Most Likely Diagnosis: Pancreatic Pseudocyst
A 9 cm cystic lesion in the lesser sac posterior to the stomach is most likely a pancreatic pseudocyst (Option D), particularly given its size and anatomic location. 1, 2
Diagnostic Reasoning
Location and Size Favor Pseudocyst
The lesser sac (omental bursa) is the classic anatomic location for pancreatic pseudocysts, which develop as a complication of acute or chronic pancreatitis and commonly extend into this space posterior to the stomach. 2, 3
Pseudocysts smaller than 6 cm resolve spontaneously in approximately 60% of cases, but a 9 cm lesion represents a mature, established collection that is less likely to resolve without intervention. 1, 2
Why Not the Other Options?
Colon neoplasm (Option A) is anatomically implausible—the colon does not typically produce cystic lesions in the lesser sac posterior to the stomach. [@General Medicine Knowledge]
Stomach neoplasm (Option B) would arise from the gastric wall itself and would not present as a discrete cystic lesion in the lesser sac; gastric tumors are typically solid or ulcerative masses. [@General Medicine Knowledge]
Pancreatic neoplasm (Option C) remains in the differential but is statistically less likely. While mucinous cystic neoplasms and intraductal papillary mucinous neoplasms can present as large cystic lesions, incidental pancreatic cysts carry a very low probability of malignancy (10–17 cases per 100,000 individuals). 1 Additionally, cystic lesions ≥3 cm are considered worrisome features with a three-fold increased risk of malignancy, but this still makes pseudocyst the more common diagnosis in the appropriate clinical context. 1
Critical Diagnostic Steps
Confirm the Diagnosis with Imaging
Contrast-enhanced MRI with MRCP is the preferred initial modality, achieving sensitivity of approximately 96.8% and specificity of 90.8% for confirming a pancreatic pseudocyst. 2
MRI identifies internal septations in 91% of cases and demonstrates pancreatic duct communication in up to 100% of cases, which are key features distinguishing pseudocysts from neoplasms. 1, 2
Look for Worrisome Features Suggesting Neoplasm
If any of the following are present, strongly consider cystic neoplasm and proceed to EUS-FNA: 1, 2
- Enhancing mural nodules or solid components within the cyst
- Thickened or enhancing cyst wall
- Main pancreatic duct diameter of 7–10 mm (worrisome) or ≥10 mm (high-risk stigmata)
- Peripheral hypervascular rim on arterial-phase CT (suggests cystic pancreatic neuroendocrine tumor)
Obtain Cyst Fluid Analysis if Diagnosis Uncertain
Cyst fluid amylase >250 IU/L is diagnostic of a pancreatic pseudocyst. 1
Cyst fluid CEA <5 ng/mL favors a pseudocyst or serous cystadenoma rather than a mucinous neoplasm. 1
Cyst fluid CEA >192–200 ng/mL indicates a mucinous cystic neoplasm with 73–80% diagnostic accuracy, and all mucinous cystic neoplasms should be surgically resected because of their malignant potential. 1
Common Pitfalls to Avoid
Do not assume all large cystic lesions in the lesser sac are benign pseudocysts—approximately 20% of radiologic diagnoses of pseudocyst are inaccurate, and it is imperative to exclude neoplasm before drainage. 4, 5
Do not drain a suspected pseudocyst before 4 weeks from pancreatitis onset, as early drainage carries a 44% complication rate compared with 5.5% when delayed to ≥4 weeks. 1, 2
Do not base intervention solely on size—symptoms, complications, and timing relative to pancreatitis onset are the primary determinants for drainage. 2, 4
Obtain a thorough history of prior pancreatitis, alcohol use, gallstones, or abdominal trauma to support the diagnosis of pseudocyst. 6, 7