These are pulmonary neuroendocrine carcinomas characterized by more cellular atypia and mitotic activity, and greater tendency for metasatasis and aggressive behavior.
In the spectrum of neuroendocrine tumours of the lung, atypical carcinoid is intermediate between typical carcinoids and small cell carcinoma.
Large Cell Neuroendocrine Tumour are a variant of atypical carcinoid, which are composed of cells larger than atypical carcinoid.
Atypical carcinoids make up about 11% of bronchopulmonary carcinoid tumours.
Most are peripheral in localization.
The peak age incidence is intermediate between that of typical carcinoid tumours and small cell carcinoma as they are more often seen in smokers than are their more benign counterparts.
Ectopic endocrine activity is commoner with atypical than typical carcinoids.
Female gender show a trend toward a poorer prognosis.
A tumor size of 3.5 cm or greater are associated with a worse prognosis.
Atypical carcinoid tumours are recognized by increased mitotic activity.
Necrosis is seen in a tumour with the characteristic cellular make-up and trabecular or mosaic architecture of a carcinoid.
The necrosis may be confined to the centers of cell groups or be more extensive, but is usually sharply demarcated.
It may lead to focal dystrophic calcification.
The atypical features may not be apparent in small biopsies as they are often focal.
Atypical tumours were originally defined as carcinoids that showed any one or a combination of features that included nuclear pleomorphism, nuclear hyperchromasia, increased nuclear cytoplasmic ratio, hypercellularity and disorganization, as well as necrosis and increased mitotic activity, but some of these additional features are of little prognostic importance.
Nuclear pleomorphism, for example, may be found in otherwise unexceptional carcinoids that behave in an indolent manner.
The important features are mitotic activity and necrosis, neither of which is seen in typical carcinoids.
Some tumours with a mosaic or trabecular pattern of carcinoids have the cytological features of small cell carcinomas and are to be regarded as further examples of atypical carcinoids.
Unlike small cell carcinomas, such tumors are usually situated at the periphery of the lung.
Many peripheral tumours diagnosed as small cell carcinomas would probably be better categorized as atypical carcinoid tumours.
A diagnosis of small cell carcinoma should,therefore be made with caution when the lesion is peripheral and the sample is small.
Resection may be more effective treatment than chemotherapy in such cases.
Electron microscopy shows that atypical carcinoids contain fewer dense-core granules than atypical carcinoids, but more than small cell carcinomas.
The immunohistochemical features are generally similar to those typical carcinoids.
Increasing atypicality is associated with loss of the S100-positive sustentacular cells that characterize typical carcinoid tumours.
Aneuploidy occurs in both typical and atypical carcinoids but is commoner in the latter.
Proliferative activity assessed by expression of the Ki-67 antigen is higher in atypical than in typical carcinoids.
The behaviour of atypical carcinoids, is intermediate between that of typical carcinoids and small cell carcinoma.
70% of the cases eventually metastasize.
Surgical resection remains the treatment of choice, and the role of chemotherapy and radiation therapy remains to be proven.
There is little effective treatment for metastatic disease as carcinoid tumours lack the chemosensitivity of small cell carcinomas.
The 5-year survival rate is 50 to 70%, which is intermediate between that of typical carcinoids and small cell carcinomas.
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