Physiological ⁶⁸Ga-RM2 uptake in patients with biochemically recurrent prostate cancer: an atlas of semi-quantitative measurements.

Abstract

⁶⁸Ga-RM2 is a bombesin (BBN) analog that targets the gastrin releasing peptide receptors (GRPR) overexpressed in many cancer cells, including prostate cancer (PC). It has been reported to successfully detect primary and recurrent PC. Here, we describe the distribution and range of physiological uptake of ⁶⁸Ga-RM2 in 95 patients with biochemically recurrent (BCR) PC. Ninety-five participants had simultaneous PET/MRI for BCR PC and were prospectively enrolled in this study. Maximum standardized uptake value (SUV_max) and mean standardized uptake value (SUV_mean) were measured in 24 normal anatomical structures for each participant. Three readers evaluated the images independently. Uptake in various normal tissues was classified into 4 different categories: no significant uptake if SUV_mean was less than SUV_mean of the aortic arch (AA); mild if SUV_mean was less or equal to 2.5, but higher than SUV_mean of the AA; moderate if SUV_mean was higher than 2.5, but less or equal to 5; intense if SUV_mean was higher than 5. The most intense uptake was observed in the urinary bladder, due to excretion of the radiotracer. No significant uptake was seen in the brain, salivary glands, lungs, myocardium, skeleton, muscles, and fat. Liver, spleen, and adrenal glands had mostly no significant uptake; the gastrointestinal tract had intense physiological uptake, with pancreas being the organ with the highest SUV_max measurements (average SUV_max 64.91). Mild and moderate uptake was measured in the esophagus (average SUV_max 3.99), while the stomach wall, duodenum, and rectum had mild uptake (average SUV_max 2.49, 3.42, and 3.58, respectively). ⁶⁸Ga-RM2 has been mostly evaluated for PC detection, but it can be used for other tumors overexpressing GRPR such as breast cancer. This atlas of normal biodistribution and SUV measurements in healthy tissues will help physicians distinguish between physiological vs. pathological uptake, as well as potentially assist with planning future studies using GRPR targeting radiopharmaceuticals.