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National Cancer Institute U.S. National Institutes of Health
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Preinvasive lesions

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This definition is for allocation of lesions with preinvasive/borderline properties. It is currently aimed at newly identified neoplasms, which may be similar to those described in humans. In mouse pathology, many adenomas may be preinvasive/borderline lesions. However, their inclusion in the preinvasive category can be justified only upon development of better diagnostic criteria.

Squamous dysplasia
Composed of airway epithelial cells with squamous metaplasia. Degree of atypia, maturation, orientation towards the basal membrane and involvement of full thickness of mucosa may vary.
Atypical adenomatous hyperplasia
Focal and diffuse lesions involving alveoli and terminal bronchioles and consisting of relatively uniform atypical cuboidal to columnar cells with dense chromatin. Degrees of cellular hypertrophy and hyperchromasia are variable. Cellular and nuclear atypia are the distinctive features as compared with hyperplasia. Their relevance to human AAH and mouse adenomas remains to be determined.
Diffuse idiopathic pulmonary neuroendocrine cell hyperplasia
Diffuse accumulation of groups of neuroendocrine cells confined to bronchiolar epithelium in the absence of airway inflammation or diffuse interstitial fibrosis. Not reported to occur spontaneously.