Feasibility of Biology-Guided Radiotherapy of Small Tumors with Extended Field of View PET Detectors
Abstract
Purpose
The RefleXion X1 system requires activity concentration (AC) >5 kBq/mL for successful biology-guided radiotherapy (BgRT). We evaluated whether the extended field-of-view (FOV) PET detector in a can lower this AC threshold and expand treatment eligibility.
Methods
A custom 3D-printed phantom containing six spherical targets with diameters of 8, 9, 11, 13, 16, and 20 mm was integrated into an ArcCHECK-compatible cylindrical insert. We focused on 9 mm and 11 mm targets to assess the impact of extended FOV on small, low-uptake lesions that were challenging to target with the X1 platform. BgRT deliverability was evaluated across target-to-background ratios (TBR) of 5:1, 10:1, 15:1 and 20:1 using 18F-FDG. AC was then measured on the X2 system. BgRT plan were generated for 9- and 11-mm targets and dosimetric accuracy of BgRT delivery was verified using ArcCHECK with gamma criteria of 3%/2mm.
Results
The X2 prototype successfully planned treatments for 7/8 experiments, even for AC values below the 5 kBq/ml threshold. BgRT safeguards prevented generating a BgRT plan for 9 mm target with 5:1 TBR, due to low signal. For 11 mm targets, all TBRs demonstrated excellent deliverability with gamma passing rates of 97.3% at 20:1 (AC = 6.9 kBq/ml), 96.9% at 15:1 (AC = 4.3), 98.8% at 10:1 (AC = 3.1), and 98.2% at 5:1 (AC = 1.2). For 9 mm targets (previously untreatable with X1) TBRs ≥10:1 demonstrated excellent deliverability with gamma passing rates of 99.2% at 20:1 TBR (AC = 4.3 kBq/ml), 96.3% at 15:1 TBR (AC = 2.8), and 99.2% at 10:1 TBR (AC = 1.48).
Conclusion
The integration of extended FOV PET has the potential to expand BgRT deliverability by enabling treatment of small, low-uptake lesions. Further studies, including motion tests, are required to establish an AC threshold for safe BgRT delivery.