Diagnosis of recurrent prostate cancer with PET/CT imaging using the gastrin-releasing peptide receptor antagonist 68Ga-RM2: Preliminary results in patients with negative or inconclusive [18F]Fluoroethylcholine-PET/CT.

Abstract

[18F]fluoroethylcholine (18FECH) has been shown to be a valuable PET-tracer in recurrent prostate cancer (PCa), but still has limited accuracy. RM2 is a gastrin-releasing peptide receptor (GRPr) antagonist that binds to GRPr on PCa cells. Recent studies suggest that GRPr imaging with PET/CT is a promising technique for staging and restaging of PCa. We explore the value of GRPr-PET using the 68Ga-labeled GRPr antagonist RM2 in a selected population of patients with biochemically recurrent PCa and a negative/inconclusive 18FECH-PET/CT. In this retrospective study 16 men with biochemical PCa relapse and negative (n=14) or inconclusive (n=2) 18FECH-PET/CT underwent whole-body 68Ga-RM2-PET/CT. Mean time from 18FECH-PET/CT to 68Ga-RM2-PET/CT was 6.1±6.8months. Primary therapies in these patients were radical prostatectomy (n=13; 81.3%) or radiotherapy (n=3; 18.7%). 14/16 patients (87.5%) had already undergone salvage therapies because of biochemical relapse prior to 68Ga-RM2-PET/CT imaging. Mean±SD PSA at 68Ga-RM2-PET/CT was 19.4±53.5ng/ml (range 1.06-226.4ng/ml). 68Ga-RM2-PET/CT showed at least one region with focal pathological uptake in 10/16 patients (62.5%), being suggestive of local relapse (n=4), lymph node metastases (LNM; n=4), bone metastases (n=1) and lung metastasis with hilar LNM (n=1). Seven of ten positive 68Ga-RM2 scans were positively confirmed by surgical resection and histology of the lesions (n=2), by response to site-directed therapies (n=2) or by further imaging (n=3). Patients with a positive 68Ga-RM2-scan showed a significantly higher median PSA (6.8ng/ml, IQR 10.2ng/ml) value than those with a negative scan (1.5ng/ml, IQR 3.1ng/ml; p=0.016). Gleason scores or concomitant antihormonal therapy had no apparent impact on the detection of recurrent disease. Even in this highly selected population of patients with known biochemical recurrence but negative or inconclusive 18FECH-PET/CT, a 68Ga-RM2-PET/CT was helpful to localize PCa recurrence in the majority of the cases. Thus, 68Ga-RM2-PET/CT deserves further investigation as a promising imaging modality for imaging PCa recurrence.