3275 - A Prospective Study on Tumor Detection and PET-guided Radiotherapy Treatment Planning with PSMA-targeted Flotufolastat F-18 Imaging on a Biology-guided Radiotherapy Machine
Presenter(s)
C. Han, Y. R. Li, Z. Huang, S. V. Dandapani, and J. Y. C. Wong; Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA
Purpose/Objective(s): A biology-guided radiotherapy (BgRT) machine (the X1) integrates a positron emission tomography (PET) imaging subsystem to its ring gantry for real-time PET signal guidance during treatment. The purpose of this single-institution prospective study is to evaluate the performance of its PET imaging subsystem for tumor detection and BgRT feasibility with PSMA-targeted 18-F-flotufolast imaging agent.
Materials/Methods: Prostate cancer patients scheduled for a standard-of-care (SOC) 18-F-flotufolastat PET/CT scan were enrolled in this study. Upon completion of the SOC PET/CT scan on a diagnostic (non-X1) PET/CT scanner, images were transferred to a radiotherapy planning system for PSMA-positive tumor target contouring. If at least one PSMA-positive tumor was identified, the patient then had a functional modeling (FM) PET scan on the X1 machine. A gross target volume (GTV) was drawn on the SOC PET/CT images for the PSMA-positive tumor, a planning target volume (PTV) was defined as a 5-mm isotropic expansion of the GTV, and a biology tracking zone (BTZ) was defined as a 5-mm isotropic expansion of the PTV. The FM PET image was reviewed to identify the tumor and BgRT treatment planning was attempted based on the FM PET image. During BgRT treatment planning, the activity concentration (AC) and normalized target signal (NTS) were calculated. To ensure sufficient PET signal strength from the tumor to guide BgRT delivery, AC must be at least 5 kBq/ml and NTS must be at least 2.7.
Results: Twenty-one patients completed both SOC PET/CT scans and FM PET scans. The average injected 18-F-flotufolastat activity was 8.5±0.6 mCi (range: 7.0 – 9.3 mCi). PSMA-positive tumors were identified in the prostate, spine, ribs, pelvic bone, hilar and pelvic nodes. The FM PET scan happened at an average of 113±26 minutes (range: 82 – 186 minutes) after the SOC PET/CT scan. The tumor could be visualized on the FM PET images in 15 patients. BgRT planning was successful for 8 patients but failed for the other 13 patients due to low activity concentration in the PTV. In the BgRT plans the average GTV volume was 8.0±11.4 cm3 (range: 0.4 – 34.6 cm3). The BgRT plans had an average AC of 10.2±8.2 kBq/ml (range: 5.1 – 29.8 kBq/ml) and an average NTS of 6.8±3.2 (range: 3.3 – 11.8). With a prescribed dose per fraction of 9 to 10 Gy, the average estimated beam-on time was 20.7±4.2 minutes (range: 13.4 – 27.1 minutes).
Conclusion: Despite lower PET image quality compared to diagnostic PET images, FM PET images allowed visualization of PSMA-positive tumors with 18-F-flotufolastat at a variety of anatomical sites. BgRT could potentially be delivered to PSMA-positive tumors at various anatomical sites provided that sufficient uptake of 18-F-flotufolastat is demonstrated in the tumor.