SUVmax/ADC Ratio as a Molecular Imaging Biomarker for Diagnosis of Biopsy-Naïve Primary Prostate Cancer

Background: Gallium-68-prostate-specific membrane antigen ( 68 Ga-PSMA) positron emission tomography/computed tomography (PET/CT) has recently been shown to be very high accuracy in biopsy-naïve prostate cancer (PCa) detection and can potentially improve the low specificity noted with diffusion-weighted magnetic resonance imaging (DW-MRI), especially in instances of prostate inflammation. We aimed to compare the diagnostic accuracy of DW-MRI and PSMA PET/CT using apparent diffusion coefficient (ADC) and maximum standardized uptake (SUV max ) values in the diagnosis of PCa. Patients and Methods: A retrospective study comparing and analyzing the diagnostic accuracy of prebiopsy DW-MRI and 68 Ga-PSMA PET/CTs done in patients with suspected PCa (raised prostate specific antigen [PSA] and/or positive digital rectal examination) from January 2019 to December 2020. The standard of reference was transrectal ultrasound-guided biopsies. Results: Sixty-seven patients were included in the study, mean age: 70 years (range 49–84), mean PSA: 23.2 ng/ml (range 2.97–45.6). Biopsy was positive for PCa in 56% ( n = 38) and negative in 43% ( n = 29). Of the benign results, benign hyperplasia was noted in 75% ( n = 22) and prostatitis in 25% ( n = 7). Of the PCa, 55% ( n = 21) of were high International Society of Urological Pathology (ISUP) grade (4–5) and 45% ( n = 17) low/intermediate ISUP grade (1–3). Overall the sensitivity/specificity/Accuracy for prediction of PCa of MRI using prostate imaging and reporting data system version 2 criteria and PSMA PET/CT using PCa molecular imaging standardized evaluation criteria was 92.1%/65.5%/80.5% and 76.3%/96.5%/85.1% respectively. Mean apparent diffusion co-efficient (mean ADC) value of benign lesions and PCa was 1.135 × 10 -3 mm 2 /s and 0.723 × 10 -3 mm 2 /s, respectively ( P = 0.00001). Mean SUV max and ADC of benign and PCa lesions was 4.01 and 16.4 ( P = 0.000246). Mean SUV max /ADC ratio of benign and malignant lesions was 3.8 × 103 versus 25.21 × 103 ( P < 0.000026). Inverse correlation was noted between ADC and SUV max values ( R = −0.609), inverse correlation noted between ADC and Gleason's score ( R = −0.198), and positive correlation of SUV max and SUV max /ADC with Gleason's score ( R = 0.438 and R = 0.448). Receiver operating characteristic curve analysis revealed a SUV max cutoff 6.03 (sensitivity/specificity - 76%/90%, area under the curve (AUC) - 0.935, Youden index (YI) - 0.66), ADC cutoff of 0.817 × 10 −3 mm 2 /s (sensitivity/specificity – 79%/86%, AUC – 0.890, YI - 0.65), and SUV max /ADC ratio cutoff of 7.43 × 103 (sensitivity/specificity – 87%/98%, AUC - 0.966, YI - 0.85) for PCa diagnosis. Conclusion: For diagnosis of biopsy-naïve PCas, the combination of diffusion-weighted MRI and PSMA PET/CT (i.e., SUV max /ADC ratio) shows better diagnostic accuracy than either used alone and the combination of PET and MRI is especially useful when distinguishing cancer from prostatitis.