Imaging differentiation of solid pseudopapillary neoplasms and neuroendocrine neoplasms of the pancreas

Purpose The present study aimed to compare the computed tomography (CT) and magnetic resonance imaging (MRI) features of solid pseudopapillary neoplasms (SPNs) and pancreatic neuroendocrine neoplasms (pNENs). Method Lesion imaging features of 39 patients with SPNs and 127 patients with pNENs were retrospectively extracted from 104 CT and 91 MRI scans. Results Compared to pNEN patients, SPN patients were significantly younger (mean age 51.8 yrs versus 32.7 yrs) and more often female (female: male ratio, 5.50:1 versus 1.19:1). Most SPNs and pNENs presented as well-defined lesions with an expansive growth pattern. SPNs more often appeared as round or ovoid lesions, compared to pNENs which showed a lobulated or irregular shape in more than half of cases (p<0.01). A surrounding capsule was detected in the majority of SPNs, but only in a minority of pNENs (<0.01). Hemorrhage occurred non-significantly more often in SPNs (p=0.09). Signal inhomogeneity in T1-fat-saturated (p<0.01) and T2-weighted imaging (p=0.046) as well as cystic degeneration (p<0.01) were more often observed in SPNs. Hyperenhancement in the arterial and portal-venous phase was more common in pNENs (p<0.01). Enlargement of locoregional lymph nodes (p<0.01) and liver metastases (p=0.03) were observed in some pNEN patients, but not in SPN patients. Multivariate logistic regression identified the presence of a capsule (p<0.01), absence of arterial hyperenhancement (p<0.01), and low patient age (p<0.01), as independent predictors for SPN. Conclusions The present study provides three key features for differentiating SPNs from pNENs extracted from a large patient cohort: presence of a capsule, absence of arterial hyperenhancement, and low patient age.


Imaging protocols
There were substantial variations in the imaging protocols due to the long study period (15 years) and due to the inclusion of external CT and MRI scans (our hospital is a tertiary referral center).
The most recent pancreatic CT protocol at our institution includes at least an arterial phase scan of the upper or whole abdomen and a portal venous phase of the whole abdomen, using a dual-source dualenergy CT scanner (SOMATOM Force, Siemens Healthineers, Forchheim, Germany).80 ml of iodinated contrast agent (monophasic bolus, adjusted for small or large patients, 300mg/ml iodine concentration) followed by a 50 ml NaCl chaser bolus are injected via a cubital vein at 4ml/s.After a trigger threshold of 140 Hounsfield units in the abdominal aorta is reached, the arterial and portal venous CT acquisitions are initiated with a delay of 10-15 s (depending on the indication of the CT exam) and 45 s, respectively.
The contrast agent gadoterate meglumine (0.2ml/kg body weight) and a subsequent 30 ml NaCl chaser bolus are injected via a cubital vein at 2ml/s.The arterial acquisition is started using a bolus-tracker.
The portal venous phase is acquired 70 s after contrast injection and the delayed phase usually 180 s after contrast injection.In brackets are the percentages referring to all included CT scans of the respective lesion entity.

Association of imaging features with histopathological grading in pNENs
High-grade (G3) neuroendocrine tumors (NETs) and NECs were less likely to show hyperenhancement in the arterial and portal venous phase than low (G1) or moderate-grade (G2) NETs (p < 0.01).G3 NETs and NECs were more often ill-defined, but the difference was not statistically significant (pMRI = 0.11, pCT = 0.24).Encasement and occlusion of adjacent vessels (p < 0.01), upstream dilatation of the pancreatic main duct (p < 0.01) with concomitant parenchymal atrophy (p < 0.01), locoregional lymphadenopathy (p = 0.01) and hepatic metastases (p < 0.01) were also observed in a significantly higher percentage of G3 NETs and NECs than G1/G2 NETs.A lobulated or irregular shape was nonsignificantly more common in high-grade lesions (G3) than in low or moderate-grade lesions (G1/G2) (p = 0.07).Two out of two G3 NETs with available DWI were hyperintense in the high b-value images; in only one of these a low b-value (0 s/mm 2 ) was available and the mean apparent diffusion coefficient (ADC) value for both readers was 918 µm 2 /s (↔ median ADC for all pNENs = 1070 µm 2 /s).

Table 3 .
ROC curves for patient age and tumor size are shown in Supplementary Figure2.