Although the rate of metastasis has not been measured in many transgenic strains, metastases have been readily found in association with the PyV-MT and some neu mammary tumors. Invasion of the tumor vasculature is common. The most metastatic tumor is the PyV-MT tumor which results in 100% metastases in the first six weeks of life. The metastases are almost exclusively found in the lung. Experimental tail vein injection of tumor cells results in pulmonary metastases and metastases are rarely observed in other organs.
The metastatic pulmonary lesions closely resemble the lesions found in the mammary gland. They are sometimes secretory and may even contain clear lipid vacuoles. They are generally found in the blood vessels but may invade surrounding tissue. These observations are very important because, in our experience, many transgenic animals that have mammary tumors may also develop pulmonary adenomas and adenocarcinomas.
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