Expanding SCINTIX Biology-Guided Radiotherapy Beyond Lung and Bone: A Planning Feasibility and Dosimetric Study on the RefleXion X1 System.

IntroductionSCINTIX® therapy, a form of biology-guided radiotherapy using real-time positron emission tomography (PET) imaging on the RefleXion® X1 platform, recently received FDA clearance for FDG-guided treatment of lung and bone tumors. This study evaluates the feasibility and dosimetric quality of Biology-Guided Radiotherapy (BgRT) plans for FDG-avid lesions in anatomical sites beyond current indications using processed diagnostic PET (dPET) images.MethodsEleven previously treated FDG-avid lesions in ten patients (sites: liver, head and neck, pancreas, kidney, pelvic/abdominal nodes) were retrospectively analyzed. dPET images were processed to emulate RefleXion X1 PET characteristics. BgRT plans were generated using a non-clinical version of the RefleXion treatment planning system, delivering stereotactic body radiation therapy-level doses (48Gy/3fx for liver, 40Gy/5fx for other sites). Plan deliverability was evaluated based on normalized target signal (NTS >2.7) and activity concentration (AC > 5 kBq/ml). Plans were categorized based on the presence of dose-limiting organs at risk (OAR) constraints. Dosimetric parameters were assessed according to institutional standards. Statistical analyses examined relationships between dPET standardized uptake value (SUV), PET-derived parameters, planning target volume (PTV) characteristics, and plan complexity.ResultsBgRT plans were successfully generated for all lesions. All met the NTS threshold (mean = 9.3); 72.7% met the AC goal (mean = 12.1 kBq/ml). Among cases with dPET SUV > 6, 87.5% met AC criterion, supporting this threshold as a reasonable selection criterion. Plans without dose-limiting OARs (n = 6) achieved higher PTV coverage (V100%=94.4%, conformity index (CI) = 1.08) compared to plans with OAR constraints (n = 5, V100%=78.0%, CI = 1.4). All plans were deemed clinically acceptable despite observed heterogeneity within target regions. MLC modulation intensity correlated strongly with target volume (r = 0.907) and superior-inferior extent (r = 0.896) rather than OAR proximity.ConclusionBgRT planning using processed dPET is feasible for non-lung/bone FDG-avid lesions. SUV > 6 may serve as a reasonable threshold, though feasibility is affected by tumor location, FDG background, and nearby critical structures. Findings support clinical potential of SCINTIX expansion into new anatomical sites.