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National Cancer Institute U.S. National Institutes of Health
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Benign tumors

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In human pulmonary pathology, benign tumors are rare and almost never progress to malignancy. The situation is quite different in mouse pathology, where a significant number of adenomas, especially after some chemical induction schemes and genetic modifications, may progress to carcinomas.

Papillary structures lined with cuboidal respiratory epithelium containing connective tissue core. Arises from the airway epithelium.
Well circumscribed areas consisting of cuboidal to columnar cells lining alveoli. The size is usually less than 5 mm in diameter. These lesions retain preexisting alveolar structure and tend to be multiple in existing mouse models. Absence of pronounced fibrovascular stroma, as well as more "plump" shape of epithelial cells, may be the reason for different appearance of mouse adenomas, as compared to their human counterparts. Differentiation between a small adenoma and focal hyperplasia can be very difficult. At the same time, no absolute criteria exist for distinguishing a large adenoma from a well-differentiated adenocarcinoma. Among features indicating benign character are a small size, and absence of vascular invasion. Well delineated demarcation and absence of lepidic growth are considered by some as indicators of a benign character. Bland character of nuclei is a main feature of human adenomas. By this criterion many mouse adenomas could be assigned to adenocarcinomas. However, unlike in humans, mouse tumors rarely metastasize during the time of their observation.