Correlation of Lesional Uptake Parameters and Ratios with miPSMA Score and Estimating Normal Physiologic Concentration: An Exploratory Analysis in Metastatic Castration-Resistant Prostatic Carcinoma Patients with 68Ga-PSMA-11 PET/CT

Abstract

The use of prostate-specific membrane antigen (PSMA)–based PET/CT has grown rapidly in recent years. This study estimated lesional uptake, normal physiologic concentrations, and temporal variation on delayed PET/CT of 68Ga-PSMA-11 across different molecular imaging PSMA (miPSMA) expression scores in patients with metastatic castration-resistant prostatic carcinoma. Methods: We retrospectively studied 50 patients who were evaluated for 177Lu-PSMA–targeted radioligand therapy and underwent 68Ga-PSMA-11 PET/CT to determine disease status. Their mean age was 67.5 6 8 y (52–84 y), and their average serum prostate-specific antigen level was 401 6 1,353 ng/mL (0.098–9,235.13 ng/mL) at the time of scanning. They underwent standard 68Ga-PSMA-11 PET/CT an average of 65 min after injection (60–90 min). Tumors (n 5 50) were correlated with miPSMA expression score and uptake. Physiologic tracer distribution was estimated by placing a volume of interest 1 cm in diameter for smaller organs (submandibular, parotid, lacrimal, and tubarial glands; renal cortices; blood pool; and bowel) and 3 cm for larger organs (liver and spleen). SUVmax and SUVmean were estimated for each region. Tumor-to-spleen (T/S), tumor-to-liver (T/L), and tumor-to-parotid (T/P) ratios were calculated for each lesion. For 16 patients who underwent a delayed scan an average of 135 min after injection (120–150 min), additional analysis evaluated the effect of the delay. Results: Uptake was maximal in renal cortices, followed by salivary glands, bowel, spleen, liver, lacrimal glands, and blood pool. SUVmax averaged 37.7 6 22.1 for renal cortices, 15.4 6 7.3 for submandibular glands, 14.4 6 7.1 for parotid glands, 9.4 6 4.9 for spleen, 6.2 6 3.7 for lacrimal glands, 5.9 6 2.3 for liver, 5.3 6 1.41 for tubarial glands, 13.8 6 7.6 for bowel, and 2.4 6 1.9 for blood pool. SUVmax averaged 10.33 6 3.27 (6.46–17) for miPSMA expression score 2 and 38.21 6 25.9 (7.68–119.08) for score 3. T/S and T/P ratios averaged 1.21 6 0.44 (0.48–2.04) and 0.6 6 0.18 (0.39–0.87), respectively, for score 2 and 5.05 6 4.46 (1.25–20.89) and 3.15 6 2.09 (1.06–9.45), respectively, for score 3. SUVmax for score 3 lesions averaged 18.85, which increased significantly to 26.24 on delayed imaging (P 5 0.0001). However, T/L, T/S, and T/P ratios did not significantly change. Temporal variation in normal organs showed SUVmax to increase significantly on delayed scans for salivary (submandibular and parotid) and lacrimal glands and renal cortices, whereas SUVmean increased significantly for spleen; liver; and parotid, tubarial, and lacrimal glands and insignificantly for other organs. Conclusion: These data form a basis for a proposed consensus on standard reference ranges for quantitative 68Ga-PSMA-11 PET/CT. The temporal variations should be kept in mind for delayed acquisitions; T/S, T/L, and T/P ratios might serve as better markers for such scenarios.