July 2012, VOLUME 199
NUMBER 1

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July 2012, Volume 199, Number 1

Residents’ Section

Pattern of the Month

Focal Cystic Abdominal Masses in Pediatric Patients

+ Affiliations:
1 Department of Radiology, Children’s Hospital Boston, Harvard Medical School, Boston, MA.

2 Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Ave, Boston, MA 02215.

Citation: American Journal of Roentgenology. 2012;199: W1-W16. 10.2214/AJR.11.6642

Keywords: abdominal cystic masses, children, pediatric

Focal abdominal cystic masses are common in pediatric patients. Affected patients present with various symptoms and physical findings depending on size, location, and mass effect on the adjacent abdominal structures. The symptoms in these patients can be abdominal pain, early satiety, bowel obstruction, or fever if the underlying cause of the mass is infection. The physical signs are abdominal distention or a palpable mass.

The sites of origin of focal cystic masses are from various abdominal organs and mesentery in pediatric patients (Table 1). For the purpose of evaluation and diagnosis, focal abdominal cystic masses in children can be divided into three categories on the basis of their region of origin: solid organs (liver, kidney, pancreas, spleen, and adrenal gland), mesentery, and bowel.

Although ultrasound is the imaging modality of choice for initial evaluation of focal abdominal cystic masses, CT or MRI is often subsequently obtained for confirmation and further characterization of focal abdominal cystic masses in pediatric patients. Understanding the characteristic imaging findings of focal cystic abdominal masses that can be symptomatic or incidental findings in pediatric patients can improve pediatric patient care by guiding the next appropriate step in management.

Solid Organ of Origin
Next section
Hepatobiliary System

Hepatic cyst—Hepatic cysts in pediatric patients can be congenital or acquired. Whereas congenital hepatic cysts occur when an intrahepatic biliary duct fails to involute, acquired hepatic cysts result from trauma or infection in the pediatric population. On histology, hepatic cysts consist of a layer of simple cuboidal epithelium. In pediatric patients, hepatic cysts are usually benign and do not require follow-up imaging evaluation. On ultrasound, a hepatic cyst is an anechoic round or oval lesion with increased posterior through-transmission and without mural nodularity.

TABLE 1: Focal Cystic Abdominal Masses in Children

Although subsequent imaging study is not necessary for further evaluation of hepatic cysts once the diagnosis is made with ultrasound, hepatic cysts also can be incidental findings on cross-sectional imaging studies, such as CT or MRI, obtained for other indications. On CT and MRI with contrast administration, a hepatic cyst is a nonenhancing well-defined homogeneous water-attenuation lesion with an imperceptible wall (Fig. 1). Administration of IV contrast material is helpful for differentiating simple hepatic cysts from other low attenuation on CT or signal intensity on hepatic lesions found on T1-weighted imaging. Hepatic cysts are typically solitary but can be multiple. The presence of more than 10 hepatic cysts in a child should suggest the possibility of associated polycystic kidney disease. Hepatic cysts in children can be complicated by hemorrhage or infection, but simple cysts without complication require no further imaging follow-up or treatment.

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Fig. 1 Hepatic cyst. Enhanced axial CT image of 1-year-old boy who presented with increasing abdominal distention shows nonenhancing well-defined homogeneous water-attenuation mass (arrows) with imperceptible wall.

Hepatic abscess—A hepatic abscess is a localized collection of pus in the hepatic parenchyma, which can result from bacterial, fungal, amebic, or parasitic infection. On ultrasound, the echogenicity and margins of hepatic abscess vary depending on the stage of the infectious process. At the early stage of the infectious process, hepatic abscesses are hypoechoic to anechoic lesions with ill-defined borders. They become more echogenic with well-defined margins as the abscesses become more organized at the later stage of the infectious process (Fig. 2A). Hepatic abscesses may also contain internal air-fluid levels, septations, gas, or debris. Although diagnosis of hepatic abscess can be obtained with ultrasound in pediatric patients who present with typical clinical signs and symptoms, such as fever, elevated WBC count, and upper abdominal pain, a cross-sectional imaging study, such as CT, may be necessary for confirmation and assessment of extent of disease, particularly before interventional procedure, such as a percutaneous drainage catheter placement.

On CT, a hepatic abscess typically appears as a complex fluid collection, which is hypodense centrally and shows peripheral rim enhancement (Fig. 2B). The best diagnostic clue for a pyogenic hepatic abscess is the “cluster” sign, in which a group of small pyogenic abscesses coalesces into a single large cavity that often has internal septations (Fig. 2B). Although small hepatic abscesses may be treated with antibiotics alone, a large hepatic abscess (> 5 cm) typically requires percutaneous catheter or surgical drainage.

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Fig. 2AHepatic abscess in 13-year-old boy with right upper quadrant pain, fever, and elevated liver function tests. Culture of drainage fluid was positive for Escherichia coli.

A, Transverse ultrasound image of liver shows large cystic mass (arrows) with internal mobile echogenic debris (D).

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Fig. 2BHepatic abscess in 13-year-old boy with right upper quadrant pain, fever, and elevated liver function tests. Culture of drainage fluid was positive for Escherichia coli.

B, Contrast-enhanced axial CT image shows complex fluid collection (arrows) with internal septations of varying thickness in right hepatic lobe.

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Fig. 3 Hydatid infection in 4-year-old boy with fever, chills, and right upper quadrant pain. Serology cultures were positive for Echinococcus granulosus infection. Contrast-enhanced axial CT image shows well-defined cystic mass with several internal endocyst membranes (arrows). (Courtesy of Ali Yikilmaz)

Hydatid infection—Hydatid disease is caused by infection with Echinococcus tapeworm (E. granulosus or E. multilocularis). E. granulosus is the most common form of hydatid disease in humans, and the liver is the primary site of infection. Hepatic hydatid infection is characterized by a large well-defined cystic hepatic mass with multiple peripheral daughter cysts. Daughter cysts can float freely within the mother cyst, and a change in their position with alterations in patient position can confirm the diagnosis of hepatic hydatid infection. Hydatid cysts are typically large, averaging 5 cm in size, and they can potentially grow 2–3 cm annually. These lesions may appear as simple cysts but sometimes contain endocyst membranes (Fig. 3). CT is helpful for evaluation of the dense peripheral calcification of a hydatid cyst, which is usually seen during the healing phase of echinococcal hepatic infection.

Mesenchymal hamartoma—Hepatic mesenchymal hamartoma is a benign hamartomatous growth of mesenchymal tissue in the liver of unknown cause. It occurs in infants and young children less than 2 years old. Histopathologically, mesenchymal hamartoma of the liver is characterized by the admixture of epithelial structures in a loose connective stroma with fluid accumulation suggestive of lymphangiomatous channels. Ultrasound frequently shows a complex cystic mass containing both cystic and solid components (Fig. 4A). The CT features of hepatic mesenchymal hamartoma depend on the amount of stromal tissue, which shows contrast enhancement whereas the cystic component does not enhance (Fig. 4B). Although there are reported cases of spontaneous regression of mesenchymal hamartomas with conservative management in children, surgical resection is usually the preferred choice of management because an increased risk of malignancy and, particularly in symptomatic patients, mass effect from large lesions.

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Fig. 4AHepatic mesenchymal hamartoma in 15-month-old girl with progressively increasing abdominal distention and palpable right upper quadrant mass.

A, Transverse ultrasound image of liver shows large mass with areas of cystic (C) and solid (arrows) components.

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Fig. 4BHepatic mesenchymal hamartoma in 15-month-old girl with progressively increasing abdominal distention and palpable right upper quadrant mass.

B, Contrast-enhanced axial CT image of liver shows large mass with multiple cysts (C) of variable size and solid components (arrows).

Choledochal cyst—Choledochal cysts are a spectrum of congenital developmental abnormalities of the biliary system in which there is fusiform dilatation of the extrahepatic and intrahepatic bile ducts. The Todani classification of choledochal cysts is as follows:

  • Type 1: Segmental or diffuse fusiform dilatation of the common bile duct (most frequent)

  • Type 2: Diverticulum of an extrahepatic duct

  • Type 3: Choledochocele

  • Type IVa: Multiple extrahepatic bile duct cysts with intrahepatic biliary involvement

  • Type IVb: Multiple extrahepatic bile duct cysts alone

  • Type 5: Cystic dilatation of the intrahepatic bile ducts (Caroli disease)

Pediatric patients with choledochal cysts typically present with jaundice, abdominal pain, and a mass. Ultrasound and MRI are the two best imaging modalities to detect and characterize choledochal malformations in children. MRI using an MRCP protocol with thick-slab T2-weighted turbo spin-echo (TSE) and HASTE in the coronal, coronal oblique, and axial planes is the MRI protocol for evaluating choledochal cysts. The findings can show a choledochal cyst that is separate from the gallbladder but communicates with the biliary ducts and shows abrupt changes in caliber at the junction between the dilated segment and normal ducts (Figs. 5A, 5B, and 5C). Dilatation of multiple intrahepatic bile ducts is typically seen in Caroli disease (Figs. 6A, 6B, and 6C). Typical complications of choledochal cysts include infection, stone formation, and bile duct carcinomas. The current treatment of choice is surgical resection.

Kidney

Hydronephrosis—Hydronephrosis, characterized by a dilated renal pelvis that communicates with dilated calyces, is the most frequent abdominal mass in neonates and infants. Imaging evaluation plays an important role by differentiating a marked hydronephrosis that may mimic a true intrarenal cystic mass. In the pediatric population, the three most frequent causes of hydronephrosis are ureteropelvic junction (UPJ) obstruction, ureterovesicular junction obstruction, and renal duplication. Additionally, patients with prune belly syndrome can also present with bilateral varying degrees of hydronephrosis and hydroureters.

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Fig. 5ACholedochal cyst in 9-year-old boy who presented with epigastric mass and jaundice.

A, Transverse ultrasound image of liver in region of porta hepatis shows large fusiform dilatation of bile duct (C). Also noted is adjacent gallbladder (GB).

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Fig. 5BCholedochal cyst in 9-year-old boy who presented with epigastric mass and jaundice.

B, Contrast-enhanced axial T1-weighted MR image shows nonenhancing large mass (C) in region of porta hepatis with mass effect on adjacent liver parenchyma. Also noted is adjacent gallbladder (GB).

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Fig. 5CCholedochal cyst in 9-year-old boy who presented with epigastric mass and jaundice.

C, Coronal MRCP image shows large cystic mass (C) corresponding well with findings seen on ultrasound image in A).

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Fig. 6ACaroli disease in 9-month-old girl who presented with increasing abdominal distention and vomiting.

A, Transverse ultrasound image of liver shows multiple cystic dilatations of intrahepatic bile ducts (arrows).

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Fig. 6BCaroli disease in 9-month-old girl who presented with increasing abdominal distention and vomiting.

B, Contrast-enhanced axial CT image shows multiple areas of low-attenuation cystic dilatation of intrahepatic bile ducts (arrows). Also noted is ascites.

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Fig. 6CCaroli disease in 9-month-old girl who presented with increasing abdominal distention and vomiting.

C, Axial T2-weighted image of liver shows cystic dilatation of multiple intrahepatic bile ducts (arrows).

UPJ obstruction is the most frequent cause of hydronephrosis in the pediatric population. UPJ obstruction typically produces severe hydronephrosis, which ends abruptly at the UPJ and is associated with a normal caliber of the downstream ureter (Fig. 7). In severe cases of UPJ obstruction, the renal pelvis is disproportionately enlarged compared with the calyces. Theoretic causes for UPJ obstruction include abnormal smooth muscle arrangement of the proximal ureter that impairs distensibility, abnormal innervation of the proximal ureter impairing peristalsis, and a crossing vessel or fibrous scar at the UPJ. UPJ obstruction can be evaluated with ultrasound and 99mTc-MAG3 (mercaptoacetyltriglycine) renography. Ultrasound can show structural dilatation of the renal collecting system, whereas 99mTc-MAG3 renography provides functional information of renal excretion in patients with UPJ obstruction. After injection of 99mTc-MAG3, sequential images of the kidney are obtained. In cases of UPJ obstruction, there is no drainage from the dilated renal collecting system into ureter and bladder despite hydration and diuretic washout. The current treatment of choice for UPJ obstruction is pyeloplasty to resect the narrowed segment at the UPJ or to reroute the crossing vessel. In pediatric patients with UPJ obstruction, pyeloplasty is indicated when there is persistent or worsening renal collecting system obstruction or deteriorating renal function on serial follow-up studies, such as ultrasound and 99mTc-MAG3 renography.

Another cause of hydronephrosis in pediatric patients is ureterovesicular junction obstruction. Megaureter is the most frequent type of ureterovesicular junction obstruction, in which the renal collecting system and ureter are dilated because of a functional aperistalsis of the distal ureter at the juxtavesical ureteral segment. Other less frequent causes of ureterovesicular junction obstruction include ureterocele and distal ureteral stricture. The amount of hydroureteronephrosis depends on the degree of obstruction. Ultrasound and voiding cystourethrography (VCUG) are the two complementary imaging modalities for diagnosing ureterovesicular junction obstruction in pediatric patients. Ultrasound shows dilatation of the renal collecting system and ureter, and VCUG shows no vesicoureteral reflux.

Duplication of the collecting system of the kidney is characterized by the presence of two separate pelvocaliceal collecting systems in one kidney. Two draining ureters may join above the bladder (partial duplication) or insert into the bladder separately (complete duplication). The ureter draining the upper pole tends to insert in the bladder inferior and medial to the ureter draining the lower pole (Weigert-Meyer rule). The upper pole ureter, which develops later than the lower pole ureter with a normal anatomic location of insertion into the bladder, is often ectopic and associated with ureterocele. Although the dysplastic upper pole collecting system tends to distend, the lower pole collecting system tends to have vesicoureteral reflux. On ultrasound, a band of renal cortex crossing the medullary portion of the kidney, which separates the upper and lower pole collecting systems, is typically seen (Figs. 8A and 8B). There may be associated hydronephrosis, especially if there is obstruction (Figs. 8A and 8B). Chronic obstruction, infection, and scarring in children with untreated renal duplication may lead to long-term complications, such as hypertension and renal insufficiency.

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Fig. 7 Newborn girl with prenatal diagnosis of ureteropelvic junction obstruction. Longitudinal ultrasound image of right kidney shows marked hydronephrosis. Renal pelvis (RP) is disproportionately enlarged compared with calyces (C). Left kidney was normal and there was no dilatation of ureters.

The prune belly syndrome is a congenital disorder of the urinary system, characterized by a triad of hypoplastic or absent abdominal wall musculature, urinary tract anomalies, and cryptorchidism. The incidence of the prune belly syndrome is approximately one in 40,000 births and occurs almost exclusively in male infants (97%). Although the underlying cause of the prune belly syndrome is unknown, high-grade urethral obstruction from a posterior urethral valve or urethral atresia, a primary defect in the mesoblast, and a complex chromosomal mutation have been implicated. Affected patients typically present with bilateral varying degrees of hydronephrosis and hypoperistaltic, tortuous hydroureters (Fig. 9). Other less frequent findings in patients with the prune belly syndrome include a dilated posterior urethra or prostatic utricle.

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Fig. 8AUreteropelvic duplication in 19-day-old girl with prenatal diagnosis of hydronephrosis and duplex kidney.

A, Longitudinal ultrasound image of right kidney shows markedly dilated upper pole (UP) moiety and mildly dilated lower pole (LP) moiety. Dilated upper pole ureter (UU) and lower pole ureter (LU) are also seen. Upper pole and lower pole collecting systems are separated by band of renal cortex (arrow).

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Fig. 8BUreteropelvic duplication in 19-day-old girl with prenatal diagnosis of hydronephrosis and duplex kidney.

B, Longitudinal ultrasound image of bladder (BL) shows upper pole ureter terminating in bladder as ureterocele (UC).

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Fig. 9 Prune belly syndrome in 1-day-old boy with prenatal diagnosis of bilateral hydronephrosis and oligohydramnios. Transverse ultrasound image of left kidney shows marked hydronephrosis (HN) and dilated ureter (DU). Right hydroureteronephrosis was also detected (not shown).

Renal abscess—Renal abscess is characterized by a collection of pus related to suppurative necrosis in the kidney. Most renal abscesses are the result of inadequately treated renal infection that ultimately liquefies. Less frequently, renal abscesses can occur from the direct spread of an infectious process from other adjacent organs, trauma, or surgery. Immunocompromised pediatric patients are at risk for developing renal abscesses, similar to adult patients. Imaging characteristics of renal abscess are similar to those seen in other organs, with ultrasound showing an avascular lesion of varying echogenicity and wall thickening (Fig. 10A). Renal abscess may be associated with low-level internal echoes that move with changes in patient position, “dirty” posterior shadowing characteristic of internal gas, or internal septations. On CT, a renal abscess often shows internal septations and a rim sign (enhancement of the abscess wall with a central low density within the collection) (Fig. 10B). Mass effect, adjacent inflammation of the perinephric or renal sinus fat, or mild dilatation of the renal pelvis and ureter may also be present. The current management of renal abscess in children includes antibiotic treatment, with or without aspiration, and follow-up imaging to confirm abscess resolution.

Multicystic dysplastic kidney—Multicystic dysplastic kidney is a nonhereditary developmental renal dysplasia. The underlying cause of multicystic dysplastic kidney is believed to be early in utero urinary tract obstruction. On pathology, there are noncommunicating cysts of varying sizes that represent the dilated collecting system. These cysts are separated by primitive dysplastic renal tissues that are nonfunctional. Unlike autosomal-dominant polycystic kidney disease, which is the most frequent hereditary cystic kidney disease involving both kidneys, multicystic dysplastic kidney almost always involves a unilateral kidney. Approximately 20–50% of patients with multicystic dysplastic kidney also have a concomitant contralateral renal abnormality, including vesicoureteral reflux, UPJ obstruction, and primary megaureter.

The imaging features of multicystic dysplastic kidney parallel the underlying pathologic features. On imaging studies, there are multiple cystic masses of variable size and shape in a random distribution with absent or dysplastic renal parenchyma (Fig. 11). Additionally, an identifiable renal pelvis or sinus is absent.

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Fig. 10ARenal abscess in 15-year-old boy who presented with fever, elevated WBC count, and right flank pain.

A, Transverse ultrasound image of right kidney shows avascular heterogeneous area in upper and mid zones.

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Fig. 10BRenal abscess in 15-year-old boy who presented with fever, elevated WBC count, and right flank pain.

B, Contrast-enhanced axial CT image shows heterogeneous lesion with multiple internal septations of varying thickness in right kidney.

Multilocular cystic nephroma—A multilocular cystic nephroma is a rare nonhereditary benign cystic renal neoplasm. It typically occurs in boys between 3 months and 2 years old, or in women between 40 and 50 years old. Multilocular cystic nephromas can range in size from a few centimeters to a mean of approximately 10 cm. In children, multilocular cystic nephroma typically presents as a palpable abdominal mass (Fig. 12A). On ultrasound, the lesion is usually a well-defined multiloculated anechoic mass with echogenic internal septa (Fig. 12B). Although CT is not usually necessary after ultrasound detection, multilocular cystic nephroma may be incidentally detected on CT obtained for other reasons. On CT, it appears as a cystic mass with water attenuation, well-circumscribed margins, and internal septations (Fig. 12C). Because imaging studies cannot reliably distinguish multilocular cystic nephroma from a multilocular cystic renal cell carcinoma or cystic Wilms tumor (Figs. 13A and 13B), surgical excision is the currently accepted management of choice.

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Fig. 11 Mutlicystic dysplastic kidney in 2-week-old boy with prenatal diagnosis of cystic right renal mass. Transverse ultrasound image of right kidney shows numerous cysts (C) of various sizes without recognizable normal renal corticomedullary architecture.

Cystic Wilms tumor—Wilms tumor is the most frequent primary renal neoplasm in the pediatric population. It accounts for approximately 95% of all pediatric renal malignancies and typically occurs in children younger than 5 years old. Pediatric patients with Wilms tumor present with a palpable abdominal mass, pain, hematuria, or hypertension. Wilms tumor may be associated with clinical syndromes or anomalies, including Beckwith-Wiedemann syndrome, Drash syndrome, hemihypertrophy, cryptorchidism, and sporadic aniridia. Approximately 5–10% of children with Wilms tumors have bilateral or multicentric tumors. Although the majority of Wilms tumors are solid renal masses, a small percentage may be purely cystic, referred to as “cystic” Wilms tumor.

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Fig. 12AMultilocular cystic nephroma in 10-month-old boy who presented with palpable left-sided abdominal mass.

A, Abdominal radiograph shows opacity (M) centered in left side of abdomen that produces mass effect on adjacent colon (arrow).

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Fig. 12BMultilocular cystic nephroma in 10-month-old boy who presented with palpable left-sided abdominal mass.

B, Transverse ultrasound image of left kidney shows cystic mass (C) with echogenic septa (arrow). Blood flow within these echogenic septa is also seen.

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Fig. 12CMultilocular cystic nephroma in 10-month-old boy who presented with palpable left-sided abdominal mass.

C, Contrast-enhanced axial CT image shows large water-attenuation mass with well-circumscribed borders and mildly enhancing internal septations (arrows).

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Fig. 13ACystic Wilms tumor in 1-year-old boy who presented with rapidly enlarging right-sided abdomen and increasing irritability.

A, Transverse ultrasound image of right kidney shows large cystic mass (C) arising from right kidney (arrows).

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Fig. 13BCystic Wilms tumor in 1-year-old boy who presented with rapidly enlarging right-sided abdomen and increasing irritability.

B, Contrast-enhanced axial CT image shows large cystic mass (C) with well-circumscribed borders and without internal solid components arising from right kidney (arrows).

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Fig. 14ARhabdoid tumor in 1-year-old girl who initially presented with low-grade fever, symptoms of prolonged upper respiratory tract infection, decreased oral intake, and increased fatigue. On admission, initial physical examination revealed palpable mass in left-sided abdomen.

A, Longitudinal ultrasound image of left kidney shows large left renal mass with subcapsular fluid collection (SF) containing internal debris. K = kidney.

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Fig. 14BRhabdoid tumor in 1-year-old girl who initially presented with low-grade fever, symptoms of prolonged upper respiratory tract infection, decreased oral intake, and increased fatigue. On admission, initial physical examination revealed palpable mass in left-sided abdomen.

B, Contrast-enhanced axial CT image shows large subcapsular fluid collection (SF) within intact left renal capsule (arrow). Also noted is left kidney (K).

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Fig. 14CRhabdoid tumor in 1-year-old girl who initially presented with low-grade fever, symptoms of prolonged upper respiratory tract infection, decreased oral intake, and increased fatigue. On admission, initial physical examination revealed palpable mass in left-sided abdomen.

C, Contrast-enhanced coronal CT image better shows relationship among tumor (T), left kidney (K), and subcapsular fluid collection (SF).

On ultrasound, cystic Wilms tumors are anechoic intrarenal masses (Fig. 13A). Although the diagnosis of cystic Wilms tumor and vascular extension can be made on ultrasound, CT may be used to confirm the renal origin of the tumor, its margins, and the local and distant metastases (Fig. 13B). The current management of Wilms tumor involves multimodal therapy, including surgery, chemotherapy, and radiation therapy for selected patients. Such multimodal therapy results in dramatic improvement in outcome in children with Wilms tumor, with overall 5-year survival rates of 90%.

Rhabdoid tumor—Malignant rhabdoid tumor is a rare and aggressive renal neoplasm in children. It is often associated with early onset of local and distant metastases as well as resistance to chemotherapy. The most frequent sites of metastases are lungs, abdominal lymph nodes, liver, brain, and bone. Although there is no pathognomonic imaging feature of malignant rhabdoid tumor of the kidney in children, it typically presents as a large renal mass composed of both cystic and solid components. The most helpful imaging feature is a peripheral subcapsular crescent-shaped fluid collection, which may be due to either hemorrhage or necrosis (Figs. 14A, 14B, and 14C). However, the definitive diagnosis of rhabdoid tumor is only made by histologic examination. Treatment includes both surgery and chemotherapy, but the prognosis is generally poor.

Pancreas

Pancreatic pseudocyst—A pancreatic pseudocyst is a collection of pancreatic fluid and inflammatory exudates encapsulated by fibrous tissue. It usually develops as a result of posttraumatic or inflammatory injury to the pancreas, when unabsorbed fluid collections organize and form a fibrous capsule within 4–6 weeks after initial injury. On ultrasound, pancreatic pseudocysts are usually well-circumscribed unilocular smooth-walled cystic masses, often with irregularly thickened walls (Fig. 15A). They are most frequently located in the body or tail of the pancreas. Pancreatic pseudocysts can sometimes be complicated by hemorrhage or infection, which is manifested by septations, internal echoes, and fluid-debris levels on imaging studies. On CT, pancreatic pseudocysts are typically round or oval homogeneously hypodense lesions with near-water attenuation (Fig. 15B). However, the attenuation and complexity of the lesion can increase when there is associated hemorrhage or superimposed infection. In pediatric patients, ultrasound is usually used to follow pancreatic pseudocysts, which may require surgical decompression if they continue to grow and cause symptoms.

Pancreatic cystadenoma—Pancreatic cystadenoma is a benign neoplasm arising from acinar cells of the pancreas. It is composed of numerous small cysts that contain proteinaceous fluid and are separated by septa of connective tissue. Depending on the size of the individual cysts, pancreatic cystadenoma can have a variable appearance on ultrasound and CT. On ultrasound, numerous small cysts can appear as solid, partially solid, or multicystic with septations (Fig. 16A). CT shows similar findings and can show enhancement of the septa separating numerous small cysts (Fig. 16B). For small cystadenomas in asymptomatic patients, conservative management with follow-up imaging is sufficient. However, large cystadenomas that cause common bile duct obstruction or atrophy of the pancreas distal to the tumor require complete surgical excision and follow-up.

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Fig. 15APancreatic pseudocyst in 17-year-old girl with history of pancreatitis who presented with new onset of epigastric pain.

A, Longitudinal ultrasound image of epigastric region shows large cystic mass (C) with irregular walls (arrows).

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Fig. 15BPancreatic pseudocyst in 17-year-old girl with history of pancreatitis who presented with new onset of epigastric pain.

B, Contrast-enhanced axial CT image shows large cystic mass (C) arising from pancreas. Mild surrounding inflammatory changes and fluid (arrows) are also seen adjacent to this large pancreatic pseudocyst.

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Fig. 16AMucinous cystadenoma of pancreas in 17-year-old girl who presented with epigastric pain.

A, Transverse ultrasound image of pancreas shows large cystic mass with multiple internal septations (arrows) in pancreatic body and tail.

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Fig. 16BMucinous cystadenoma of pancreas in 17-year-old girl who presented with epigastric pain.

B, Contrast-enhanced axial CT image shows large low-attenuation cystic mass with several mildly enhancing internal septations (arrows) arising from body and tail of pancreas.

Spleen

Splenic cyst—Splenic cysts can be either congenital or acquired, although they appear identical on imaging studies. Congenital splenic cysts (true or epidermoid cysts) contain an inner cellular lining. Acquired splenic cysts (false or pseudocysts) are usually posttraumatic and due to liquefactive necrosis. Both congenital and acquired splenic cysts usually appear as anechoic round lesions with imperceptible walls on ultrasound, although they can show internal echoes if they contain debris or hemorrhage. Posttraumatic splenic cysts often have thicker walls that may be calcified, unlike the imperceptible walls of congenital cysts. There is no internal enhancement on contrast-enhanced CT images (Fig. 17).

Splenic abscess—A splenic abscess is a collection of pus within the splenic parenchyma. On ultrasound, it is typically a hypoechoic lesion containing low-level internal echoes that represent debris and internal septations. On CT, splenic abscesses typically appear as complex fluid collections that are often associated with internal septations, peripheral contrast rim enhancement, and surrounding inflammatory changes (Fig. 18). Splenic abscesses can be divided into microabscesses (≤ 1.5 cm), which are often due to fungal infection (Fig. 19), and larger abscesses (> 1.5 cm), which are typically due to bacterial infection (Fig. 18). Splenic abscesses in children also can be due to cat scratch disease, which is a regional, infectious lymphadenitis caused by a pleomorphic gram-negative bacillus. Children affected with cat scratch disease may present with multiple hypoechoic lesions on ultrasound in the spleen or hypoattenuating lesions on CT of the spleen, representing underlying granulomatous inflammation with abscess formation. Imaging evaluation with ultrasound or CT can be helpful in pediatric patients with splenic abscess for guiding surgical drainage procedures in managing large splenic abscesses, particularly when the abscesses are not responding to medical management with either antibacterial or antifungal medications.

Vascular malformation—Splenic vascular malformations are characterized by abnormal vascular channels lined with a single layer of dysplastic endothelium. Capillary and venous malformations of the spleen have a similar imaging appearance and may be solitary or multiple. Multiple vascular malformations in the spleen often occur in patients with Klippel-Trenaunay-Weber or Beckwith-Wiedemann syndrome. For assessment of the underlying vascular component of splenic vascular malformation, Doppler ultrasound is a helpful imaging technique. Splenic vascular malformation may be an incidental finding on CT performed for other reasons in pediatric patients. On CT, splenic vascular malformations are often multiple, homogeneous, and hypodense and are sometimes associated with central or peripheral calcification.

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Fig. 17 Simple splenic cyst in 10-year-old girl who presented with right lower quadrant pain. Contrast-enhanced axial CT image of abdomen obtained for possible appendicitis shows well-circumscribed round splenic cystic lesion (arrows) without contrast enhancement. Follow-up imaging study showed interval stability of this incidentally detected lesion.

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Fig. 18 Splenic abscess in 11-year-old boy with fever, elevated WBC count, and left flank pain. Contrast-enhanced axial CT image shows large complex fluid collection with internal septations (straight arrows) and perisplenic inflammatory change (curved arrow). Fluid culture obtained after drainage catheter placement was positive for Staphylococcus aureus infection.

Lymphatic malformations are rare benign cystic splenic lesions that can be solitary or multiple. On contrast-enhanced CT images, they typically have low attenuation and do not enhance (Fig. 20).

Adrenal Gland

Adrenal hemorrhage—Adrenal hemorrhage, which typically occurs in infants during the perinatal period, is usually due to birth trauma, anoxia, neonatal sepsis, or dehydration. The right adrenal gland is more often affected than the left. Left adrenal gland hemorrhage is associated with renal vein thrombosis. Clinical findings of neonatal adrenal hemorrhage include a palpable mass, anemia, and jaundice due to underlying hyperbilirubinemia.

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Fig. 19 Splenic abscess in 15-year-old girl with leukemia who presented with fever. Contrast-enhanced axial CT image shows multiple small low-attenuation splenic lesions (arrows). Blood culture was positive for Candida albicans infection.

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Fig. 20 Lymphatic malformation. Contrast axial CT image in 18-year-old girl shows multiple low-attenuation cystic lesions of varying size in spleen.

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Fig. 21ANeonatal adrenal hemorrhage in 5-day-old boy with prenatal diagnosis of cystic mass of right upper quadrant.

A, Longitudinal ultrasound image of suprarenal region shows cystic mass with internal debris (arrow), likely representing blood product and consistent with adrenal hemorrhage. K = kidney.

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Fig. 21BNeonatal adrenal hemorrhage in 5-day-old boy with prenatal diagnosis of cystic mass of right upper quadrant.

B, Follow-up ultrasound image obtained 3 months later shows interval decrease in size of adrenal hemorrhage (arrows). K = kidney.

The imaging appearance depends on the stage of the hemorrhage. On ultrasound, acute adrenal hemorrhage typically presents as a suprarenal cystic mass with variable echogenicity and complexity. In the subacute stage, adrenal hemorrhage becomes more hypoechoic because of liquefaction of the hemorrhage (Figs. 21A and 21B). Later, adrenal hemorrhage becomes hyperechoic as the clot retracts and calcification develops. Follow-up ultrasound is helpful for documenting interval resolution of adrenal hemorrhage in neonates because sonographic differentiation between neonatal adrenal hemorrhage and neuroblastoma can be difficult or impossible. In contrast to neonatal adrenal hemorrhage, which decreases in size within 1–2 weeks, neuroblastoma remains stable or increases in size on follow-up ultrasound.

Cystic neuroblastoma—Neuroblastoma is the most frequent malignant tumor in infancy and most often originates from the adrenal gland. Although cystic neuroblastoma, which can arise from the adrenal gland or any neural crest element of the abdomen, is a rare form of this neoplasm, it should be considered in the differential diagnosis of a cystic adrenal mass, particularly in neonates. On ultrasound, cystic neuroblastoma typically presents as an anechoic or complex echogenic suprarenal mass that may have associated calcification (Fig. 22). Because the imaging appearance of cystic neuroblastoma is often very similar to that of adrenal hemorrhage in neonates, follow-up ultrasound is currently recommended for management. Although adrenal hemorrhage rapidly decreases in size and usually resolves within several weeks, cystic neuroblastoma in the neonate is unlikely to show any substantial interval change in size on follow-up ultrasound.

Mesentery
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Mesenteric Cyst or Lymphatic Malformation

A mesenteric lymphatic malformation, also known as mesenteric cyst, arises from the small or large bowel mesentery. It develops from a proliferation of lymphatic tissue that fails to communicate with the central lymphatic system. A mesenteric cyst can range in size from a few millimeters to 40 cm. On imaging studies, mesenteric lymphatic malformations can be unilocular or multilocular cystic lesions. They sometimes show thin septations, fine calcifications, and variable attenuation or internal echogenicity depending on the composition of the internal fluid (Figs. 23A and 23B). Complications of mesenteric lymphatic malformations include hemorrhage and infection.

CSF Collection

A collection of CSF at the distal end of a ventriculoperitoneal shunt is termed a “CSFoma” and typically occurs secondary to adhesion of the ventriculoperitoneal shunt catheter in the peritoneal cavity and subsequent blockage of peritoneal absorption of the shunted CSF. Children with CSFomas typically present with abdominal distention or shunt malfunction, which manifests as increased intracranial pressure. The diagnosis can be made by ultrasound or CT, which typically show an anechoic abdominal cystic mass located near the tip of the ventriculoperitoneal shunt catheter (Fig. 24). Although CSFomas sometimes resolve spontaneously, shunt revision or aspiration of the lesion should be considered as other treatment options, particularly in symptomatic children.

Abscess

Mesenteric abscesses are localized collections of pus within the mesentery. In children, they are often secondary to infectious or inflammatory processes of the large and small bowel, such as inflammatory bowel disease or ruptured appendicitis. Ultrasound and CT can show a fluid collection that is often complex with internal gas, air-fluid level, or septations (Fig. 25).

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Fig. 22 Cystic neuroblastoma. Longitudinal ultrasound in 2-year-old boy shows complex suprarenal cystic mass (arrows). Also noted is mild pelvocaliectasis of right kidney.

Cystic Teratoma

Intraabdominal mesenteric cystic teratomas are extremely rare. Radiographs often show a large abdominal opacity that displaces bowel loops. Calcifications associated with mesenteric cystic teratoma can sometimes be seen. On ultrasound, the mass has variable cystic and solid components (Fig. 26A). CT shows similar imaging findings, but this modality better shows the fat and calcifications that are often associated with an intraabdominal mesenteric cystic teratoma (Fig. 26B).

Bowel
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Duplication Cyst

Enteric duplication cyst is a rare focal congenital cystic malformation of the gastrointestinal tract. They can occur anywhere along the mesenteric border of the bowel but most frequently involve the jejunum or ileum. Although affected patients may be asymptomatic, the typical clinical presentation includes a palpable abdominal mass, abdominal distention, vomiting (secondary to bowel obstruction), or hemorrhage (secondary to peptic ulceration due to the presence of gastric mucosa in some patients). Pathologically, duplication cysts are characterized by a well-developed coat of smooth muscle, epithelial lining that represents some part of the alimentary tract, and contiguity with some part of the alimentary tract.

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Fig. 23AMesenteric cyst in 14-year-old girl who presented with abdominal pain.

A, Transverse ultrasound image of midabdomen shows large anechoic cystic mass (C) with imperceptible wall.

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Fig. 23BMesenteric cyst in 14-year-old girl who presented with abdominal pain.

B, Contrast-enhanced axial CT image shows well-circumscribed cystic mass (C) without enhancement in midabdomen.

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Fig. 24 CSF collection (“CSFoma”) in 16-year-old boy with ventriculoperitoneal shunt catheter placed after posterior fossa dermoid tumor resection who presented with abdominal pain and headache. Transverse ultrasound image of midabdomen shows loculated fluid collection in midabdomen. Ventriculoperitoneal shunt catheter (arrow) lies within CSFoma.

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Fig. 25 Mesenteric abscess in 2-year-old boy who presented with fever, elevated WBC count, increased irritability, and decreased appetite. Contrast-enhanced axial CT image shows complex fluid collection (straight arrows) with air-fluid level. Note appendicolith (curved arrow) within complex fluid collection. BL = bladder.

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Fig. 26AMesenteric cystic teratoma in 9-month-old girl who presented with palpable abdominal mass.

A, Longitudinal ultrasound image of midabdomen shows complex mass composed of cystic (C) and solid (arrows) components.

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Fig. 26BMesenteric cystic teratoma in 9-month-old girl who presented with palpable abdominal mass.

B, Contrast-enhanced axial CT image shows cystic component (C), fat component (F), and calcification (arrow).

On ultrasound, duplication cysts typically have a spherical, ovoid, or dumbbell shape (Fig. 27). They have a characteristic bowel wall signature, which includes echogenic mucosa, hypoechoic muscular layer, and echogenic serosa (Fig. 27). On CT, gastrointestinal duplication cysts typically appear as round masses with water attenuation and a mildly enhancing wall (Fig. 28). Information regarding complications of gastrointestinal duplication cysts is limited, particularly in the pediatric population. However, reported complications include perforation, intussusception, bowel obstruction, and volvulus. The current treatment of choice for gastrointestinal duplication cysts in symptomatic pediatric patients is surgical excision, whereas there is currently no standardized evidence-based guideline for managing incidentally detected gastrointestinal duplication cysts in asymptomatic pediatric patients.

Conclusion
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Focal cystic abdominal masses are frequent disorders that can be symptomatic or incidental findings in pediatric patients. Imaging evaluation can provide precise information regarding their location, appearance, size, and mass effect on adjacent abdominal structures. This information is crucial for early and correct diagnosis, which, in turn, can lead to optimal patient management.

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Fig. 27 Duplication cyst of ileum in 9-month-old boy who presented with increasing abdominal distention and crying. Transverse ultrasound image of right lower quadrant shows spherical cystic mass with echogenic mucosa (long arrow), hypoechoic muscular layer (short arrow), and echogenic serosa (curved arrow). This complex of findings represents gut signature.

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Fig. 28 Colonic duplication cyst. Contrast-enhanced axial CT image in 2-year-old boy with right lower quadrant pain to evaluate of possible appendicitis shows round cystic mass (C) within right colon surrounded by oral contrast material.

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Selected Reading
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Address correspondence to R. L. Eisenberg ().

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