Author: Muhammad Ramish Ashraf, Girish Bal, Daniel Pham, Murat Surucu 👨🔬
Affiliation: RefleXion Medical, Department of Radiation Oncology, Stanford University School of Medicine, Stanford University 🌍
Purpose: To systematically evaluate the relationship between Activity Concentration (AC) obtained from the RefleXion PET-Linac with the maximum standardized uptake value (SUVmax) derived from a diagnostic PET/CT scanner for Biology-guided Radiotherapy (BgRT) across varying target sizes and contrast levels.
Methods: A PET-Linac consists of two 90° PET arcs rotating with 60 RPM around the same isocenter with the Linac to capture PET emissions and direct radiation towards the tumor. To simulate small tumors, a customized phantom containing six spherical targets (8, 9, 11, 13, 16 and 20mm diameter) was 3D-printed and integrated within an ArcCHECK-compatible cylindrical insert. Targets were injected with varying target-to-background ratios (TBR: 5:1, 10:1, 15:1 and 20:1) using 18F-FDG while the background activity was kept at 5 kBq/ml. Images were acquired on both the RefleXion X1 system for AC measurements and a Siemens Biograph mCT scanner for SUVmax quantification. The BgRT screening guidelines is to have diagnostic SUVmax > 6 whereas the AC must be > 5 kBq/ml to generate a BgRT plan and deliver BgRT treatments using the RefleXion system.
Results: For the highest TBR ratio of 20:1, all target sizes yielded SUVmax>6 whereas, the RefleXion measured AC was not satisfactory for 8mm and 9mm targets. For 15:1 and 10:1 TBRs, targets ≥11mm met SUVmax criteria, but the 11mm target at 10:1 TBR failed AC requirements despite adequate SUVmax. In the challenging 5:1 case only targets ≥16mm satisfied both metrics. The mean AC/SUVmax ratios were 0.34, 0.48, 0.45, 0.60, 1.05, and 1.73 for targets sizes 8, 9, 11,13,16, and 20mm.
Conclusion: Target size influences the relationship between diagnostic SUVmax and RefleXion AC measurements. To qualify for BgRT based BgRT, tumors smaller than 10 mm need to exhibit higher SUVmax values exceeding 13-15 as higher SUVmax is better to satisfy the AC criteria for BgRT planning.