⁶⁸Ga-FAPI PET/CT Interobserver Agreement on Tumor Assessment: An International Multicenter Prospective Study

Area of Science:

• Nuclear Medicine
• Radiomics
• Oncology Imaging

Background:

• Gallium-68 fibroblast activation protein inhibitors (68Ga-FAPIs) are emerging radiotracers for PET/CT cancer imaging.
• Understanding interobserver agreement in 68Ga-FAPI PET/CT interpretation is crucial for clinical adoption.
• Previous studies have not sufficiently evaluated the impact of observer experience on interpretation reliability.

Purpose of the Study:

• To assess the interobserver agreement and diagnostic accuracy of 68Ga-FAPI PET/CT interpretation across different levels of reader experience.
• To determine the minimum experience required for reliable interpretation of 68Ga-FAPI PET/CT scans in various cancer types.

Main Methods:

• 50 cancer patients underwent 68Ga-FAPI PET/CT scans.
• Fifteen masked observers, stratified by experience (low, intermediate, high), interpreted scans for tumor involvement.
• Agreement was assessed using Fleiss κ statistics and compared against a standard of reference (SOR).

Main Results:

• Highly experienced observers (>300 scans) demonstrated substantial agreement (κ > 0.6) across all assessed categories (primary tumor, nodal, metastasis) with 85% accuracy.
• Intermediate experience observers showed substantial agreement for primary tumor and metastasis but only moderate agreement for nodal involvement (κ = 0.55).
• Low experience observers exhibited moderate agreement across all categories (κ < 0.6) and lower accuracy (78%).

Conclusions:

• Substantial interobserver agreement and diagnostic accuracy for 68Ga-FAPI PET/CT cancer imaging are achieved only by highly experienced readers.
• Experience with at least 300 representative scans is recommended for accurate interpretation, particularly for nodal and metastatic disease assessment.
• Standardized training and quality control are essential to ensure reliable clinical application of 68Ga-FAPI PET/CT.