Cancer can occur in any organ in the body; however, cancer is much more common in some organs than others (although this varies from country to country).
A cancer death in a 40-year-old woman, for example, represents about 42 years of life lost (because the typical 40-year-old woman is expected to live to age 82). A cancer death in an 80-year-old woman, however, represents about nine years of life lost (because the typical 80-year-old woman is expected to live to age 89). So if two cancers cause roughly the same number of deaths, but one generally causes death in younger people, it is ranked higher in this column.
Skin cancer is one of common form of cancer, but most skin cancers are not deadly. That's the reason skin cancer does not appear in the other two columns. Lung cancer might not be the most typical cancer, but more people die of it than of any other cancer (in fact, more than from breast, prostate, and colon cancer combined). Prostate cancer is the second most common form of cancer, but it often doesn't cause death. And because it is generally a disease of older men, prostate cancer is not in the top five when it comes to years of life lost. Breast and colon cancer, which are relatively common, frequently cause death, and often occur in the middle of life, appear in all three columns.
But just knowing where and how often cancer occurs does not answer the question "What is cancer?" Although it's a simple question, there is no simple answer. Indeed, the word cancer encompasses a broad spectrum of disorders, making the cancer/not cancer distinction fuzzy. Because this complexity is responsible for many of the problems outlined in the following paragraphs, I want to begin by attempting to define cancer.
Let's start with the dictionary. Mine reads, "Cancer: a malignant tumor of potentially unlimited growth that expands locally by invasion and systematically by metastasis." Malignant in turn means "tending to infiltrate, metastasize, and terminate fatally," while metastasis (or the verb metastasize) refers to the spread of disease to a distant site. My medical dictionary is a little more terse, defining cancer as "a cellular tumor the natural course of which is fatal."
This is the way most of us think about cancer. It is bad. It grows relentlessly (local invasion). It pops up in unexpected places (metastasis). It spreads throughout the body. And it kills you. If cancer is thus defined, it seems logical that it must be found and it must be treated.
But doctors diagnose almost all cancer before people die from it, and they diagnose most cancer before any metastasis occurs. So the diagnosis can be made without the foregoing criteria, that is, without death and without metastasis. There must be another definition for cancer, then, and there is: the microscopic definition.
The diagnosis of cancer is made by pathologists those physicians who specialize in the performance of laboratory tests and the examination of human tissue under the microscope. Physicians who are worried about a cancer diagnosis generally take a piece of the organ in question (a biopsy) and send it to the lab. The pathologist then examines thin sections of the tissue under the microscope.
The microscopic definition depends on the look of individual cells. The less individual cells look like the normal cells found in the organ, the much more likely they are cancer. The more individual cells vary in decoration, the much more likely they are cancer. And the more the pathologist sees individual cells in the midst of dividing, the more likely they are cancer.
The microscopic definition also depends on the appearance of the collection of cells the architecture that results from how individual cells fit together. Growths that are clearly walled off from surrounding tissue, as if in a cocoon, are less likely to be cancer. Conversely, growths that extend into surrounding tissue and invade nearby blood vessels are more likely to be cancer.
So again: what is cancer? Is it the lethal disease that has spread throughout the body, or is it the microscopic abnormality seen in several cells? You might reasonably ask, "What's the difference? People who develop the microscopic abnormality are bound to develop the lethal disease if they are not treated." In fact, sometimes that is true, but sometimes it's not. The natural course of untreated cancer what epidemiologists call the "natural history" turns out to be highly variable. Cancer is a dynamic process that may proceed in many different ways.
But the process may proceed at very different speeds, and it may even be stopped by the body's immune system. So what pathologists call cancer in fact encompasses a broad range of entities that may evolve in very different ways. Some may grow very fast, others may grow very slowly, and some may even go away. This heterogeneity in the natural history of cancer is the reason for most of the problems raised in the following paragraphs: why tests miss some cancers, why there are others you would rather not know about, why it's hard to know who really has early cancer, and why our cancer statistics can be so misleading.
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