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 determine minimum target size and FDG contrast thresholds for reliable Biology-guided Radiation Therapy (BgRT) delivery using a systematic phantom study.
Methods: A custom 3D-printed phantom containing six spherical targets of 8mm, 9mm, 11mm, 13mm, 16mm and 20mm diameter was integrated within an ArcCHECK-compatible cylindrical insert. The phantom was used to evaluate BgRT delivery across varying target-to-background ratios (TBR: 5:1, 10:1, 15:1 and 20:1) using 18F-FDG. All targets were imaged first on Siemens PET-CT with standardized acquisition protocols for SUV quantification, followed by RefleXion X1 measurements of activity concentration (AC) and normalized target signal (NTS). Treatment plans (50 Gy/5 fractions) were generated using the RefleXion SCINTIX system, with delivery accuracy validated using ArcCHECK (gamma criteria: 3%/2mm and 3%/3mm). Twenty-four BgRT plans were evaluated to establish operational thresholds.
Results:
The study revealed distinct delivery thresholds based on target size and contrast requirements. Small targets (8-9mm) targets were not plannable and hence not treatable at any contrast level. Large targets (16-20mm) demonstrated reliable delivery (>90% gamma passing rates) even at low contrast (5:1). Medium-sized targets (13mm) required at least moderate contrast (10:1), while 11mm target was only treatable at high contrast (≥15:1) but achieved excellent accuracy (>93%) when treated. All successful deliveries maintained clinically acceptable accuracy, with gamma passing rates of 81.9% to 100% (3%/2mm) and improved to 93.8% to 100% using 3%/3mm criteria.
Conclusion: This study establishes practical BgRT delivery thresholds: targets ≥11 mm at TBR ≥15:1, ≥13 mm at TBR ≥10:1, and ≥16 mm at TBR ≥5:1. These findings will be useful for multi-target treatment sequencing, suggesting prioritization of smaller targets when FDG signal is higher over treating larger targets during multi-target BgRT treatment.