Optimization of injected ⁶⁸Ga-PSMA activity based on list-mode phantom data and clinical validation

Area of Science:

• Nuclear Medicine
• Medical Imaging
• Radiopharmaceutical Dosimetry

Background:

• Gallium-68 (⁶⁸Ga) prostate-specific membrane antigen positron emission tomography/computed tomography (⁶⁸Ga-PSMA PET/CT) is crucial for prostate cancer management.
• Current guidelines for optimal injected ⁶⁸Ga activity are lacking, impacting image quality, radiation safety, and costs.
• Determining the highest acceptable coefficient of variation (COV) is key for reliable ⁶⁸Ga-PSMA PET/CT interpretation and quantification.

Purpose of the Study:

• To establish an objective method for optimizing injected ⁶⁸Ga activity in ⁶⁸Ga-PSMA PET/CT.
• To determine the maximum allowable COV for reliable image interpretation (based on contrast-to-noise ratio, CNR) and quantification (based on SUVmax percentage difference, PD).

Main Methods:

• A phantom study using the NEMA Image Quality phantom was conducted to evaluate the relationship between COV, CNR, and PD.
• Image quality was assessed using the Rose criterion (CNR > 5) and quantification reliability was set at PD_SUVmax ≤ 20%.
• List-mode PET data were reconstructed with varying acquisition times to simulate different activity levels and COV.

Main Results:

• A COV of 25% was found to meet both CNR_SUVmean ≥ 5 and PD_SUVmax ≤ 20% criteria for a 10-mm sphere.
• This translates to a clinical activity regimen of 3.5 MBq/kg·min, reducing activity by nearly 50% compared to current practices.
• Clinical verification in 15 patients confirmed that no lesions were missed with the proposed activity regimen.

Conclusions:

• An injected activity regimen of 3.5 MBq/kg·min is acceptable for ⁶⁸Ga-PSMA PET/CT at the institution, enhancing efficiency and safety.
• The proposed phantom-based method provides an objective approach for determining optimal activity levels for various PET/CT systems and tracers.
• This optimization can lead to significant reductions in radiopharmaceutical use and radiation exposure without compromising diagnostic accuracy.