Cancer, from the Latin word for “crab,” and from the Greek word for crab, karkinos or carcinos, used by Hippocrates to describe tumors, has plagued humanity since antiquity, and probably before recorded history. The name make sense, since the swollen blood vessels that surround, infiltrate, and feed the tumor mass, reminded Hippocrates of the claws of a crab.

As you can see from the picture on the left, this is an understandable comparison. The tumor in the picture is of an invasive mouse breast cancer. It is large, chock-a-block full of blood vessels, and looks like a disorganized blob. Not very pretty, and definitely deadly. Those blood vessels feed the tumor at the expense of the host (i.e. the patient’s body) and can help tumor cells that break off from the primary mass to travel through the bloodstream and colonize other organs in a process called metastasis.

But what is cancer? Where does it come from? Why is it so difficult to treat? Let’s start with the first question. According to Google, cancer is defined as “a disease caused by an uncontrolled division of abnormal cells in a part of the body.” Uncontrolled growth is a hallmark of cancer. Where does it come from? Cancer comes from the transformation of a normal cell, which works in harmony to fulfill its assigned function in the body, into a cell that abandons its normal function and growth constraints to divide (make copies of itself), displacing and destroying normal cells and tissue, hijacking resources (e.g. oxygen and nutrients delivered by blood vessels), and if untreated or undetected, spreading to other parts of the body and destroying normal cells in tissues outside of the site of origin. Why is it so difficult to treat? It’s complicated, but it relates to at least three inherent properties of cancer: (1) cancer comes from normal cells, which makes it difficult for the immune system to recognize it as a threat; (2) cancer cells are genetically unstable and prone to collecting mutations in DNA, the genetic blueprint that controls all cellular functions – see Figure on the left; and (3) because there are many genes that control normal cell growth, survival, and other processes exploited by cancer, each cancer is unique – cancer isn’t a single disease, even within the same tissue. There are at least 5 distinct types of breast cancer (and subtypes within those types), and each is as unique as the patient in which they grow. More on that in a future post.

Unlike infectious diseases caused by viruses and bacteria, pathogens that the immune system can recognize and defend against, cancer cells are seen by the immune system as “self” in many cases (more on anti-tumor immunity and immune checkpoint inhibitors on the market in a future post). Even worse, when immune cells do enter tumors, the tumors can adapt and send signals to immune cells instructing them to protect rather than destroy the tumor. The same genetic instability that enables mutations and changes that allow cancer cells to grow uncontrollably also allow cancer cells to adapt to attacks from the immune system and therapies including chemotherapy, molecularly targeted therapies (like estrogen and HER2 blockers in breast cancer). The rapidly growing tumor mass also tricks the surrounding tissue into sending new blood vessels to infiltrate and feed the tumor, allowing tumor cells to grow, survive, and invade to metastasize.

So, in a nutshell – normal cells + mutation(s) leading to uncontrolled growth + more mutations leading to transformation into malignant cells + more mutations + a blood supply + tricking the immune system = cancer. It’s more complicated than that, but this is a good starting point for understanding cancer.

Want more information? I’ll be posting a LOT more on this topic. In the meantime, here are some really amazing resources on the subject: The Emperor of All Maladies: A Biography of Cancer (Book by Siddhartha Mukherjee and PBS documentary); SciShow’s excellent video on YouTube; Cancer Research UK’s video overview. is also an excellent resource.