A 58-year-old man from rural India presents with a 3-month history of epigastric pain radiating to the back, weight loss of 8 kg, and jaundice. Imaging shows a 4 cm solid mass in the pancreatic head with upstream ductal dilation. Which finding best distinguishes pancreatic ductal adenocarcinoma from chronic pancreatitis with pseudocyst?

## PDAC vs. Chronic Pancreatitis: Key Discriminator ### Clinical Context Both PDAC and chronic pancreatitis can present with: - Epigastric pain radiating to the back - Weight loss - Jaundice (from CBD obstruction) - Upstream pancreatic duct dilation However, the **presence of a solid, enhancing mass with loss of fat planes** is the hallmark of PDAC and distinguishes it from chronic pancreatitis. ### Imaging Comparison Table | Feature | PDAC | Chronic Pancreatitis | |---------|------|----------------------| | **Mass lesion** | Solid, enhancing, well-defined or infiltrative | Diffuse parenchymal atrophy; no discrete mass (unless pseudocyst) | | **Enhancement pattern** | Hypoattenuating in arterial phase; may enhance in portal venous phase | Diffuse, homogeneous enhancement if parenchyma present | | **Fat plane loss** | Yes — tumour infiltrates peripancreatic fat | No — fat planes usually preserved | | **Ductal dilation** | Abrupt transition ("double duct sign") | Gradual, irregular, "beads-on-string" appearance | | **Parenchymal calcification** | Rare | Common (>30% cases) | | **Atrophy** | Focal (distal to mass) | Diffuse | | **Pseudocyst** | Rare; if present, associated with mass | Common | ### Why This Feature Discriminates **Key Point:** A **discrete solid mass with loss of peripancreatic fat planes** is pathognomonic for PDAC. Chronic pancreatitis presents with diffuse parenchymal changes, atrophy, and ductal irregularity — not a focal enhancing mass. **High-Yield:** The "double duct sign" (dilated CBD + dilated pancreatic duct) can occur in both, but a solid mass at the transition point is PDAC until proven otherwise. **Clinical Pearl:** In a patient with risk factors for chronic pancreatitis (alcohol, smoking, chronic inflammation) who develops a focal pancreatic mass, always suspect PDAC — it can coexist with or mimic chronic pancreatitis. ### Why Other Options Are Not Discriminating 1. **Upstream ductal dilation** — occurs in both conditions. In PDAC, there is abrupt obstruction at the tumour; in chronic pancreatitis, dilation is diffuse and irregular. 2. **Parenchymal calcification** — actually favours chronic pancreatitis (>30% of cases); PDAC calcification is rare and, when present, suggests advanced disease. 3. **Elevated bilirubin** — both PDAC and chronic pancreatitis can cause CBD obstruction and jaundice. Not discriminating. ### Diagnostic Algorithm ```mermaid flowchart TD A[Pancreatic mass on imaging]:::outcome --> B{Solid enhancing mass<br/>with fat plane loss?}:::decision B -->|Yes| C[PDAC]:::outcome B -->|No| D{Diffuse parenchymal<br/>atrophy + calcification?}:::decision D -->|Yes| E[Chronic pancreatitis]:::outcome D -->|No| F[Further characterisation<br/>EUS-FNA, MRI]:::action ```