Approximately 11% to 14% of subjects with neuroendocrine neoplasms (NENs) have metastatic lesions with unknown primary origin (UPO), with the majority of UPO-NENs found in the small bowel. Herein, we assessed the available literature on UPO-NENs, focusing on clinical presentation and diagnostic techniques to identify the primary site. The identification of the primary tumor is important as it affects the prognosis; however, the clinical presentation can be non-specific in non-functioning forms. In the presence of metastatic disease, the histological sample is fundamental to obtain immunohistochemical markers that might orientate the clinician in the search for the primary tumor through radiology, functional imaging and endoscopic techniques. In summary, multidisciplinary management plays a key role in UPO-NENs, even more than in other NENs. Molecular biology and gene-expression profiling represent areas of great interest which might be developed in the near future for both the diagnosis and the treatment of these neoplasms.

Among neuroendocrine neoplasms (NENs), a non-negligible proportion (9–22%) is represented by sufferers of NENs of unknown primary origin (UPO), a poor prognostic group with largely unmet clinical needs. In the absence of standard therapeutic algorithms, current guidelines suggest that the treatment of UPO-NENs should be based on tumor clinical-pathological characteristics, disease burden, and patient conditions. Chemotherapy represents the backbone for the treatment of high-grade poorly differentiated UPO-NENs, usually providing deep but short-lasting responses. Conversely, the spectrum of available systemic therapy options for well-differentiated UPO-NENs may range from somatostatin analogs in indolent low-grade tumors, to peptide receptor radioligand therapy, tyrosine kinase inhibitors (TKIs), or chemotherapy for more aggressive tumors or in case of high disease burden. In recent years, molecular profiling has provided deep insights into the molecular landscape of UPO-NENs, with both diagnostic and therapeutic implications. Although preliminary, interesting activity data have been provided about upfront chemoimmunotherapy, the use of immune checkpoint inhibitors (ICIs), and the combination of ICIs plus TKIs in this setting. Here, we review the literature from the last 30 years to examine the available evidence about the treatment of UPO-NENs, with a particular focus on future perspectives, including the expanding scenario of targeted agents in this setting.

Neuroendocrine tumors (NETs) are a heterogeneous group of neoplasms with increasing incidence, particularly within the gastroenteropancreatic (GEP) system. The liver represents the most common site of metastasis, and neuroendocrine liver metastases (NELMs) significantly impact prognosis, symptom burden, and therapeutic decision-making. Surgical management remains a cornerstone in the treatment of NELMs and encompasses a spectrum of strategies, including curative liver resection, cytoreductive surgery, and, in selected cases, liver transplantation (LT). Hepatic resection, although potentially curative when technically feasible, is applicable only to a highly selected subset of patients, and its benefits in terms of long-term survival and symptom control remain limited by recurrence rates and patient-related factors. Cytoreductive surgery has emerged as a valuable alternative in patients with unresectable disease, with increasing evidence supporting a ≥70% debulking threshold as sufficient to achieve meaningful clinical benefit. This approach may improve survival and quality of life, notably in symptomatic patients, and can be combined with parenchymal-sparing techniques and locoregional therapies. Liver transplantation represents a radical but potentially curative strategy for highly selected patients with liver-only disease, favorable tumor biology, and stable disease. Outcomes are strongly dependent on strict selection criteria, and appropriate patient selection remains critical. The incorporation of systemic treatments, such as somatostatin analogues, targeted therapies, and peptide receptor radionuclide therapy (PRRT), has broadened the available therapeutic options and contributed to redefining current treatment strategies. Overall, the management of NELMs requires a multidisciplinary, individualized approach guided by tumor biology, disease distribution, and patient-specific factors, with the goal of optimizing survival outcomes and preserving quality of life.