Gallium-68 (Ga-68) prostate-specific membrane antigen (PSMA)-11 radiotracer may be a viable option for guiding positron emission tomography (PET)-based SCINTIX radiation therapy in patients with bony metastases from prostate cancer. New data presented at the American Society for Radiation Oncology (ASTRO) 2024 Annual Meeting suggests the tracer can effectively guide SCINTIX therapy, even though the radiation procedure's software was initially developed for F-18 tracers. This approach, investigated by a California-based team supported by RefleXion Medical, could improve targeted radiation delivery and reduce adverse events.
Feasibility Study Details
The study, led by Chunhui Han, PhD, from the City of Hope National Medical Center, retrospectively analyzed data from patients who underwent diagnostic scans using Ga-68 PSMA-11 PET or computed tomography. The analysis included five patients with PET-avid bone metastases. Gross tumor volumes (GTVs) were delineated for a PET-avid bone lesion in each patient, and planning target volumes (PTVs) were created by adding 5 mm margins to the GTVs. A biology tracking zone (BTZ) was defined to account for potential tumor motion.
To simulate SCINTIX treatment plans on the X1 machine, diagnostic PET images were converted to simulated treatment planning PET images to account for differences in PET detector size and reconstruction algorithms. Activity concentration (AC) and normalized target signal (NTS) were then calculated.
Key Findings
The mean Ga-68 injection dose was 4.86 mCi (range, 4.1 – 5.6) per patient. The average maximum PET activity in the GTVs was 25.2 kBg/ml (range, 13.3 – 43.5), and the mean PET activity was 16.44 kBg/ml (range, 14.3 – 24.2). In the simulated treatment planning PET images, the mean AC value was 11.5 kBg/ml (range, 6.2 – 18.9), and the mean NTS value was 12.6 kBg/ml (range, 8.7 – 22.2).
The average treatment beam-on time was 1678 seconds (range, 1416 – 2120), and the mean PTV volume was 15.7 cc (range, 7.0 – 30.9). The mean PTV coverage was 93.6%, the conformity index was 1.14, and the homogeneity index was 1.63.
Implications for SCINTIX Therapy
These findings suggest that Ga-68-based tracers can be used for planning SCINTIX treatment, offering an alternative to F-18 radiotracers. According to Han and colleagues, metastatic bony lesions from prostate cancer exhibit sufficient positron emission strength for SCINTIX treatment when Ga-68 PSMA-11 radiotracer is used at diagnostic dose levels. This study demonstrates the feasibility of using Ga-68 PSA-11 radiotracer for future PET-guided SCINTIX treatments on the X1 machine, potentially improving outcomes for patients with metastatic prostate cancer.
