Considerations When Comparing SUV Numbers from Gallium-68 and Fluorine-18 PSMA PET Scans
When comparing SUV numbers between Gallium-68 and Fluorine-18 PSMA PET scans for prostate metastasis detection, clinicians must recognize that F-18 DCFPyL PET/CT demonstrates higher sensitivity (88%) compared to Ga-68 HBED-CC PET/CT (66%) for PSA values ≥3.5 mg/L, which can significantly impact interpretation of results. 1
Tracer-Specific Differences
- F-18 PSMA tracers generally show higher SUVmax values compared to Ga-68 PSMA tracers, with studies showing significantly higher mean SUVmax in concordant lesions (14.5 vs. 12.2, p=0.028) 2
- Tumor-to-background ratios are significantly higher for F-18 DCFPyL compared to Ga-68 PSMA-HBED-CC when using kidney, spleen, or parotid as reference organs 2
- Ga-68 PSMA demonstrates better contrast ratio between tumor and normal prostate tissue (median SUV%-CR: 3.3) compared to F-18 PSMA (median SUV%-CR: 1.8) 3
Technical Factors Affecting SUV Measurements
- Reconstruction algorithms significantly impact SUV measurements, with Bayesian Penalized Likelihood (BPL) reconstructions showing different variability compared to standard OSEM reconstructions 4
- SUV repeatability varies by lesion size, with smaller lesions exhibiting poorer repeatability on both standard and BPL reconstructions (p<0.0001) 4
- SUV measurements have upper and lower limits of agreement of approximately +43% and -46% for standard reconstructions, indicating substantial variability even when using the same tracer 4
Clinical Factors Influencing SUV Interpretation
- PSA levels significantly affect detection rates and SUV intensity for both tracers, with higher PSA associated with higher SUVmax values 5
- Patients with PSA ≥10.0 ng/ml exhibit significantly higher PSMA uptake than those with PSA levels <10.0 ng/ml (median SUVmax: 17.6 versus 7.7; p<0.001) 5
- Gleason score correlates with SUVmax values, with higher Gleason scores (>7) showing significantly higher tracer uptake (median SUVmax: 21.2) compared to lower Gleason scores (median SUVmax: 5.9-8.3 for GS 6-7) 5
- Androgen deprivation therapy (ADT) status affects detection rates, with ADT-positive patients showing higher detection rates (97.7% vs. 86.3%, p=0.038) 1
Practical Recommendations
- When comparing sequential scans using different tracers, consider that a change in SUVmax of less than 50% may be within the normal variability of the measurement and not represent true disease progression or response 4
- For small lesions (<1cm), expect greater variability in SUV measurements regardless of which tracer is used 4
- Consider using tumor-to-background ratios rather than absolute SUVmax values when comparing scans with different tracers 2
- When switching between tracers, recognize that F-18 PSMA may detect additional lesions not visualized on Ga-68 PSMA scans due to higher sensitivity 2
Pitfalls to Avoid
- Avoid direct numerical comparison of SUVmax values between different tracers without accounting for the systematic differences in biodistribution and uptake patterns 3, 2
- Do not interpret changes in SUV measurements less than 50% as definitive evidence of disease progression or response when comparing different tracers 4
- Be cautious when comparing lesion detection between tracers in patients with low PSA values (<3.5 ng/mL) where sensitivity differences are most pronounced 1
- Remember that technical factors such as acquisition parameters, reconstruction algorithms, and time post-injection can significantly affect SUV measurements independent of tracer differences 4