Ampullary adenoma or carcinoma: The location of the hypodense lesion at the ampulla of Vater, along with the dilatation of the common bile duct and pancreatic duct, strongly suggests a lesion originating from the ampulla. Adenomas and carcinomas in this area can cause obstructive symptoms and ductal dilatation.

Choledocholithiasis: A stone in the common bile duct could cause dilatation of the bile duct and potentially affect the pancreatic duct, depending on its location and the presence of a stone in the ampulla of Vater.
Pancreatic adenocarcinoma: Although less likely given the specific location at the ampulla, a pancreatic adenocarcinoma could cause ductal dilatation and a mass lesion. However, the primary location of pancreatic adenocarcinoma is more commonly in the pancreatic head, body, or tail.
Intraductal papillary mucinous neoplasm (IPMN): IPMNs can cause dilatation of the pancreatic duct and can be associated with a mass at the ampulla if the IPMN involves the main pancreatic duct and extends to the ampulla.

Cholangiocarcinoma: A malignancy of the bile duct that could present with similar findings, including ductal dilatation. It's crucial to consider this diagnosis due to its significant implications for treatment and prognosis.
Pancreatic neuroendocrine tumor (PNET): Although less common, PNETs can occur in the head of the pancreas and cause ductal obstruction. They have a varied prognosis depending on their characteristics and potential for malignancy.

Gastrointestinal stromal tumor (GIST) of the duodenum: A rare tumor of the duodenum that could potentially cause extrinsic compression of the bile and pancreatic ducts, leading to dilatation.
Lymphoma involving the ampulla of Vater: A rare condition where lymphoma involves the ampulla, causing obstructive symptoms and ductal dilatation.
Metastasis to the ampulla of Vater: Metastatic disease from other primary sites can rarely involve the ampulla, mimicking primary ampullary lesions.