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Carcinoid tumours are rare, malignant neuroendocrine neoplasms first described in 1888 in the ileum1 and called ‘Karzinoide’ by Oberndorfer in 1907.2 Neuroendocrine cells were originally called clear cells and later amine precursor uptake and decarboxylation (APUD) system cells.3 The term neuroendocrine was introduced with the finding that these cells are capable of producing bioactive amines and that a number of these cells are identical to those of the nervous system. Neuroendocrine tumours of the lung arise from bronchial mucosal cells known as enterochromaffin cells or Kulchitsky cells, which are part of a diffuse neuroendocrine system. For many years the carcinoid tumour of the lung was called bronchial adenoma, which comprised other bronchial tumours with benign behaviour. Today, bronchial carcinoids (BCs) are classified as well-differentiated malignant neuroendocrine tumours in two distinctive forms – typical carcinoid (TC) and atypical carcinoid (AC) – with different histological features, clinical course and prognosis. The differences in histological criteria between TC and AC were first described by Arrigoni et al.4 and later modified by Travis et al.5 They were fixed in 1999 by the World Health Organization (WHO).6
TC is a variant of neuroendocrine tumours with a low-grade histological malignancy profile (<2 mitoses/10 high-power field [HPF], nuclear pleomorphism and absence of necrosis) that rarely metastasises.7,8 AC is considered to be an intermediate grade of malignancy; it presents with ≥2 but <10 mitoses/HPF and/or coaugulative necrosis.5 Several immunohistochemical markers have been considered for the histological assessment and risk stratification of carcinoids. In terms of tumour cell commitment, carcinoids are consistently associated with the immunohistochemical expression of neuroendocrine markers (chromogranin A, neuron-specific enolase [NSE], synaptophysin, Leu7).9 Mib1 and Bcl2 expression is an independent variable associated with tumour prognosis (with no statistical interaction between the two). In the future, such a biologically plausible immunohistochemical pattern could also be suitable in the routine histological assessment of BC.10
Recently, researchers have studied the importance of genetics in BC. A recent paper reports that in TC and AC DNA, under-representations of 11q are frequent and that in AC there are also frequently losses of 10q and 13q, as in high-grade neuroendocrine malignant tumours (large- and small-cell lung cancer). Losses of 10q and 13q probably suggest a more aggressive behaviour of AC.11