Author: Girish Bal, Thomas I. Banks, Bin Cai, Neil Desai, Aurelie Garant, Orhan Oz, Elizeva Phillips, Rameshwar Prasad, Chenyang Shen, Robert Timmerman 👨🔬
Affiliation: Department of Radiation Oncology, UT Southwestern Medical Center, University of Texas Southwestern Medical Center, UT Southwestern Medical Center, RefleXion Medical 🌍
Purpose: This study reports findings from the first-in-human imaging-only trial evaluating the feasibility of using the novel PET tracer 68Ga-PSMA-11 (Illuccix) to guide external beam radiotherapy on a PET-linac platform.
Methods: The RefleXion X1 PET-linac system enables BgRT by using real-time PET signals to guide the linac beamlets for targeted radiation delivery. The FDA has only approved BgRT on X1 platform with FDG. To assess the feasibility of 68Ga-PSMA-guided radiotherapy, a non-significant risk study was opened. Our first patient was injected with 5.3 mCi of 68Ga-PSMA-11. A patient with an ultra-central tumor (GTV: 25mm × 25mm × 35mm) was injected with 5.3 mCi of 68Ga-PSMA-11. A diagnostic PET-CT scan was performed 60 minutes post-injection for target and OAR contouring, followed by a functional modeling PET acquired 160 minutes post-injection on the RefleXion platform. A BgRT plan based on RefleXion PET data was created, and IMRT QA assessed plan accuracy and deliverability.
Results: Diagnostic PET showed SUVmax of 18.3, while RefleXion PET revealed an activity-concentration (AC) of 3.0 kBq/ml, normalized-target-signal (NTS) of 9.51 and target-to-background ratio (TBR) of 5.08. A BgRT plan delivering 35 Gy in 5 fractions with 95% PTV coverage and meeting OAR constraints was successfully generated (Dmax = 40.4 Gy, Dmean = 37.33 Gy, CI = 1.18, HI = 1.18, treatment time = 25 minutes). Plan QA passed with 0.3% point dose error measured with ionization chamber and 100% Gamma passing rate (2mm/2%) measured with SNC ArcCHECK phantom
Conclusion:
This study demonstrates the feasibility of using 68Ga-PSMA-11(Illuccix) as a novel agent for BgRT planning with RefleXion PET-linac. Unlike FDG-based BgRT requiring 15 mCi, this study showed that clinically acceptable plans can be created using 5.3 mCi of 68Ga-PSMA due to its higher TBR. Further evaluation is ongoing to establish minimum AC, NTS, and TBR thresholds.