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Similarly to the human classification, the term "tumor" is used synonymously with neoplasm.

Benign tumors
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.
Preinvasive lesions
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.
Malignant tumors
Main criteria for malignancy include size over 5 mm in diameter, invasion of airways, blood or lymphatic vessels, regional and distant metastasis, and ability to grow upon transplantation. Nuclear and cellular atypia, and loss of architecture should be considered as ancillary criteria for defining malignancy.