Gemcitabine Hydrochloride With or Without Erlotinib Hydrochloride Followed by the Same Chemotherapy Regimen With or Without Radiation Therapy and Capecitabine or Fluorouracil in Treating Patients With Pancreatic Cancer That Has Been Removed by Surgery

Inclusion Criteria: * Histologic proof of primary head of pancreas invasive adenocarcinoma managed with a potentially curative resection (i.e., removal of all gross tumor) involving a classic pancreaticoduodenectomy (Whipple) or a pylorus preserving pancreaticoduodenectomy; patients with invasive adenocarcinoma that also contains a component of intraductal papillary mucinous neoplasm (IPMN) are eligible * The operating surgeon must document in the operative note that a complete gross excision of the primary tumor was achieved; the pathology report must include documentation of the margin status and the size of the tumor; the pathology report must also include the status of the three major margins-bile duct, pancreatic parenchyma, and retroperitoneal (uncinate) * For patients who have not started their chemotherapy prior to registration, the interval between definitive tumor-related surgery and 1st step registration must be between 21-70 days; for patients entering on the study who have already received up to 3 months of adjuvant chemotherapy as per the treating institution, the interval between definitive tumor-related surgery and day one of adjuvant chemotherapy must be between 21-77 days * Patients will be staged according to the 6th edition American Joint Committee on Cancer (AJCC) staging system with pathologic stage T1-3, N0-1, M-0 being eligible. Pathologic reporting using the College of American Pathologists (CAPS) format is strongly encouraged * Age \>= 18 * Zubro…