ACTH-secreting atypical carcinoid lung tumour expanding the Lynch syndrome spectrum

Neuroendocrine tumours (NETs) are increasingly associated with Lynch syndrome (LS). In this autosomal dominant cancer predisposition syndrome, a somatic mutation in addition to a germline pathogenic variant is required for tumour development. We describe the case of a middle-aged female patient with LS with a known germline <i>MLH1</i> mutation who was diagnosed with Cushing's syndrome. An ectopic adrenocorticotropic hormone (ACTH) producing carcinoid tumour of the lung with lymph node metastases was found and resected. Immunohistochemical analysis showed loss of <i>MLH1/PMS2</i> expression, and genetic analysis confirmed a deletion of the entire <i>MLH1</i> gene, acting as the second hit for tumour formation. This provides unequivocal evidence of the tumour's association with LS. Only 30 cases of NETs in LS have been described in the literature, most of them of gastrointestinal origin. We describe the first bronchopulmonary NET in a patient with LS, broadening the spectrum of LS tumours, and the first ACTH-producing tumour in LS.