2235 - Feasibility Study of Treating Small Targets with Different PET Uptake Concentration Using BgRT

J. Chang¹, R. J. Lalonde¹, D. Dimitriadou¹, A. Abel¹, R. Yang², A. Maniyedath², and G. Bal²; ¹Department of Radiation Oncology, UPMC Hillman Cancer Center, Pittsburgh, PA, ²RefleXion Medical, Inc., Hayward, CA

Purpose/Objective(s): The introduction of Biology-guided radiotherapy (BgRT) on the medical technology company platform currently necessitates PET-guided treatment of multiple metastases to be carried out in separate treatment plans. The decay of FDG may impose limits on the number of lesions that can be treated in a single day. This study examines the feasibility and dosimetric performance of treating a constellation of small targets within the time constraints imposed by FDG decay. Materials/

Methods: A custom patient-specific quality assurance insert containing five 1-cm diameter sphere targets was filled with FDG with target-to-background ratios of 20:1, 30:1, 40:1, 50:1, and 60:1. Treatment plans were created with the PTV defined as GTV+5mm expansion and the biology-tracking zone (BTZ) defined as PTV+10mm expansion. 10Gy was prescribed to all targets. The PET modeling procedure (which involves optimizing the plan based on PET signal), along with pre-scan PET and treatment delivery, were completed on the same day. The absolute-dose gamma passing rates were evaluated using the patient-specific quality assurance detector.
Results: With five different target-to-background ratios, the mean Activity Concentration (AC in kBq/ml), defined as the contrast between the intended and the background signal, was 18.06, 20.11, 29.52, 39.68, and 51.06, respectively. The signal-to-noise ratio, called Normalized Target Signal (NTS) ratios, for five targets measured 9.64, 13.81, 15.13, 34.21, and 63.69 during PET modeling. In the pre-scan PET phase, the AC was 10.86, 11.52, 11.68, 13.34, and 13.87, with corresponding NTS for the five targets at 7.60, 11.27, 11.35, 16.23, and 18.22. The absolute gamma with 3%/2mm criteria was 95.4%, 97.1%, 99.0%, and 99.7% for the peripheral four targets. For the centrally located target, it achieved 95.2% with 3%/3mm criteria.
Conclusion: The administered FDG dose allowed for the delivery of the treatment to all five sequential plans. The beam delivery results showed acceptable passing rates in absolute dose mode.