EGFR Mutated Lung Adenocarcinoma Metastasis to the Pancreas Mimicking Primary Pancreatic Ductal Carcinoma

Background: The occurrence of lung adenocarcinoma metastasizing to the pancreas is overall rare and can histologically imitate primary pancreatic ductal carcinoma (PDAC). Case Report: This is a case report of a 70-year-old female with a history of surgically resected right lung adenocarcinoma presenting for routine follow up without symptoms. CT scans revealed a pancreatic cystic mass with ductal dilatation that was sampled via endoscopic ultrasound-guided fine needle aspiration (EUS-FNA) and thought to be a primary pancreatic mucinous neoplasm with high grade dysplasia suspicious for carcinoma based on smear cytology. On repeat EUS-FNA and biopsy (FNB) with additional immunohistochemical testing for lung adenocarcinoma markers thyroid transcription factor (TTF1) and Napsin A and molecular testing, the lesion was identified as a metastasis of lung adenocarcinoma with an epidermal growth factor receptor (EGFR L858R) mutation; subsequently, the patient underwent targeted therapy that yielded an almost complete response. Conclusion: To the best of our knowledge, this is the first documented case in English literature of a lung adenocarcinoma metastasis to the pancreas mimicking a pancreatic primary neoplasm and highlights the potential pitfalls of EUS-FNA for the diagnosis of certain metastases to the pancreas.