Author: Angela J Da Silva, Andrew Groll, Manoj Narayanan, Jeffrey Schmall, Sanchit Sharma, Timothy Pok Chi Yeung 👨🔬
Affiliation: Reflexion Medical Inc,, RefleXion Medical 🌍
Purpose: PET-based biology-guided radiotherapy (BgRT) is a radiation therapy technique that uses PET imaging to deliver tracked dose to the tumor in real-time. We characterized the delivery accuracy of BgRT on static targets having varying PET image characteristics: target-to-background ratio (TBR), radioactivity, and agreement between planning and delivery TBR across fractions.
Methods: An 18F-fluorodeoxyglucose fillable phantom was inserted into the ArcCHECK with a 22 mm diameter spherical target and a C-shape organ-at-risk. Two plans with identical constraints and two unique planning PET images (32 mm PTV, 42 mm BTZ) were delivered to on the RefleXion X1:
Twenty-five fractions, in groups of 5 consecutive fractions per fill, were delivered to a plan with a planning PET image of 8.2:1 TBR at 6.5 kBq/ml in the background. The phantoms used for delivery were prepared with a TBR of 9.9, 8.1, 6.2, 3.93, and 2.06 with initial background activities of 6.7, 6.1, 5.7, 5.4, and 6.5 kBq/ml, respectively.
Five fractions were delivered to a plan with a PET image of 3.8:1 TBR with 5.4 kBq/ml in the background. The delivered fractions were prepared with a TBR of 3.8 and a background of 7.4 kBq/ml.
Each fraction was characterized by the target activity concentration (AC), normalized target signal (NTS), and effective TBR. Treatment was evaluated using the 3%/2mm gamma index.
Results: The effective TBR at 9.9, 8.1, 6.2, 3.9, and 2.1 was 3.70±0.7, 3.3±0.4, 2.7±0.3, 1.8±0.8, and 1.4±0.7. The AC range was 11.5, 11.5, 5.0, 2.4, and 2.1. NTS range was 3.9, 4.1, 1.7, 0.7, and 2.0. Gamma was 95.9%±1.9%, 99.8%±1.2%, 95.3%±2.9%, 91.7±5.1%, and 95.9±3.18%. The 3.8 TBR planning PET plan delivered to a 3.8 TBR phantom achieved a mean delivery gamma of 88.9%±6.7%.
Conclusion: Standalone TBR and AC may be more influential for delivery accuracy than matching PET characteristics between planning and delivery.