Pulmonary Pathology Online
Pathology Pulmonary Carcinoid Tumorlet
A pulmonary carcinoid tumorlet (PCT) is a nodular proliferation of neuroendocrine cells smaller than 0.5 cm.
The term tumorlet was coined by Whitwell for minute, tumour-like proliferations that are only detectable by microscopy and are generally discovered incidentally.
Liebow had earlier noted their resemblance to carcinoid tumours and referred to them as "atypical carcinoid proliferations".
According to some authors tumourlets represent hyperplasia while others consider them to be neoplastic lesions. The former view is generally favoured.
Tumorlet rarely if ever, metastasize.
If similar deposits are found in the hilar lymph nodes or the pulmonary lesions are identified they should be best regarded as small carcinoids.
The distinction between tumorlets and carcinoids is imprecise.
Arbitrarily a diameter of 0.5 cm has been taken as the dividing point, with those above this being classed as carcinoids.
Tumourlets arise in close proximity to bronchioles as multiple nests of cells separated by narrow connective tissue septa.
Their cells are uniform and have regular, round, oval, or spindle-shaped nuclei with finely dispersed chromatin.
The cytoplasm contains granules of typical dense-core type averaging 100 nm in diameter.
The immunohistochemical features are identical to those of normal bronchopulmonary neuroendocrine cells and of carcinoid tumours.
Localized regenerative proliferations of bronchiolar epithelium have sometimes been termed tumorlets, but these cell collections usually consist of stratified squamous epithelium and lack the pattern of a true tumorlet. They also do not possess any neuroendocrine features.
Tumourlets are most frequently associated with bronchiectasis.
These are often multiple, particularly when the pulmonary parenchyma is extensively scarred.
Lungs resected for carcinoid tumours or carcinoma may show a significant increase in the number of neuroendocrine cells.
These may include clusters large enough to disturb and narrow small airways.
Occasionally, otherwise typical carcinoid tumours may be accompanied by multiple fully formed tumorlets.
Other conditions associated with tumorlets include emphysema and various causes of focal pulmonary scarring.
Tumorlets are seldom seen in normal lungs, nor are they particularly associated with diffuse interstitial fibrosis.
There have been reports of clinically significant impairment of lung function associated with multiple tumourlets.
Histological classification in such cases has shown obliteration of bronchioles by multiple tumourlets,many of which have been associated with fibrosis.
It has been suggested that in these cases the tumorlets are the cause rather than an effect of the fibrosis, and that the fibrosis is perhaps mediated by the secretion of fibrogenic cytokines such as bombesin.
Differential diagnosis: Tumourlets may be confused with multiple Minute Pulmonary Meningothelial-like Nodules (so-called minute pulmonary chemodectomas)- these are not related to air spaces but are situated within the interstitium, generally near septal veins; they lack neuroendocrine features.
Pulmonary tumourlets must be considered in the differential diagnosis of minute lesions suspected to be small cell carcinoma or peripheral carcinoid tumor.
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