Small discrete nodular proliferations of neuroendocrine cells less than 4 mm in diameter are termed tumorlets. They typically grow along alveolar walls or small conducting airways.
Neuroendocrine (Kulchitsky) cells are normally present in the airways. These nodular proliferations -- "tumorlets" -- are sometimes seen in reaction conditions such as cystic fibrosis, chronic bronchitis, bronchiectasis, asthma, COPD, primary pulmonary hypertension.
These nodules can be up to 4 mm in diameter and if larger, they are best described as carcinoid tumors. These tumorlets are often multiple and peripheral. The clusters of neuroendocrine cells are often invested by hyalinized stroma. They have rarely been reported to secrete ACTH.