A patient has breast cancer which has metastasized to their lungs. This is a very common route of metastasis, in which cancer cells are drawn into lymphatic capillaries with lymph, move through the lymphatic system, then gain access to general circulation as the lymphatic ducts empty into venous circulation. Often, the cancer cells then “seed” the first hospitable organ they encounter (often the lungs, for non-GI cancers). Trace the route that cancerous cells took from the left breast, through the lymphatic vessels, and then through circulation, to the point where they invaded the lung tissue. Solution Cancer cells break away from the left breast and attach themselves to the outside wall of any lymphatic capillary. Then, they accomplish themselves in the vessel wall to flow with the lymph carried away for infiltration to reach the lymph nodes. There they start multiplying and blocking the pleural sac (surrounds the lung) which in turn gets filled with fluid or forms a tumor and then slowly these cells invade the lung tissue..