Histological comparison between predictive value of preoperative 3‐T multiparametric MRI and 68Ga‐PSMA PET/CT scan for pathological outcomes at radical prostatectomy and pelvic lymph node dissection for prostate cancer

Abstract

Objective To evaluate the ability of pre‐operative multiparametric MRI (mpMRI) and a positron emission tomography prostate specific membrane antigen tracer (68Ga‐PSMA PET/CT) scan to predict pathological outcomes and also identify a group of men with a <5% risk of histological pelvic lymph node metastasis (LNM) at pelvic lymph node dissection (PLND) performed during a robot assisted laparoscopic radical prostatectomy (RALP) for prostate cancer. We then aimed to compare these results to known risk calculators for LNM, including CAPRA score, MSKCC and Briganti nomograms. Methods Between July 2014 and September 2019 only men who had both a pre‐operative mpMRI and staging 68Ga‐PSMA PET/CT at our institution followed by a radical prostatectomy with PLND referred to a single specialist uro‐pathology laboratory were considered for inclusion. The data was collected retrospectively prior to February 2019 and in a prospective manner thereafter. A model was built to allocate probabilities of the men with a negative 68Ga‐PSMA PET/CT scan having a <5% risk of histologically LNM at RALP based on the pre‐operative radiological staging. Results 233 consecutive men met the inclusion criteria of which 58 men (24.9%) had a pelvic LNM identified on PLND histology. The median International Society of Urological Pathology (ISUP) grade was 5 (range 1‐5) and the median PSA was 7.4 ng/mL (1.5‐72). The median number of resected lymph nodes was 16 (1‐53) and the median number of positive nodes identified on histology was 2 (1‐22). Seminal vesicle invasion on mpMRI was more common in node positive men than in the absence of LNM (31% vs 12%). The SUVmax of the primary tumour on 68Ga‐PSMA PET/CT was higher in men with LNM (median 9.2 vs 7.2, p=0.02). Suspected LNM were identified in 42/233 (18.0%) men with 68Ga‐PSMA PET/CT compared with 22/233 (9.4%) men with mpMRI (p=0.023). The positive and negative predictive value for 68Ga‐PSMA PET/CT is 66.7% and 84.3% respectively, compared to 59.1% and 78.7% for mpMRI. A predictive model showed only 2 men (4.2%) with a negative pre‐operative 68Ga‐PSMA PET/CT would be positive for a histological LNM if they are ISUP <4 and PIRADS <4; or ISUP 5 with PIRADS <3. An inspection of three additional variables: CAPRA score, MSKCC and Briganti nomograms did not improve the predictive probability for this group. However, of the 61 men with ISUP grade 4‐5 malignancy and also a PIRADS 5 mpMRI, 20 (32.8%) men had a microscopic node metastasis despite a negative pre‐operative 68Ga‐PSMA PET/CT. Conclusion Pre‐operative 68Ga‐PSMA /PET CT was more sensitive in identifying histological pelvic lymph node metastases than 3T mpMRI. Men with a negative 68Ga‐PSMA PET/ CT have a lower risk of node metastases than predicted with CAPRA scores or MSKCC and Briganti nomograms. We identified that the combination of a negative pre‐operative 68Ga‐PSMA PET/CT, ISUP biopsy grade <4 and PIRADS <4 prostate mpMRI, or an ISUP grade 5 with PIRADS <3 on mpMRI is associated with a 30% risk of microscopic node metastasis despite a negative pre‐operative 68Ga‐PSMA PET/CT and this high‐risk group would appear suitable for an extended PLND at the time of a radical prostatectomy.