Intrapancreatic Accessory Spleen: Findings on MR Imaging, CT, US and Scintigraphy, and the Pathologic Analysis

Intrapancreatic Accessory Spleen: Findings on MR Imaging, CT, US and Scintigraphy, and the Pathologic AnalysisAlthough the tail of the pancreas is the second most common site of an accessory spleen, intrapancreatic accessory spleen (IPAS) has rarely been noted radiologically.However, as the imaging techniques have recently advanced, IPAS will be more frequently detected as an incidental pancreatic nodule on CT or MRI.Because accessory spleens usually pose no clinical problems, it is important to characterize accessory spleens as noninvasively as possible.An IPAS has similar characteristics to those of the spleen on the precontrast and contrastenhanced images of all the imaging modalities.In particular, inhomogeneous enhancement of an IPAS in its early phases may be a diagnostic clue.Superparamagnetic iron oxide (SPIO)-enhanced MRI and Levovist-enhanced US, and the mechanisms of which are theoretically similar to that of Tc-99m scintigraphy, can be used as alternative tools to confirm the diagnosis of IPAS.An IPAS shows a significant signal drop similar to the spleen on the SPIOenhanced T2 or T2*-weighted imaging and prolonged enhancement on the delayed hepatosplenic phase of contrast-enhanced US.We review and illustrate the differential points between IPAS and hypervascular pancreatic tumors in this manuscript.ctopic splenic tissue can be categorized as two entities: splenosis that is due to autotransplantation of splenic tissue, and this usually happens after splenectomy (1); and accessory spleens that are congenital foci of healthy splenic tissue that are separate from the main body of the spleen (1).Accessory spleens are structurally identical to the spleen and they arise from the failure of fusion of the splenic anlage located in the dorsal mesogastrium during the fifth week of fetal life (1).The presence of an accessory spleen has been reported in 10% to 30% of cases in postmortem studies (2) and in 45% to 65% of patients after splenectomy (3).The locations of accessory spleens vary, i.e., the splenic hilum, the tail of the pancreas, the greater omentum, the splenic ligament, the small and large intestinal mesentery, the wall of the small intestine, the female adnexa and the scrotum in descending order of prevalence.Although intrapancreatic accessory spleen (IPAS) has rarely been noted radiologically because the spatial resolution of conventional images was too low to detect them, it is not uncommon.Indeed, in an autopsy study of 3,000 patients, 61 (16.8%) of 364 accessory spleens were found in the pancreatic tail (2).It is important to characterize accessory spleens as noninvasively as possible as they usually pose no clinical problems and no treatment is necessary except in the following three circumstances: first, an accessory spleen may mimic lymphadenopathy or tumors in other abdominal organs, including the pancreas.Second, accessory spleen occasionally may become symptomatic because of torsion, spontaneous rupture, hemorrhage