RESUMEN
OBJECTIVES: Two 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 µSPECT images with high-resolution 7 T USPIO µMR images and histopathology of resected LN specimens from prostate cancer patients to assess the degree of correspondence at subnodal level. MATERIALS AND METHODS: Twenty 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 γ-probe detection or palpation. µSPECT and µMRI acquisition were performed immediately after resection. Overlay of µSPECT images on MR images was performed, and the level of correspondence (LoC) between µSPECT and µMR findings was assessed according to a 4-point Likert classification scheme. RESULTS: Forty-one LNOIs could be matched to an LN on ex vivo µMRI. Coregistration of µSPECT 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 histopathology. CONCLUSIONS: Coregistration of µSPECT and USPIO-µMRI 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.
