The imaging features are those of a pancreatic head large cystic lesion suggestive of an IPMN, possible a mixed type given its apparent focal communication with the main pancreatic duct, with other tiny side branches IPMNs scattered along the pancreatic body and tail. The main pancreatic duct is not grossly dilated, as we usually see in the intraductal or mixed types of IPMN.  

Given the size and possibility of a mixed type on imaging, surgical resection was performed. The gross specimen has not shown any communication with the main duct and found a large side branch IPMN. 

Macroscopy:  Labeled "Whipple's".  A pancreaticoduodenectomy specimen comprising pancreatic head with attached fat 85 x 50 x 38mm, duodenum 200 x 45 mm and common bile duct 35 x 7 mm (length x width). The duodenum is filled with mucinous tan material.  Proximal duodenal margin inked blue.  Distal duodenal margin inked black.  Bile duct margin amputated, resulting surface inked black.  Pancreatic neck margin amputated, resulting surface inked yellow. Anterior surface inked blue. Posterior surface inked black. Groove for mesenteric vessels inked green. Probes are passed through the pancreatic duct and the common bile duct, and the tissue is sectioned along this plane.  There is a 42 x 23 x 10 mm cystic lesion probe patent to the pancreatic duct, 33 mm from the pancreatic neck/duct margin. The bulk of the lesion is located within the peripancreatic fatty tissue between the common bile duct and the pancreatic neck margin. The lesion is multiloculated and filled with mucinous material.  The surface fatty tissue surrounding the neoplasm is inked red. The length of the intra-pancreatic cystic duct is 70 mm, common bile duct 80 mm. The lesion appears mostly confined to the posterior portion of the pancreatic head. The background pancreas has a lobulated tan appearance.  There are no other lesions.  Several lymph nodes are identified, the largest up to 25 mm in the region of the CBD.  The node is admixed with firm calcified nodules.  These are submitted for decalcification. 
Microscopy: Sections taken through the cystic lesion show a lesion adjacent to the common bile duct and adjacent the pancreatic duct composed of multiple cysts of varying size lined predominantly by a single layer of columnar mucin-secreting epithelium with mildly dysplastic epithelium. A complex branching papillary architecture is not observed. There is no adjacent invasive neoplasm. The tumor is clear of all surgical resection margins. The pancreatic duct shows minor epithelial crowding and mucinous cells. The common bile duct shows a columnar epithelium. There is some adjacent increase in fibrosis around the tumor. Away from these areas, the pancreas shows focal areas of atrophy resection margins show no evidence of tumor. 13 lymph nodes show no evidence of a metastatic tumor. Also present is one fragment, showing a multicystic structure lined by a bi-layered cuboidal epithelium focally showing squamous metaplasia. This small cystic structure with adjacent and present on the serosal surface of the duodenum. There is no adjacent lymph node structure is no mucinous differentiation.

Conclusion: Whipple's–42 mm branch duct type intraductal papillary mucinous neoplasm, completely excised, with no evidence of invasive malignancy, no metastatic malignancy within 13 lymph nodes. Also present is a 4 mm mesothelial cyst of the duodenal serosal surface.