Objectives: Assessment of a pancreaticoduodenectomy specimen by pathologists requires specialized knowledge of anatomy. Standardized assessment, description and documentation of the retroperitoneal margin are crucial for the accurate interpretation of studies evaluating adjuvant therapy for pancreatic cancer patients. Methods: Twenty-five patients who underwent a pancreaticoduodenectomy for pancreatic adenocarcinomas had their pathological specimens examined prospectively, using an anatomical-based mapping approach. All margins, including the bile duct, pancreatic neck, superior mesenteric artery, superior mesenteric vein and posterior surface of the uncinate process, were microscopically examined in their entirety. The assessment of an R1 margin in terms of distance was assessed in two ways: first defining it as a tumour at the margin or secondary as tumour within 1 mm (1 mm rule). Results: If the existing College of American Pathologists recommendations were applied (assessing only the bile duct, pancreatic neck and superior mesenteric artery margins), a R1 status would be achieved in only 9 of 25 patients. Extending the examination by assessment and reporting of the entire retroperitoneal resection margin, including the Superior Mesenteric Vein margin and the Posterior surface of the uncinate process margin, increased the number of patients with a R1 resection to 14 out of 25. Applying the 1-mm rule further increased the number of patient with a R1 resection to 20 of 25 patients. Conclusions The above findings illustrate that different approaches to the assessment and reporting of the retroperitoneal margin can change the results and adversely affect the final statistics used in pancreatic cancer studies and clinical trials.