotspots on Gaussian plans matched pre-specified hotspot within 2.2% on average. Gaussian-based planning led to a statistically significant increase in biologically effective dose of 20 Gy (α/β=10) compared against the homogenous plans. On nominal plans, healthy liver and gastrointestinal organ-at-risk and planning-risk-volumes showed no statistically significant differences, though liver D mean trended lower with the Gaussian approach (10.8 Gy vs 11.9 Gy, p=0.01). In evaluating plan robustness, Gaussian plans had slightly higher dose spill outside of the PTV, but lower dose to GI OARs. In a subset of 3 plans evaluated for deliverability, Gaussian plans took 10 seconds longer to deliver per field and patient-specific QA results passed institutional criteria. Conclusions A mathematically driven treatment planning technique was developed and tested for generating liver SBPT plans. The proposed technique was derived to mirror photon SBRT dose distributions, allowing for enhanced harmonization of SBPT with historical SBRT clinical data and potentially allowing safe radiation dose escalation during SBPT.