Subnodal Correspondence of PSMA Expression and USPIO-MRI in Metastatic Pelvic Lymph Nodes in Prostate Cancer

ObjectivesTwo advanced imaging modalities used to detect lymph node (LN) metastases in prostate cancer patients are prostate-specific membrane antigen (PSMA) positron emission tomography/computed tomography and ultrasmall superparamagnetic iron oxide (USPIO)-enhanced magnetic resonance imaging (MRI). As these modalities use different targets, a subnodal comparison is needed to interpret both their correspondence and their differences. The aim of this explorative study was to compare ex vivo 111In-PSMA mu SPECT images with high-resolution 7 T USPIO mu MR images and histopathology of resected LN specimens from prostate cancer patients to assess the degree of correspondence at subnodal level.Materials and MethodsTwenty primary prostate cancer patients who underwent pelvic LN dissection were included and received USPIO contrast and 111In-PSMA. A total of 41 LNs of interest (LNOIs) were selected for ex vivo imaging based on gamma-probe detection or palpation. mu SPECT and mu MRI acquisition were performed immediately after resection. Overlay of mu SPECT images on MR images was performed, and the level of correspondence (LoC) between mu SPECT and mu MR findings was assessed according to a 4-point Likert classification scheme.ResultsForty-one LNOIs could be matched to an LN on ex vivo mu MRI. Coregistration of mu SPECT and USPIO-enhanced water-selective multigradient echo MR images was successful for all 41 LNOIs. Ninety percent of the lesions showed excellent correspondence regarding the presence of metastatic tissue and affected subnodal site (LoC 4; 37/41). In only 1 of 41 LNOIs, a small metastasis was misclassified by both techniques. Three LNOIs were classified as LoC 3 (7%) and 1 LNOI as LoC 2. All LoC 2 and LoC 3 lesions had PSMA-expressing metastases on final histopathologyConclusionsCoregistration of mu SPECT and USPIO-mu MRI showed excellent subnodal correspondence in the majority (90%) of LNs. Ex vivo imaging may thus help localize small cancer deposits within resected LNs and could contribute to improved interpretation of in vivo imaging of LNs.