Nonfunctioning and functioning islet-cell tumors have similar enhancement characteristics . Nonfunctioning islet-cell tumors are more easily detected by the mass effect they produce. However, smaller tumors are often discovered by producing secondary effects to a strategic location , such as obstruction of the bile or pancreatic ducts.
Because of their rich vascular supply , islet -cell tumors classically are hyperattenuating compared with the surrounding pancreatic parenchyma on contrast-enhanced CT .Hypervascular enhancement is one of the typical imaging features of islet -cell tumors and often helps distinguish them from the much more common pancreatic adenocarcinoma, which tend to be hypovascular and desmoplastic. Capturing the vascular blush supply is essential for the diagnosis of small tumors , which often do not distort the contour of the pancreas. Many small lesions enhance more prominently and thus are easier to detect in the arterial phase or become incospicuous in the venous phase ( Case 1, Fig. C, D). Although islet-cell tumors are classically considered hypervascular in the arterial phase , the degree, uniformity and timing of enhancement can be highly variable (Case 2, Fig.P, Q ). In fact some islet-cell tumors may be seen best on venous or delayed phase images. Often, a characteristic ring-like enhancement is seen on early or delayed contrast-enhanced images (Case 2, Fig. J-O). Also, the uniformity of enhancement tends to be variable , with larger lesions exhibiting more heterogeneous enhancement , conversely, small lesions are often homogeneous in enhancement.
Ring-like enhancement and cystic degeneration may be present in larger lesions. It is important to carefully evaluate the venous phase, as tumors with cystic components may exhibit delayed enhancement best seen in this phase.
Islet-cell tumors are usually bright on T2-weighted sequences, however lesions with intermediate or low T2-weighted signal intensity may be seen They can occasionally present a cystic mass; T2-weighted sequences are effective at demonstrating these tumors secondary to high fluid content.Rarely, islet-cell tumors may be desmoplastic, rendering them low in signal intensity on T2-weighted images, with negligible enhancement in arterial and venous phases (Case 2, Fig. C-E). The unenhanced T1- weighted fat-suppressed sequence provide excellent contrast between the low-signal-intensity tumor and the normal pancreas, which is bright secondary to the abundance of acinar protein. This may be the best sequence to detect subtle tumors (Case 5, Fig. A, B).
The signal intensity and enhancement patterns of lymph nodes and liver metastases are similar to the primary (Case 4, Fig. J, M).
POTENTIAL PITFALLS AND DIFFERENTIAL DIAGNOSIS
Potential false negative include tumors adjacent to vessels, non hyperattenuating lesions, peduncolated morphology (Case 3, Fig.J), and tumors immediately adjacent to small bowel.
Differentiating features from ductal adenocarcinoma include presence of calcification, a decreased incidence of vascular encasement, central necrosis, increased T2 signal, homogeneous enhancement and rare obstruction of the ductal system (Case 5, Fig.C-F; Case2, Fig.C-F).
Pancreatic metastases from hypervascular primary tumors, particularly renal cell carcinomas, display enhancement characteristics similar to those of islet cell tumors. Patients with previously diagnosed renal cell carcinoma may present a diagnostic dilemma because metastases can occur as late as 22 years after the initial diagnosis and because of the association of renal cell carcinoma and islet cell tumors with von Hippel-Lindau syndrome (Case 6, Fig.A-C).
SOMATOSTATIN RECEPTOR SCINTIGRAPHY ( SRS)
The presence of somatostatin (SS) receptors at the cell membrane of neuroendocrine tumours ( NETs) constitutes the basis of the clinical use of radiolabeled SS analogs, both for imaging and therapy.
Among the five subtypes of human SS receptors that have been cloned, sstr2 is the predominant one in NETs and clinically used SS analogs bind predominantly to sstr2.
The information of SRS is exquisitely functional, reflecting the physiological and pathological distribution of SS receptors and cannot be obtained by morphologic imaging techniques. SRS has the advantage of giving a panoramic view of the whole body.
Multiple lesions are more difficult to detect. In our experience SRS helps to localize multifocal islet cell tumors (Case 1, Fig. G-I; Case 2, Fig.P-T).
Sensitivity of SRS is related to various factors such as type and density of SS receptors expressed by the tumor, tumor-to-background ratio, tumor site and tumor histology. SRS sensitivity in endocrine pancreatic tumors is very high, raging from 80 to 100% ( the best results are obtained in gastrinomas), with the only exception of insulinomas ( approximately 60%) due to a lower incidence of sstr2.
Moreover, in the presence of pancreatic mass, a negative SRS suggests a pancreatic adenocarcinoma (see Case 5 ) or a poorly differentiated tumor.
Also SRS specificity is high, even though tumors other than NETs as well as granulomas or autoimmune diseases can be visualized due to SSTR expression in peritumoral vessels and in inflammatory and immune cells. So, in order to avoid false-positive results, it is very important that interpretation of SRS images is performed in close relation to the clinical history and also by the combined reading of SRS and CT/MR images.
CASE 1
HYPERVASCULAR ISLET CELL TUMOR
Fig.: CASE 1 - Fig A, B:Two aneurysms recognized as focal dilatations of the splenic artery result from increased flow in the splenic artery in a 45-year-old male liver transplantation candidate with hepatocarcinoma in cirrhosis and portal hypertension secondary to hepatitis C. Oblique volume-rendered images of the arterial abdomen enhanced with intravenous contrast material show focal dilatations of the splenic artery (white arrows in A) and a small homogeneous hyperattenuating area adjacent to vessel in the tail of the pancreas ( yellow arrows in A and B). Lesion was mistaken for vessel on preliminary review. Oblique reconstruction image clearly shows lesion anterior to splenic artery.
Fig.: CASE 1 - Fig C, D: Axial CT images of the pancreas show mass to be less conspicuous in venous phase ( yellow arrows in D ) than in arterial phase ( yellow arrow in C ) emphasizing importance of arterial phase and optimal timing of enhancement. Note the arteriola adjacent to the hypervascular lesion (*). Note hepatocarcinoma hyperdense in arterial phase and hypodense in portal phase ( red arrows in C and D).
Fig.: CASE 1 - Fig E, F:Fig. E: fused images SPECT-CT; transaxial view.Fig. F: fused images SPECT-CT; coronal viewSRS shows intense SS receptor expression in the tail of the pancreas, corresponding to a 8 mm lesion seen at CT ( red marker in E and F). An additional area of tracer uptake is evident in the cephalo-pancreatic lesion ( yellow arrow in F) .
Fig.: CASE 1 - Fig G-I:Retrospectively reviewed CT images reveal localization of abnormal uptake as exophytic and cystic mass ( 8 mm) in all phases of enhancement arising from head of pancreas (yellow arrows in G and H, red marker in I). Patient underwent liver transplantation and surgical enucleation of masses confirmed diagnosis of benign multifocal nonhyperfunctioning islet cell tumor.
CASE 2
MULTIFOCAL ISLET CELL TUMOR
Fig.: CASE 2 - Fig A, B: Multifocal malignant nonhyperfunctioning islet cell tumor in 70 years old man with pancreatic lesion incidentally discovered at sonography. No jaundice and normal results of liver function.Axial T1-weight fast-sat spoiled gradient-echo MR images show an ill defined bright iuxtapapillar periductal lesion ( white arrows in A and B) in the head of the pancreas.
Fig.: CASE 2 - Fig C-F:Axial and coronal T2-weighted MR images at same level show hypointense lesion ( white arrows in C , D and E ) which produce at magnetic resonance cholangiopancreatography ( MRCP) stricture of distal common bile duct and stricture of main pancreatic duct ( white arrows in F). The common bile duct and main pancreatic duct distal to the stenosis are subtle dilated. Visualization of the accessory Santorini duct ( * ).
Fig.: CASE 2 - Fig. G-I:The duodenal filling 10 minutes after secretin stimulation reveals duodenal submucous distortion ( yellow arrows in G) which corresponds at trasverse T2 weighted images as intermediate signal intensity paraduodenal mass ( yellow arrows in H and I). Visualization of the accessory Santorini duct which vicariant overdistension ( * ). Note posteriorly the vena cava ( white arrow in H and I).
Fig.: CASE 2 - Fig. J-O: The dynamic 3D LAVA MR sequences clearly demonstrate ring enhancement in paraduodenal lesion, that varies in thickness depending on enhancing phases and is more conspicuous in venous and delayed phases (yellow arrows in J, L and N). This pattern is usual for islet-cell tumors gadolinium-enhanced. 3D LAVA-sequences show poorly marginated hetereogeneously enhancing mass of pancreatc head. The lesion is nearly iso-attenuating relative to pancreas in arterial phase (white arrows in K) and is nearly incouspicuos except for subtle low intensity in T1-sequences. Lesion enhancement is more evident in venous and delayed phases (white arrows in M and O). This pattern is unusual for adenocarcinoma and islet-cell tumors.
Fig.: CASE 2 - Fig. P, Q: Coronal and axial 3D-LAVA sequences , obtained 10 minutes after gadolinium-contrast administration, show homogeneous bright enhancement in pancreatic and paraduodenal lesions ( white arrows in P, yellow arrows in Q).Delayed sequences are useful in the detection of islet-cell tumors, due to the fact that some islet-cell tumors were proved at histopathologic examination to be schirrous tumors with abundant fibrous tissue, like in this case.
Fig.: CASE 2. Fig. R-T: Fig. R: fused images SPECT-CT; view coronal Fig. S and T: images SPECT; view coronalSRS shows two adjacent areas of abdominal uptake in the rightmesogastric region ( white arrows in R, S, and T) corresponding to the pancreatic and paraduodenallesions evidenced at MRI.
CASE 3
PEDUNCULATED ISLET CELL TUMOR
Fig.: CASE 3- Fig A-D:35 years-old woman with history of pancreatic lesion incidentally detected on MR imaging at outside institution diagnosed as lymph-node on basis of findings at previous MRI exam.Axial T1 weighted fat-suppressed spoiled gradient-echo MR images show 2-cm exophytic mass arising from body of pancreas. The lesion is hypointense, oval, adjacent to the stomach ( white arrows in A, B, C and D ).
Fig.: CASE 3 - Fig E-J: Axial and coronal T2-weighted images show lesion with central high signal intensity ( white arrows in E, F and G ) subtle rim enhancement in arterial phase gadolinium –enhanced 3D LAVA sequence ( arrows in H), more prominent in venous and delayed oblique volume-rendered images (arrows in I and J) emphasizing importance of delayed phase.Middle segment pancreasectomy confirmed presence of intermediate malignant pedunculated nonhyperfunctioning islet cell tumour.
CASE 4
EXTRAPANCREATIC METASTATIC GASTRINOMA
Fig.: CASE 4 - Fig A, B : Malignant functioning with hepatic metastasis islet cell tumor ( gastrinoma ) within the “gastrinoma triangle” in a 45 years-old man.Axial T1 weighted fat-suppressed spoiled gradient-echo MR images show extrapancreatic lesion ( yellow arrows in A e B) hypointense to the pancreatic parenchyma medially to the second portion of the duodenum ( D ).
Fig.: CASE 4 - Fig C, D: Axial and coronal T2-weighted images show a small , well-defined hyperintense mass ( yellow arrows in C and D).
Fig.: CASE 4 - fig E-G: Tumor conspicuity is greater in T1 and T2 sequences than in gadolinium enhanced T1 weighted spoiled gradient-echo MR images because both the mass ( yellow arrows in E, F and G) and pancreatic parenchyma (P) show the same homogeneous enhancement in all phase (arterial and venous phases in E and F).Note homogeneously enhancing lesion in delayed coronal T1 weighted fat sat spoiled gradient-echo (yellow arrow in G).
Fig.: CASE 4 - fig H, I: SRS shows intense SS receptor expression in the retro-pancreatic lesion found at MR (yellow arrow in H).SPECT images show an additional small area of abnormal uptake in the right liver lobe ( red arrow in I)
Fig.: CASE 4 - Fig J-M: Retrospectively reviewed MR images reveal area of altered uptake at SRS as small, 5 mm, high hyperintense lesion on T2 weighted sequences (red arrow in J) with subtle rim enhancement in all phases of enhanced T1 weighted images ( arrows in arterial K , portal I and delayed L phases ).Hepatic lesion was mistaken for cyst on preliminary review.
CASE 5
PITFALL
Fig.: CASE 5 - Fig A, B: 50 years-old woman with pancreatic lesion incidentally discovered at sonography.Axial T1 weighted fat-suppressed spoiled gradient-echo MR images show 2 cm low-signal-intensity lesion in the pancreatic body, which is well seen relative to normal hyperintense pancreatic parenchyma due to excellent soft-tissue contrast resolution of MRI (yellow arrows in A and B).
Fig.: CASE 5 -Fig C-F : T2- weighted imaging reveals the heterogeneously increased signal intensity within the mass ( yellow arrows in D and E) ; center of lesion is high signal intensity (yellow arrows E).MRCP reveals stricture of main pancreatic duct ( yellow arrows in F).Note dilatation distal to the stenosis of main pancreatic duct (white arrows in C and F).
Fig.: CASE 5 - Fig G-L: Gadolinium-enhanced 3D LAVA show marginated heterogeneously enhancing mass in arterial phase (white arrows in G and H) which has marked enhancement in venous and delayed phases ( white arrows in I, J, K and L). Lesion appearance is non-specific and can be seen with nonhyperfunctioning islet cell tumor as well , unlike adenocarcinoma , which tends to be hypovascular.No focal uptake at SRS.Retrospective evaluation proved that the distal ductal dilatation is more conspicuous in this case than case 2.
Fig.: CASE 5 - Fig. M-O:Diagnosis of schirrous adenocarcinoma was established at distal spleno-pancreatectomy. Black arrows in N and O mark the pancreatic lesion.
CASE 6
DIFFERENTIAL DIAGNOSIS
Fig.: CASE 6 - Fig A- C:Hypervascular pancreatic metastasis in 45 years-old woman with history of right nephrectomy for renal cell carcinoma 10 years earlier.Axial CT images of pancreas in arterial phase of enhancement show hyperattenuating mass ( yellow arrows in A and B) in body and tail of the pancreas. Note hypervascular hepatic metastasis ( white arrow in C).
Fig.: CASE 6 - fig D-I: Axial CT images obtained at same level as D and F in venous phase of enhancement show that enhancement in pancreatic mass ( yellow arrows) is not as pronounced as in A. Note that hypervascular hepatic metastases ( white arrows in E and G) has become inconspicuous , collateral veins are present ( red arrow in H) , and splenic vein is dislocated (*).
