The ancient view regarding breast cancer as a metastasis has not been supported so far by experimental evidence. We have implanted nephroblastoma tumor cells resulting in a rat metastatic kidney capsule-parathymic lymph node (PTN) model. India ink implantation confirmed the lymphatic connection between the primary tumor of the kidney and PTNs. (18)F-FDG glucose analog distribution provided further evidence that the first metastatic sites of distant tumor progression are PTNs. Tumor invasion caused disruptions in the tissue of the primary renal tumor, releasing cancer cells into the peritoneal cavity. Colloidal particles, among them bacteria and India ink, crossed transdiaphragmatic channels drained from the peritonel cavity to the thoracic lymphatics and entered not only in the parathymic lymph nodes but also in the anterior mammary lymph nodes. The kidney capsule-PTN complex is reflecting a so far unknown mechanism of tumor development and suggests a similar tumor progression directed towards mammary lymph nodes. The mammalian tumor model provides a reasonable explanation for breast cancer development viewed as a metastasis, rather than a primary tumor.