Prospective evaluation of ⁶⁸Ga-RM2 PET/MRI and ⁶⁸Ga-PSMA11 PET/CT in patients with biochemical recurrence of prostate cancer

Objectives: Early diagnosis of biochemically recurrent prostate cancer (BCR PC) after primary treatments remains a challenge; however, new prostate cancer specific radioligands have been developed and early results are encouraging. Here, we compare the gastrin releasing peptide receptors (GRPRs) - targeting ⁶⁸Ga-RM2 and the prostate specific membrane antigen (PSMA) - targeting ⁶⁸Ga-PSMA11. Materials and Methods: We prospectively enrolled 29 patients, 60 - 83 year-old (mean±Standard Deviation (SD): 71.34±5.73), who had both ⁶⁸Ga-RM2 PET/MRI and ⁶⁸Ga-PSMA11 PET/CT. ⁶⁸Ga-RM2 imaging started 45.1 - 91 minutes (mean±SD: 55.84±11.01) after injection of 3.7 - 4.2 mCi (mean±SD: 3.98±0.13) using a TOF-enabled, SiPM-based, simultaneous PET/MRI scanner. ⁶⁸Ga-PSMA11 imaging started at 45 - 107.9 minutes (mean±SD: 67.31±15.10) after injection of 3.5 - 5.4 mCi (mean±SD: 4.03±0.35) using a SiPM-based PET/CT scanner. The standardized uptake value (SUV_max) was collected in each patient in up to 6 lesions with the highest uptake. We compared the semi-quantitative measurements in the detected lesions grouped by location between ⁶⁸Ga-RM2 and ⁶⁸Ga-PSMA11.

Results: Four patients had negative ⁶⁸Ga-RM2 and ⁶⁸Ga-PSMA11. ⁶⁸Ga-RM2 PET/MRI identified 50 lesions (average SUV_max 11.40) in 21 patients with a mean PSA value of 8.41. Eight patients had negative ⁶⁸Ga-RM2 PET/MRI; their average PSA value was 2.89. ⁶⁸Ga-PSMA11 PET/CT identified 56 lesions (average SUV_max 13.49) in 23 patients with a mean PSA value of 7.97. Six patients had a negative ⁶⁸Ga-PSMA11 PET/CT; their average PSA value was 2.72. A total of 48 lesions in 19 patients were detected by both ⁶⁸Ga-RM2 and ⁶⁸Ga-PSMA11. SUV_max ranged 2.46 - 77.07 (mean±SD:11.95±13.93) and 1.61 - 58.38 (mean±SD:13.80±12.37) for ⁶⁸Ga-RM2 and ⁶⁸Ga-PSMA11, respectively. ⁶⁸Ga-RM2 PET/MRI detected 3 more lesions in 2 patients compared to ⁶⁸Ga-PSMA11 PET/CT (all lymph-nodes); mean PSA value of these patients was 3. ⁶⁸Ga-PSMA11 PET/CT detected 10 more lesions in 7 patients compared to ⁶⁸Ga-RM2 (8 lymph-nodes, 1 lung nodule, 1 bone metastasis); mean PSA values of these patients was 3.69. The semi-quantitative measurements comparison is shown in Table 1. CONCUSIONS: ⁶⁸Ga-RM2 and ⁶⁸Ga-PSMA11 are useful radiotracers in the evaluation of BCR PC, with different localization of lesions but similar semi-quantitative measurements. In the age of personalized medicine and theragnostics, it is important to identify which patient will benefit from one class of radiopharmaceutical vs the other. Further evaluation in larger cohorts are needed to confirm our data.

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