Elliot K. Fishman M.D.
Johns Hopkins Hospital

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Macroscopic anatomy

• An intraperitoneal organ, almost entirely surrounded by the peritoneum, which firmly adherent to the capsule.
• The splenic hilum is usually directed anteromedially, and the splenic artery and vein enters the spleen in this region through the splenorenal ligament.
• The splenorenal and gastrosplenic ligaments are the two folds of peritoneum that hold the spleen in its position.
• Normal splenic size: approximately 200g, 12 cm length, 3-4 cm thickness, 7 cm width.

Histological anatomy

• Red pulp: Complex network of sinusoids and splenic cords
  • Functions as a filter and blood flow regulator
  • The site of erythrocyte storage and macrophage proliferation and differentiation
• White pulp: Consists of lymphatic tissue, contains germinal centers.
  • The site of the spleen’s immunological and cytopoietic function
• Marginal zone: An ill-defined interphase between red/white pulp.
• Transient heterogeneous pattern of contrast enhancement is thought to be related to the variable rates of blood flow through the red/white pulp.

Accessory spleen

• Found in 10-20% of individuals.
• Within the tail of the pancreas is the second common site of the accessory spleen, and can mimic hypervascular pancreatic tumor (e.g. neuroendocrine tumor)
• Accessory spleen can be a site of relapse of hypersplenism after splenectomy in patients with a hematologic disorder with hypersplenism.

Splenosis

• Ectopic splenic tissue caused by autotransplantation of splenic cells resulting from traumatic disruption of the splenic capsule via trauma or surgery.
• More numerous and widespread than accessory spleens.

Polysplenia

• A rare complex syndrome, consists of situs ambiguous with features of left isomerism (bilateral left-sidedness)
• Multiple spleen in right or left upper quadrant, single, lobulated spleen, or a normal spleen
• Anomalous position of abdominal viscera, short pancreas, abnormal bowel rotation, cardiovascular anomalies

Asplenia

• Absent spleen, situs ambiguous, multiple anomalies including cardiovascular anomalies (typically more complex than those with polysplenia), bowel malrotation, genitourinary tract anomalies.

Wandering spleen

• The spleen migrate from its normal position due to congenital or acquired laxity of the splenic suspensory ligament.
• At risk of vascular pedicle torsion and splenic infarct.

“ Typically, accessory spleens appear on CT scans as well marginated, round masses that are smaller than 2 cm and enhance homogeneously on contrast-enhanced images.”
CT Features of the Accessory Spleen
Mortele KJ et al.
AJR 2004; 183:1653-1657

An accessory spleen, also called a supernumerary spleen, a splenule, or a splenunculus, is a benign and asymptomatic condition in which splenic tissue is found outside the normal spleen. Accessory spleens are a relatively common phenomenon with an estimated 10% to 30% of the population having one.

“IPAS are the result of splenic tissue buds failing to fuse during embryologic development and are quite common, found in 10%-20% of individuals. Accessory splenic tissue is usually asymptomatic and found incidentally with the most common location in the splenic hilum. However, 10%-15% are found in the pancreatic tail where they pose a diagnostic predicament.”
Pancreatic Incidentalomas: A Management Algorithm for Identifying Ectopic Spleens 
Baugh KA et al.
J Surg Res. 2019 Apr;236:144-152

“After traumatic splenic injury or splenectomy, small isolated spleens may develop. These implants are not limited to the left upper quadrant, and splenosis in other locations can mimic other pathologic entities. This pictorial essay presents the range of appearances of intraabdominal and pelvic splenosis.”
CT of splenosis: patterns and pitfalls.
Lake ST, Johnson PT, Kawamoto S, Hruban RH, Fishman EK.
AJR 2012 Dec;199(6)W686-93

“ Differentiation from a hypervascular pancreatic neoplasm (e.g. islet cell tumor) is, therefore sometimes challenging.”
CT Features of the Accessory Spleen
Mortele KJ et al.
AJR 2004; 183:1653-1657

“CT can be used to differentiate between IPAS and PanNET with good specificity and sensitivity. The IPAS mirrors the spleen’s enhancement and is usually located along the dorsal surface of the pancreas.”
Intrapancreatic Accessory Spleen: Possibilities of Computed Tomography in Differentiation From Nonfunctioning Pancreatic Neuroendocrine Tumor 
Coquia SF,Kawamoto S, Hruban RH, Fishman EK
J Comput Assist Tomogr. 2014 Nov-Dec;38(6):874-8

“The reader should look for enhancement of the IPAS matching the enhancement pattern of the spleen on multiphase CT examination. Furthermore, routine evaluation of the splenic vein should be performed with each lesion as occlusion of the vein has been associated with non- functioning PanNETs.”
Intrapancreatic Accessory Spleen: Possibilities of Computed Tomography in Differentiation From Nonfunctioning Pancreatic Neuroendocrine Tumor 
Coquia SF,Kawamoto S, Hruban RH, Fishman EK
J Comput Assist Tomogr. 2014 Nov-Dec;38(6):874-8

“Although not statistically significant, several other findings are also helpful to differentiate IPAS and neuroendocrine tumors. All IPASs in this study were located at the tip or within 3 cm of the tip of the tail of the pancreas. Therefore, if an enhancing mass is seen more than several centimeters from the tip of the tail of the pancreas, it is less likely to represent IPAS and more likely a neuroendocrine tumor.”
Intrapancreatic Accessory Spleen: Possibilities of Computed Tomography in Differentiation From Nonfunctioning Pancreatic Neuroendocrine Tumor 
Coquia SF,Kawamoto S, Hruban RH, Fishman EK
J Comput Assist Tomogr. 2014 Nov-Dec;38(6):874-8

“ In conclusion, CT can be used to differentiate between IPASs and PanNETs with a high degree of sensitivity and specificity. Specific findings on CT are more prevalent with IPASs and can help increase diagnostic confidence. These findings include a lesion that is not completely embedded in the pancreatic parenchyma, a lesion that is located along the dorsal surface of the pancreas, a lesion that shows heterogeneous enhancement at the arterial phase, and a lesion that has the same degree of en- hancement of the spleen at the venous phase.”
Intrapancreatic Accessory Spleen: Possibilities of Computed Tomography in Differentiation From Nonfunctioning Pancreatic Neuroendocrine Tumor 
Coquia SF,Kawamoto S, Hruban RH, Fishman EK
J Comput Assist Tomogr 2014 (in press)

“In cases where the reader finds the lesion as indeterminate, although most were ultimately PanNETs in our study, given the associated decline in overall reader specificity seen in our study, the CT reader should recommend confirmatory testing such as 99mTc-labeled heat-damaged red blood cell scintigraphy or MRI rather than an observation with fine needle aspiration as needed for confirmation.”
Intrapancreatic Accessory Spleen: Possibilities of Computed Tomography in Differentiation From Nonfunctioning Pancreatic Neuroendocrine Tumor 
Coquia SF,Kawamoto S, Hruban RH, Fishman EK
J Comput Assist Tomogr. 2014 Nov-Dec;38(6):874-8