KEY POINTS
- PARTY prospectively enrolled 10 patients with oligometastatic or oligoprogressive tumors involving the abdomen, pelvis or chest wall. Nine had received previous radiotherapy, including five with overlapping or adjacent prior fields.
- Patients received pencil-beam scanning proton SBRT of 30–50 CGE in 5 fractions, using CT-on-rails imaging and same-session recontouring, dose recalculation and re-optimization.
- The primary feasibility endpoint was clearly met: 50/50 fractions were successfully completed, and 42/50 fractions (84%) were treated using an adapted plan.
- Adaptation was clinically consequential rather than routine replanning alone: 20 fractions had critical OAR constraint violations on the original plan, 16 had meaningful target-coverage improvement, and 6 had both. All OAR violations identified during online evaluation were resolved through adaptation.
- Median door-to-door treatment time was 88 minutes, including 12 minutes for setup, 41 minutes for adaptation, and 35 minutes from plan approval through treatment completion. Gated treatments took a median 100 minutes.
- Treatment was generally well tolerated. One patient developed grade 3 pancreatitis considered at least possibly treatment-related; no other grade 3 toxicities occurred. At a median 7.8-month follow-up, local control was 90% at 3 months and 72% at 6 months.
CLINICAL TAKEAWAY
This is an important proof of feasibility for true online adaptive proton SBRT: daily anatomy frequently made the original plan unacceptable or suboptimal, and adaptation allowed all treatments to proceed. The major current limitation is operational—nearly 90 minutes per fraction—along with the very small and heterogeneous phase I cohort.
SOURCE
International Journal of Radiation Oncology, Biology, Physics