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The ability of a cancer cell to spread - to break away from the original tumor and grow in another location in the body - is a defining trait of cancer. In normal tissues, the cells are tightly bound and have snug “stitching” between them, like the patchwork pieces of a quilt. Benign growths, such as moles or warts on our skin, are also made up of cells that are tightly bound and cannot travel or invade the healthy tissue underneath them. For this reason, benign tumors are cured once they are removed.

Inside a cancerous growth, however, are cells that have found a way to separate from their neighbors, to loosen and dissolve the stitching that binds them. The freed cells can then be swept away by the flow of blood in a nearby blood vessel to other locations in the body. The ability of cancer to form new blood vessels is called “angiogenesis”; this property is essential for a cancer to grow beyond a very small size and to have a chance at gaining access to the circulation. Cancer cells also travel through another network of vessels, called lymphatic vessels. These channels drain our organs of excess fluid and debris and also serve as a highway on which immune cells travel throughout the body. Once cancer cells enter the bloodstream or lymphatic channels, few of them actually form metastases (fewer than 0.1 percent of circulating cancer cells). Some are killed in the circulation, whereas others die after a period of attachment to a distant organ. Even those that do establish a home in a new location may never develop into harmful tumors. Some will enter a period of prolonged dormancy, whereas others will multiply and die at equal rates, yielding a lack of net growth. These varied fates of disseminated cancer cells help explain how some metastases can manifest themselves ten or more years after the original cancer was treated: for unknown reasons, the dormant or nonproductive cancer cells suddenly acquire the ability to duplicate themselves successfully without limit.

If metastasizing cancer cells can be thought of as the “seed,” then the new environments in which they are trying to grow (most often the liver, lungs, bones, and brain) can be thought of as the “soil.” The “seed versus soil” hypothesis of cancer growth was first introduced more than a hundred years ago. For the past century of cancer research, the focus has been on understanding how the seed grows, ignoring the contribution of the soil, but this is changing. We now know that the organs to which cancer cells spread must provide growth-promoting chemicals as well as a blood supply if the cells are to grow there; similarly, plants grow better in fertile rather than barren soil. Provocative new research suggests that one person’s soil may be different from another’s. In other words, one person’s liver may be more hospitable to cancer metastases than another person’s liver. The factors that determine these differences await further discovery. The final and most straightforward way that cancer spreads is by direct invasion of nearby tissues. A growing tumor can expand beyond the site of origin and take root in nearby structures. For example, a bladder cancer can break down the bladder wall and invade the nearby rectum; a large thyroid cancer can extend out of the thyroid and into the muscles of the neck; a lung cancer can invade an overlying rib and cause severe pain.

In these situations, the cancer is said to be “locally advanced,” in contrast to metastatic cancer, which is defined by distant spread in the body. The distinct ways in which locally advanced and metastatic cancers are treated we speak later. There are some common misconceptions about how cancer spreads. Some people avoid undergoing operations to remove cancerous tumors because of a mistaken belief that the disease is spread through the air during surgery. And I have heard patients wonder if cutting into a tumor as part of a biopsy before its removal, or even in the more definitive surgery, can spread cancer in the body. With regard to the first belief, cancer cells cannot become airborne, nor can they survive in the air even if they were to become airborne. Certainly, if this were the case, coughing or the sharing of body fluids could spread cancer.

Infections can be spread in these ways, but cancer cannot. With regard to the second concern, isolated reports of cancers tracking along biopsy routes have been reported, but this is not believed to be a significant way cancer is spread. In sum, a cancer diagnosis represents the point of discovery of a process that began with the conversion of a healthy, controllable cell into one that can grow (multiply), spread, and survive without regulation. These properties constitute the foundation of cancer. They are central to understanding the diagnosis, behavior, and treatment of any cancer.