Imaging Brain Metastases of Neuroendocrine Tumors

Neuroendocrine tumors (NETs) are rare tumors with a low incidence, characterized by a slow growth with often an indolent course. These tumors can arise in nearly all organs, with gastrointestinal and bronchopulmonary sites being the most common primary sites. The occurrence of primary neuroendocrine brain tumors is very rare, but metastases of NET to the brain are more common. The incidence of brain metastasis in patients with NET is approximately 1.5-5% with a median age of diagnosis of 60 years. Detection of NET brain metastases occurs with a median of 1.5 years after diagnosis of primary NET with a likely underdiagnosis. Brain metastases most commonly develop in patients with bronchopulmonary NET. Brain metastases grow globoid with a microglial boundary and are often surrounded by vasogenic edema. Metastatic brain lesions are made up of a subgroup of cells with a different biological behavior compared to the primary NET and other metastases which can be explained by the difficulties these cells have to face before being able to successfully grow within the brain. These patients suffer from a more aggressive NET behavior, and in many patients, carcinoid syndrome is also present. Clinical presentation is mostly characterized by headaches, personality changes, and gait disturbances. According to the European Neuroendocrine Tumor Society (ENETS) consensus guidelines, MRI is the preferred modality for detection of NET brain metastases. However, PET has gained a very important role in the staging of NET, and it is likely that PET will become very helpful for the detection of neuroendocrine brain tumors and metastases. Several tracers are available at this moment, of which somatostatin receptor PET and 18F-DOPA PET seem the most promising at this moment. Due to the high background uptake in the brain and the relatively low glucose metabolism in NET cells, the indication of 18F-FDG PET seems very limited for the detection of neuroendocrine brain tumors. However, due to the rarity of NET overall and thus especially for NET brain metastases, data on these tumors is limited, and additional research is warranted.