Spontaneous metastasis: random or selective?

Spontaneous lung metastases from an infrequently metastasizing fibrosarcoma and a more highly metastatic mammary carcinoma showed no increased capacity for generating primary tumors with greater metastatic potential when transplanted into either normal or whole-body-irradiated syngeneic mice for up to six isotransplant generations. Rather, successive generations varied in their metastatic behavior, tending towards decreasing metastatic efficiency. These data support the view that spontaneous metastases are determined more by random survival of intravasated tumor cells than by the selection of preexisting metastatic cell variants.