Cancer of Unknown Primary: When Imaging, Pathology, and Molecular Biology Do Not Match.

Introduction: Cancers of unknown primary are aggressive and rare malignancies with a complex diagnosis and management. Here we present a case in which imaging, pathology, and molecular biology did not match for a specific tumor site and the importance of a multidisciplinary team for these complicated cases. Case Presentation: A man in his 70s with strong smoking history under workup for suspicion of metastatic lung cancer underwent lung mass biopsy. Immunohistochemical stains corresponded to hepatocellular/cholangiocarcinoma or germ cell tumor; however, dedicated liver and testicular studies including imaging and iscochrome 12p FISH were negative. Additionally, somatic variant profiling was not specific for any malignancy nor targetable variants. Given the pattern of disease, risk factors, and patient history, the patient received treatment for lung adenocarcinoma (carboplatin, pemetrexed, and pembrolizumab). The patient had a drastic improvement in dyspnea, weight gain, and was able to return to work. Conclusion: This report describes a case in which immunohistochemistry and molecular profiling did not identify the tissue of origin and highlights the importance of a multidisciplinary team to reach a diagnosis and guide treatment without delaying patient care in patients with these diagnoses.