Rationale: To formally determine the repeatability of Ga-68 PSMA lesion uptake in both relapsing and metastatic tumor. In addition, it was hypothesized that the BPL algorithm Q. Clear has the ability to lower SUV signal variability in the small lesions typically encountered in Ga-68 PSMA PET imaging of prostate cancer. Methods: Patients with biochemical recurrence of prostate cancer were prospectively enrolled in this single center pilot test-retest study and underwent two Ga-68 PSMA PET/CT scans within 7.9 days on average. Lesions were classified as suspected local recurrence, lymph node metastases or bone metastases. Two datasets were generated: one standard PSF + OSEM and one with PSF + BPL reconstruction algorithm. For tumor lesions, SUVmax was determined. Repeatability was formally assessed using Bland–Altman analysis for both BPL and standard reconstruction. Results: A total number of 65 PSMA-positive tumor lesions were found in 23 patients (range 1 to 12 lesions a patient). Overall repeatability in the 65 lesions was −1.5% ± 22.7% (SD) on standard reconstructions and −2.1% ± 29.1% (SD) on BPL reconstructions. Ga-68 PSMA SUVmax had upper and lower limits of agreement of +42.9% and −45.9% for standard reconstructions and +55.0% and −59.1% for BPL reconstructions, respectively (NS). Tumor SUVmax repeatability was dependent on lesion area, with smaller lesions exhibiting poorer repeatability on both standard and BPL reconstructions (F-test, p < 0.0001). Conclusion: A minimum response of 50% seems appropriate in this clinical situation. This is more than the recommended 30% for other radiotracers and clinical situations (PERCIST response criteria). BPL does not seem to lower signal variability in these cases.

Purpose To formally determine the repeatability of 68Ga-PSMA lesion uptake in both relapsing and metastatic tumour. In addition, it was hypothesized that the Bayesian penalized likelihood algorithm Q.Clear has the ability to lower SUV signal variability in the small lesions typically encountered in PET images of early relapse or early metastatic disease of prostate cancer.Methods Patients with biochemical recurrence of prostate cancer were prospectively enrolled in this pilot test-retest study and underwent two 68Ga-PSMA PET/CT scans within on average 7.9 days. Lesions were classified by two independent readers as suspected local recurrence, lymph node metastases or bone metastases. Two datasets were generated: one standard ordered subset expectation maximization (OSEM) reconstruction incorporating both time-of-flight (TOF) and point spread function (PSF) modelling, and one with the Bayesian penalized likelihood (BPL) reconstruction algorithm, called Q.Clear. For tumour lesions the maximum standardized uptake value (SUVmax) were determined. Repeatability was formally assessed using Bland-Altman analysis for both BPL and standard reconstruction. Results A total number of 65 PSMA-positive tumour lesions were found in 23 patients (range 1 to 12 lesions a patient). Another 7 patients had no detectable lesions and were therefore excluded. Overall repeatability in the 65 lesions was -1.5% ± 22.7% (SD) on standard reconstructions and -2.1% ± 29.1% (SD) on BPL reconstructions. The difference was not statistically significant. 68Ga-PSMA SUVmax had upper and lower limits of agreement of +42.9% and -45.9% for standard reconstructions and +55.0% and -59.1% for BPL reconstructions, respectively. Tumour SUVmax repeatability was dependent on lesion area, with smaller lesions exhibiting poorer repeatability on both standard and BPL reconstructions (F-test, p < 0.0001).Conclusion 68Ga-PSMA SUVmax has relatively high upper and lower limits of agreement for both standard and BPL reconstructions in the small lesions typical for early relapse or early metastatic disease of prostate cancer. BPL does not seem to lower signal variability in these cases.