Poorly Differentiated Neuroendocrine Tumors of the Pancreas: A Comparative Analysis of Primary Versus Secondary Tumors—A Literature Review

Background: Poorly differentiated neuroendocrine tumors of the pancreas (pd-PNETs) are very rare tumors. Differentiating primary pd-PNET from neuroendocrine carcinomas, which metastasize to the pancreas, can be difficult. We will refer to any neuroendocrine carcinoma with pancreatic metastasis as secondary pd-PNETs. This study evaluates the differences in incidence, clinical picture, outcomes, and treatment between primary and secondary pd-PNETs. Methods: A comprehensive search of the pd-PNET database was performed to gather data on incidence, race, age, gender, clinical picture, and outcomes for primary and secondary pd-PNETs. The emphasis was on small-cell lung cancer (SCLC) and Merkel cell carcinoma (MCC) due to their associations with secondary pd-PNET. Additional data from the PubMed database were analyzed, and 12 case reports of primary pd-PNETs were added for clinical characteristic analysis. Results: Primary and secondary pd-PNETs exhibit highly similar profiles in terms of age, gender, race, and clinical features. However, treatment strategies are significantly different. Primary pd-PNETs are managed with tumor resection and platinum-based chemotherapy. Primary tumors usually have poor prognosis, with a median survival of 12 months. Treatment for secondary pd-PNETs varies based on the primary tumor. The treatment strategy for metastatic MCC was changed to immune checkpoint inhibitors (ICIs), and survival improved. Tarlatamab also recently showed a good response in the management of SCLC. These findings highlight the need for accurate and timely diagnosis to provide correct treatment. Conclusions: Patients with primary and secondary pd-PNETs exhibit similar clinical presentations and epidemiological characteristics. However, when a poorly differentiated neuroendocrine pancreatic mass is identified, it is critical to exclude MCC or small-cell lung carcinoma metastasis, as treatments may be different and prognosis may also be different.