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 ± 6.8 months. 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 ± 53.5 ng/ml (range 1.06-226.4 ng/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 (6.8 ng/ml, IQR 10.2 ng/ml) value than those with a negative scan (1.5 ng/ml, IQR 3.1 ng/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.