An Investigation into Planned Dose Preservation of Biology-guided Radiotherapy Under Various Radiopharmaceutical Uptake Conditions

IntroductionPositron emission tomography-based biology-guided radiotherapy is an advanced radiation therapy technique that utilizes real-time imaging data from radiopharmaceuticals to deliver a tracked dose to a targeted area. The uptake of positron emission tomography radiopharmaceuticals in the same patient can vary due to changes in physiological condition and response to treatment. Thus, the robustness of the therapeutic approach under various uptake conditions is a property that needs to be understood.MethodsAn 18F-fluorodeoxyglucose fillable cylindrical phantom, which contained a spherical target measuring 22 mm in diameter and a c-shaped organ-at-risk, was placed inside the ArcCHECK device. Two treatment plans with identical constraints were developed, each based on unique planning positron emission tomography images, using the RefleXion X1 system. Plan 1 was based on a planning image with a target-to-background ratio of 8:1 while plan 2 was based on a planning image with a target-to-background ratio of 4:1. All pre-treatment go-no-go thresholds were disabled to allow all fractions to proceed to treatment regardless of PET characteristics. Twenty-five fractions of plan 1 were delivered in groups of five consecutive fractions, using phantoms prepared with different target-to-background ratios ranging from 10:1 to 2:1. Five fractions of plan 2 were delivered based on a phantom prepared with a similar target-to-background ratio as the plan. Each treatment fraction was characterized by its target activity concentration, normalized target signal, and effective target-to-background ratio. Treatment accuracy was evaluated using the 3%/2mm absolute gamma criteria.ResultsFor plan 1 and plan 2, 91.7% and 40% of deliveries exceeded a 90% 3%/2mm absolute gamma threshold, respectively. Of these deliveries which would be clinically allowed by the machine, 100% exceeded the 90% 3%/2mm.ConclusionFor deliveries meeting the minimum activity concentration and normalized target signal requirements for target-to-background ratios of 6:1 to 10:1, positron emission tomography-based biology-guided radiotherapy demonstrated robust, and accurate treatment results for static targets. For target-to-background ratios of 2:1 and 4:1, which are not clinically deliverable, the treatment gamma results demonstrated signal-limited instability.