AQA 4.2.2.6 Metastasis: How Cancer Spreads

Welcome to the show everybody! I'm Lottie, here with Mr H. And Mr H, I want to start with a concept that sounds like a sci-fi escape movie, but it's happening at a microscopic level inside the human body. We're talking about metastasis, specifically how a cancer cell from a primary tumour manages to break free and travel to a completely different organ. A grim but essential topic for the AQA specification, Lottie. Specifically, reference 4.2.2.6. Now, before we get carried away with movie metaphors, we must be scientifically precise. It begins with adhesion. In a malignant tumour, the cells lose their stickiness--or, to use the correct terminology, they lose cell-to-cell adhesion. Because they aren't bound tightly to one another, individual rogue cells detach from the primary tumour and actively invade the surrounding healthy tissue. So they literally unstick themselves, crawl through the local tissue, and then they need a gateway to the rest of the body. And that gateway is our transport network, right? The blood vessels and the lymphatic system. Precisely. They squeeze through the walls of nearby capillaries or lymphatic vessels in a process called intravasation. Once they penetrate those walls and enter the bloodstream, they are swept into the general circulation. It is like they are catching a bus to a different city. They hop on the highway of the bloodstream and can go absolutely anywhere. I must object to the bus analogy. The bloodstream is not a public transit system waiting to serve them. In fact, it is a highly hostile environment. The vast majority of these circulating tumour cells are actually recognized and destroyed by the host's immune system while they are in transit. The shear forces of the blood flow alone kill many of them. Right, so the body's security system is actively hunting them down. But the terrifying part is that it only takes one or two hardy survivors to slip through the dragnet to cause a massive problem. Exactly. A single surviving cell is all it takes to colonise a distant site. So, let's follow that surviving cell. It's floating along in this high-speed blood river, but eventually, the vessels have to get smaller, right? Indeed. As the bloodstream reaches distant organs, the vessels branch into incredibly narrow capillary beds. The cancer cell, which is relatively large, literally gets physically lodged in a capillary of a distant organ. Very frequently, this occurs in the lungs, the liver, or the bones, because these organs have incredibly dense, high-volume capillary networks. The lungs and liver make perfect sense because all our blood is constantly filtered through them. So the rogue cell gets physically stuck in a tiny lung capillary, and then what? Does it just start growing inside the blood vessel? No, it must exit. It squeezes back out through the capillary wall into the surrounding tissue of the new organ. Once established in this new soil, the cell resumes what cancer cells do best: rapid, uncontrolled cell division by mitosis. This forms a secondary tumour. So, a patient could start with breast cancer, but because of this journey, they end up with secondary tumours growing in their lungs or bones. It's the exact same type of cancer cell, just living in a new home. Correct. A secondary lung tumour from breast cancer is still comprised of malignant breast cancer cells. And this is precisely why early detection is so critical. If a clinician can diagnose and treat a malignant tumour before it undergoes metastasis--before those cells ever enter the blood or lymphatic vessels--the prognosis is vastly improved because the disease is localized. It really highlights the race against time in oncology. Catch it while it's still in one place, before it finds the gateway. Well, that is a sober reminder of how dynamic and dangerous cellular biology can be. Next time, we'll be looking at how we can use our knowledge of these pathways to target treatments. See you then! Goodbye.