Walled-off necrosis (WON) is a collection containing heterogeneous fluid and solid necrotic debris, encapsulated by a mature inflammatory wall that develops ≥4 weeks after necrotizing pancreatitis. According to the revised Atlanta classification (2012), it is the matured form of acute necrotic collection (ANC). Unlike pseudocyst, it contains solid necrotic components (non-liquefied pancreatic or peripancreatic fat necrosis). Infection risk is high (30-70%) and infected WON significantly increases mortality. The step-up approach (percutaneous drainage → endoscopic necrosectomy → surgery) is preferred for treatment.
Age Range
30-70
Peak Age
50
Gender
Male predominant
Prevalence
Uncommon
WON develops as a result of necrosis of pancreatic and peripancreatic tissues in necrotizing pancreatitis. In acute necrotizing pancreatitis, ischemic necrosis of the pancreatic parenchyma and lipase-mediated necrosis of peripancreatic fat tissue occurs. In the first 4 weeks, this necrotic tissue accumulates as a mixture of fluid and solid debris known as 'acute necrotic collection' (ANC). Over time, the inflammatory response of surrounding tissues forms a fibrous capsule (mature wall) that delimits the collection — this maturation process takes ≥4 weeks and is defined as WON. WON content is HETEROGENEOUS unlike pseudocyst: liquefied necrotic debris (fluid), non-liquefied solid necrotic tissue fragments, and fat necrosis material coexist. This heterogeneous content is reflected on imaging as characteristic fluid-debris levels and solid components. Infection risk is high because necrotic tissue provides a substrate for bacterial colonization — gut translocation is the primary infection mechanism.
Non-enhancing solid debris islands (non-liquefied necrotic tissue fragments) within the encapsulated collection on CT — the pathognomonic finding distinguishing WON from pseudocyst. These solid islands represent 'floating' necrotic tissue fragments in the fluid medium and show 20-50 HU density on CT. Solid debris amount determines treatment strategy: minimal debris → drainage sufficient, abundant debris → necrosectomy required. On MRI, they appear as low-signal islands on T2, and MRI is superior to CT for evaluating solid debris amount.
In the portal venous phase, a heterogeneous collection surrounded by a thick (>4 mm), irregular/lobulated contoured, enhancing inflammatory wall. The content contains areas of fluid density (0-20 HU) coexisting with solid/semi-solid necrotic debris (20-50 HU). Fluid-debris levels may be visible.
Report Sentence
A heterogeneous collection (fluid + solid necrotic debris) measuring [X×Y] mm, surrounded by a thick irregular enhancing wall, is observed in the peripancreatic area following necrotizing pancreatitis, consistent with walled-off necrosis (WON).
Gas-density (-1000 HU) foci within the collection — strong indicator of infected WON. Gas bubbles typically accumulate in the upper part of the collection (fluid-gas level) or are scattered within the debris.
Report Sentence
Gas bubbles are observed within the collection, suggesting infected walled-off necrosis; clinical correlation and urgent treatment planning are recommended.
On T2-weighted sequences, heterogeneous signal pattern within the collection: hyperintense fluid areas coexisting with intermediate-to-low signal solid necrotic debris islands. Fluid-debris levels may be visible. Distinctly different from the homogeneous T2 signal of pseudocyst.
Report Sentence
Heterogeneous signal pattern is observed within the collection on MRI, with T2-hyperintense fluid areas coexisting with low-signal solid necrotic debris islands; consistent with walled-off necrosis.
Thick (>4 mm), prominently enhancing inflammatory wall around the collection in arterial and portal venous phases. Wall contours may be irregular or lobulated. Wall enhancement reflects neovascularization and inflammatory hyperemia.
Report Sentence
A thick ([X] mm), irregular, prominently enhancing inflammatory wall is observed around the collection, consistent with matured walled-off necrosis wall.
On transabdominal US, a collection with heterogeneous echogenicity: mixture of anechoic fluid areas with hyperechoic/isoechoic solid debris areas. Internal debris may form fluid-debris levels. The thick, irregular wall appears as a hyperechoic band.
Report Sentence
A collection with heterogeneous echogenicity containing a mixture of fluid and solid debris is observed in the peripancreatic area on US; consistent with walled-off necrosis.
Criteria
No signs of infection: no gas, no fever/leukocytosis or mild, FNA/aspiration negative.
Distinct Features
Asymptomatic sterile WON can be managed with conservative follow-up — most resolve over time. Step-up approach is applied for symptomatic (pain, obstruction, feeding difficulty) or enlarging sterile WON. Initial intervention is usually percutaneous or EUS-guided drainage.
Criteria
Signs of infection: intracollection gas, clinical sepsis (fever, leukocytosis, organ failure), positive gram stain/culture on FNA.
Distinct Features
Mortality rate is much higher than sterile WON (15-35%). Antibiotherapy + intervention is mandatory. Step-up approach: percutaneous/EUS drainage → video-assisted retroperitoneal debridement (VARD) or endoscopic necrosectomy → open surgery (last resort). Intervention is generally preferred after 4 weeks (wall maturation awaited) but early intervention is performed in life-threatening sepsis.
Criteria
Pancreatic duct completely disrupted at the necrosis site — proximal and distal duct segments separated from each other. Duct disruption can be demonstrated on MRCP.
Distinct Features
High risk of collection recurrence after drainage (duct leak continues). Long-term transmural stent placement or distal pancreatectomy may be required. EUS-guided transmural drainage with permanent stent placement is the preferred approach.
Distinguishing Feature
Pseudocyst has HOMOGENEOUS fluid content (NO solid debris) surrounded by thin smooth wall (<2 mm). WON has HETEROGENEOUS content (fluid + solid necrotic debris) surrounded by thick irregular wall (>4 mm). This distinction is more evident on MRI (solid debris as low-signal islands on T2). Treatment approach differs: pseudocyst → simple drainage, WON → may require necrosectomy.
Distinguishing Feature
Ductal adenocarcinoma appears as a hypovascular solid mass, shows enhancement, and causes vascular invasion/duct obstruction. WON is a heterogeneous collection accompanying pancreatitis history with non-enhancing solid debris. However, atypical situations exist such as periampullary carcinoma mimicking WON or WON mimicking carcinoma — clinical context is critical.
Distinguishing Feature
Main duct IPMN is visualized as a dilated pancreatic duct (tubular structure), filled with mucinous fluid, and may contain mural nodules. WON is a large extra-pancreatic or pancreatic parenchyma-involving collection, duct dilatation is not the dominant finding, and pancreatitis history is present. On MRCP, IPMN shows duct morphology while WON shows collection morphology.
Urgency
urgentManagement
interventionalBiopsy
Not NeededFollow-up
specialist-referralWON management requires a multidisciplinary approach (gastroenterology, surgery, interventional radiology, ICU). Sterile asymptomatic WON may be followed conservatively but symptomatic or infected WON requires intervention. Step-up approach is preferred: 1) Percutaneous or EUS-guided drainage (least invasive, first step), 2) Endoscopic necrosectomy or VARD (if drainage insufficient), 3) Open surgical necrosectomy (last resort). Intervention is generally deferred to ≥4 weeks (wall maturation) but early intervention is performed in life-threatening sepsis. In infected WON, antibiotherapy (carbapenem or quinolone) is initiated concurrently. In the presence of disconnected duct syndrome, long-term transmural stent placement is considered.
WON may become infected and require endoscopic or percutaneous drainage. A step-up approach (percutaneous drainage -> endoscopic necrosectomy -> surgery) is recommended for treatment. Asymptomatic WON may be managed conservatively.