Diagnosis of prostate cancer can be improved by using MRI together with prostate-specific membrane antigen (PSMA) PET, suggests the PRIMARY trial. The combined use of the two imaging modalities also reduced the number of biopsies needed for diagnosis.

The prospective multicentre phase II PRIMARY trial aimed to find out the diagnostic efficacy of PSMA + MRI in detecting clinically significant prostate cancer compared to use of MRI alone in 291 men with suspected prostate cancer (PSA <20 ng/mL or an abnormal digital rectal examination). The trial was conducted across three centers in Australia. The study participants were subject to MRI, pelvic-only PSMA, and systematic ± targeted biopsy.

PSMA PET + MRI was defined as negative for PSMA-negative Prostate Imaging Reporting and Data System (PI-RADS) 2/3 and positive for either MRI PI-RADS 4/5 or PSMA-positive PI-RADS 2/3.

Any International Society of Urological Pathology (ISUP) grade group ≥2 was defined as clinically significant prostate cancer.

Results showed that 56% (162/291) patients were found to have clinically significant prostate cancer. Of these 67% had MRI PI-RADS score 3-5, 73% were PSMA positive and 81% were combined PSMA + MRI positive. Twenty-seven of 95 patients with PI-RADS 2 and 25 of 53 patients with PI-RADS 3 had clinically significant prostate cancer, and PSMA PET was positive in 90% of them.

Combining MRI and PSMA improved the negative predictive value for detecting prostate cancer (91%) compared with MRI alone (72%). The sensitivity increased to 97% vs 83% (p < 0.001), although the specificity was reduced (40% vs 53%, p = 0.011), while the positive predictive value was similar (67% vs 65%).

Five clinically significant prostate cancers were missed with PSMA + MRI (four ISUP grade group 2 cancers and one ISUP grade 3 cancer).

Fifty six of the 291 subjects (19%) were negative on PSMA + MRI (38% of PI-RADS 2 and 3 patients) and could potentially have avoided biopsy result in a risk of delayed detection of clinically significant cancer in 3.1% men (5/162) or 1.7% (5/291) overall.

The study authors concluded that “the combination of magnetic resonance imaging (MRI) + prostate-specific membrane antigen positron emission tomography reduces false negatives for clinically significant prostate cancer (csPCa) compared with MRI, potentially allowing a reduction in the number of prostate biopsies required to diagnose csPCa”. But they also call for further studies to determine if it is safe to avoid biopsy in men with negative combined imaging but high clinical suspicion of clinically significant prostate cancer.

Reference

1. Emmett L, et al. The additive diagnostic value of prostate-specific membrane antigen positron emission tomography computed tomography to multiparametric magnetic resonance imaging triage in the diagnosis of prostate cancer (PRIMARY): a prospective multicentre study. Eur Urol. 2021 Aug 28;S0302-2838(21)01946-1. doi: 10.1016/j.eururo.2021.08.002.