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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.

Solid adenoma
Round to oval cells fill alveolar spaces. Fixation of the lung without inflation results in predominance of solid over alveolar pattern. Cells usually have abundant eosinophilic cytoplasm with fine granularity and/or vacuoles.
Papillary adenoma
Consists primarily of papillary structures lined by cuboidal to columnar cells. Cells forming papillary structures are frequently more hyperchromatic and atypical, which is regarded as indication of potential progression towards malignancy.
Adenoma with mixed subtypes
Both papillary and solid structures are present.