Feasibility, Workflow, Dosimetry, and Positron Emission Tomography Signal Trends in Lung Cancer Treated With Biology Guided Radiation Therapy

Abstract

Purpose: Biology guided radiation therapy (BgRT), a novel treatment approach, integrates real-time positron emission tomography (PET) imaging with external beam delivery to enable tumor-targeted dose modulation. Lung tumors, given their high ¹⁸F-fluorodeoxyglucose (FDG) uptake and low background activity, are considered ideal candidates for BgRT. This study presents the first clinical experience of BgRT in lung cancer, evaluating treatment workflow, plan quality, PET signal characteristics, and delivery timing.

Methods and materials: A consecutive cohort of 27 patients with lung tumors was evaluated for BgRT using the RefleXion X1 platform between December 2023 and January 2025. Of these, 14 patients received stereotactic body radiation therapy in 5 or fewer fractions. PET modeling and pretreatment PET evaluation were performed to assess activity concentration and normalized target signal. Key metrics included treatment completion rate, plan quality parameters, temporal trends in FDG signal across fractions, and a timing analysis of treatment delivery encompassing imaging, planning, and delivery workflows.

Results: Among the 14 patients treated with BgRT, 12 completed all planned BgRT fractions. Two were transitioned to conventional image guided radiation therapy due to insufficient PET signal and machine interlock, respectively. PET-based metrics demonstrated stable FDG avidity in most patients with gradual declines in activity concentration and normalized target signal that remained within clinically acceptable thresholds. BgRT plans met institutional dosimetric criteria, achieving a median planning target volume V100% = 95.6% (range, 72.2%-97.7%) and appropriate organ sparing. The median beam-on time was 29 minutes (range, 10-48 minutes), and the median total procedure time was 116 min (range, 91-136 min), supporting clinical feasibility.

Conclusions: BgRT using the RefleXion X1 system is clinically feasible, with preliminary indicators of short-term tolerability among evaluable fractions in lung cancer patients. This initial experience demonstrates reliable PET signal acquisition, consistent plan quality, and efficient delivery. These findings support the potential integration of BgRT into routine practice and justify further prospective studies in other disease sites.