Neuroendocrine tumours (NETs) are a broad group of complex tumours that arise from neuroendocrine cells.

NETs which occur in the lungs, bronchi, thymus, small intestine, appendix  or rectum, and pancreatic neuroendocrine (PNET or PET) tumours, which arise in the endocrine tissues of the pancreas.

NETs may also arise in the  parathyroid, adrenal and pituitary glands, as well as in specific cells of the thyroid.

Less frequently, NETs may also occur in the ovaries, cervix, testicles, spleen and breast but these are considered very rare.

NETs are classified as being functional or non-functional.

Functional tumours produce hormones and/or peptides that cause symptoms.
Non-functional tumours generally do not produce hormones – but even if they do – the patient will not experience symptoms as a result.

Many neuroendocrine tumours are non-functional and do not cause any symptoms.