Intrapancreatic accessory spleen: CT appearance and differential diagnosis
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- Kawamoto, S., Johnson, P.T., Hall, H. et al. Abdom Radiol (2012) 37: 812. doi:10.1007/s00261-011-9830-x
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Although autopsy studies report that the second most common site of the accessory spleen is in the tail of the pancreas, intrapancreatic accessory spleens (IPASs) are rarely recognized radiologically. With recent improvements in imaging techniques, IPASs are more commonly detected on imaging studies. IPAS can be mistaken for other type of mass-forming lesions in the tail of the pancreas, particularly an asymptomatic small neuroendocrine neoplasm. Rarely, an epidermoid cyst originating from IPAS may simulate other cystic pancreatic lesion. Accurate preoperative diagnosis would obviate unnecessary surgery. IPAS should be considered when a hypervascular mass is seen in the tail of the pancreas on CT. Typical location, similar attenuation of the lesion to the spleen on noncontrast, and postcontrast CT at different phases are helpful to make diagnosis of IPAS. In particular, characteristic heterogeneous contrast enhancement of IPAS on the arterial phase may be helpful for correct diagnosis. However, when it remains difficult to exclude the other diagnosis, 99mTc labeled heat-damaged red blood cell scintigraphy or superparamagnetic iron oxide-enhanced MRI can be used to confirm the diagnosis of IPAS.
KeywordsIntrapancreatic accessory spleenPancreatic neuroendocrine neoplasmCTContrast enhancement
Accessory spleen is a congenital abnormality consisting of normal splenic tissue in ectopic sites seen in approximately 10 to 20% of individuals [1, 2]. The most common location of an accessory spleen is the splenic hilum. The second most common site of the accessory spleen is in the tail of the pancreas. However, intrapancreatic accessory spleens (IPASs) are rarely recognized radiologically [3, 4].
With recent technical improvement of imaging studies, IPASs are more commonly detected on CT and MR imaging [3, 5, 6]. IPAS can be mistaken for other mass-forming lesions in the pancreas, particularly asymptomatic pancreatic neuroendocrine neoplasms. Despite advances in CT resolution, criteria to discriminate IPASs from small pancreatic neuroendocrine neoplasms have yet to be defined.
IPAS is usually asymptomatic and found incidentally on imaging studies, and generally does not require therapy. The challenge lies in reliably distinguishing a small enhancing intrapancreatic splenic rests from neuroendocrine neoplasm. Accurate preoperative distinction would obviate unnecessary surgery. The purpose of this article is to review the typical and atypical CT appearances in a series of patients with IPAS, and to discuss the characteristic appearance on CT and other imaging modalities that can be used to distinguish IPAS from other mass-forming lesions in the pancreas.
Embryology and incidence of IPAS
Accessory spleen is a congenital abnormality consisting of normal splenic tissue in ectopic sites. An accessory spleen arises as a result of failure of fusion of some of the multiple buds of splenic tissue in the dorsal mesogastrium during embryologic life . Accessory spleens are usually single, but approximately 10% of patients with one have a second focus . More than two deposits are seen infrequently [1, 2].
In imaging studies, IPAS is less commonly encountered. In a retrospective review of CT examinations in 1,000 patients, Mortele et al. reported that a total of 180 accessory spleens were found in 156 patients (15.6%), and IPASs were found in 2 patients (0.2%) . In this study, the most common location of accessory spleen was inferior third of the posteromedial portion of the splenic hilum, which was adjacent (lateral, posterior, and superior) to the tail of the pancreas .
Microscopically, an IPAS reproduces the splenic pattern . Usually, they have a well-defined fibrotic capsule which separates the surrounding normal pancreatic tissue [5, 8]. The capsule has a varying width, with septa . In some of the IPAS, the capsule may be thin, incomplete, or missing, in which cases lymphoid tissue can be located outside and adjacent to the capsule . When superimposed lesions, such as chronic congestion, hemosiderosis, lymphoproliferative disorder, are present within an IPAS, they usually have an identical appearance to the same lesions in the main spleen .
CT findings of IPAS
Halpert et al reported by autopsy data that 23 of 37 (62%) IPASs were partly embedded in the substance of the pancreas, whereas 14 (38%) were completely surrounded by the pancreatic tissue and their presence could be ascertained only on cur surfaces . This autopsy study included 36 IPAS that ranged from 0.3–3.5 cm. In our experience, when IPAS is observed on CT, it is not often completely surrounded by the pancreatic parenchyma, and a part of the surface of IPAS is exposed to the peripancreatic fat (Figs. 2, 3, 4, 5, 6, 7, 8). IPASs that are completely surrounded by pancreas are likely small, but those small IPAS may not be visualized on CT.
Also in our cases, when an IPAS is located close to the tip of the tail of the pancreas, the exposed surface of IPAS (i.e., incompletely surrounded by the pancreatic tissue) is often seen along the dorsal surface of the pancreatic tail (Figs. 7, 8), than along the anterior surface of the pancreatic tail. However, further data are needed to confirm this finding.
CT attenuation and enhancement pattern
The blood supply to an accessory spleen is usually derived from the splenic artery, with drainage occurring into the splenic vein . Accessory splenic tissue tends to exhibit the similar attenuation both on noncontrast and postcontrast scans, and same pattern of contrast enhancement as does the spleen itself on all phases [6, 12, 13].
Arterial phase CT imaging may be particularly helpful, especially when the characteristic heterogeneous, serpiginous enhancement pattern of normal spleen is also observed within IPAS (Figs. 2, 5, 7). This characteristic heterogeneous enhancement pattern is likely related to the unique histology of the vascular system of the spleen and the different flow rate through the cords of the red pulp and white pulp [6, 12]. It needs to be noted that heterogeneous enhancement pattern may not be detected or appreciated in IPAS at arterial phase CT (Figs. 6, 8) even when the main spleen has a serpiginous enhancement pattern , especially when the IPAS is small.
Differential diagnosis of IPAS
The most important differential diagnosis of IPAS is asymptomatic pancreatic neuroendocrine neoplasm. In review of surgically resected cases of IPAS, the most common preoperative diagnosis was nonfunctioning pancreatic neuroendocrine neoplasm . Both IPASs and pancreatic neuroendocrine neoplasms are typically hyperenhancing on contrast enhanced CT (Figs. 9, 10, 11). Pancreatic neuroendocrine neoplasms are typically hypervascular, particularly on arterial phase, and may demonstrate uniform or ring-like enhancement  (Fig. 10). However, in some cases, pancreatic neuroendocrine neoplasms will be more enhancing on the venous phase acquisition (Figs. 9, 11). Furthermore, when the mass within the tail of the pancreas is small, the pattern of contrast enhancement may be difficult to analyze on CT (Fig. 11).
An alternative but uncommon differential diagnostic consideration is metastasis to the pancreas. Metastases from renal cell carcinomas often have a hypervascular appearance (Fig. 12), which can have an appearance similar to IPAS . Clinical history of renal cell carcinoma helps to make the correct diagnosis.
Technetium-99m (99mTc) labeled sulfur colloid or 99m Tc labeled heat-damaged red blood cell (HDRBC) scintigraphy has been used to diagnose ectopic splenic tissue (Fig 2). However, use of 99mTc labeled HDRBC scintigraphy is preferred because of reduced uptake by normal liver tissue resulting in improved target-to-background ratio  and higher sensitivity of 99mTc labeled HDRBC than 99mTc sulfur colloid scans to detect accessory spleen or splenosis [17, 18]. In reported cases and in our experience, IPASs that are less than 2 cm can result in false negative 99mTc sulfur colloid SPECT scintigraphy . On the other hand, 99mTc labeled HDRBC scintigraphy is a highly specific method for detecting IPAS, as up to 90% of the injected HDRBCs are trapped by splenic tissue . Nonetheless, when minimal functioning splenic tissue is present, or the lesion is small, visualization of IPAS may be still difficult due to inferior spatial resolution compared to other cross-sectional imaging modalities . For example, Kim et al have reported a 1.1 cm IPAS that was missed on 99mTc labeled HDRBC SPECT scintigraphy .
There are a few reported cases of IPAS or splenosis in the pancreas, in which 111In somatostatin receptors scintigraphy (Octreoscan) was performed due to suspicion of pancreatic neuroendocrine neoplasm resulting in false positive results [9, 21–23]. The likely cause for these false positives is the presence of somatostatin receptors within the splenic tissue, which also can bind octreotide with high affinity .
Superparamagnetic iron oxide (SPIO)-enhanced MRI can also be used for the diagnosis of IPAS. With SPIO-enhanced MRI, IPAS has a signal drop similar to that of the spleen [6, 10, 24]. SPIOs are phagocytosed by the reticuloendothelial system (RES), and SPIO-enhanced MR imaging is reported to be an excellent alternative to 99mTc labeled HDRBC scintigraphy for confirming the diagnosis of IPAS because of its higher spatial resolution than scintigraphy .
Reports describe the use of contrast-enhanced sonography, with a microbubble contrast agent, to reveal the characteristic enhancement feature of IPAS and allows the specific diagnosis of IPAS [3, 25]. Kim et al. observed six patients with IPAS who showed inhomogeneous enhancement in the early vascular phase, and prolonged enhancement in the hepatosplenic parenchymal phase (3–5 min after contrast injection) on contrast enhanced sonography, using galactose and palmic acid (Levovist; Schering, Germany) . This agent is exclusively accumulated by the hepatic and splenic parenchyma due to RES activity on the delayed phase [3, 25].
Epidermoid cyst within IPAS
In conclusion, IPAS should be considered when a hypervascular mass is seen in the tail of the pancreas. Radiologist should recognize the imaging overlap with hypervascular masses in the tail of the pancreas, particularly asymptomatic pancreatic neuroendocrine neoplasms. Typical location, similar attenuation of the lesion to the spleen on noncontrast, and postcontrast CT at different phases are helpful to make diagnosis of IPAS. In particular, characteristic heterogeneous contrast enhancement of IPAS during the early postcontrast phase may prove most helpful for correct diagnosis. However, when it remains difficult to exclude the other diagnosis, 99mTc labeled HDRBC scintigraphy or SPIO-enhanced MR imaging can be used to confirm the diagnosis of IPAS, with the caveats described above. Future research should be aimed at systematically comparing the CT appearance of IPAS and pancreatic neuroendocrine neoplasm, in terms of location within the pancreatic tail, percentage of exposure to the peripancreatic fat and enhancement relative to both pancreas and spleen, in an attempt to define reliable criteria for preoperative diagnosis.