5.1 An Unlimited Capacity for Growth
The growth of cancer is very much on the minds of all those affected by this disease: patients, physicians, and researchers.
When a cancer patient wonders how long it has been from the time his or her cancer first started to when it was diagnosed, he or she is asking about the growth rate of the cancer. Another way to phrase this question is: “How fast is the cancer growing?”
When a patient asks if the cancer is in “remission,” he or she is really asking if the cancer has stopped growing and, more to the point, started to shrink. On the other hand, if the patient is told that the cancer has “relapsed,” then it means the cancer is growing again.
The Meaning of Remission
There are two main types of remission:
Partial or complete. In a partial remission, the cancer shrinks in size by at least 30 percent; in a complete remission, the cancer becomes undetectable. In the past, cancer doctors and researchers believed that only treatments that achieved remission could benefit patients.
However, some newer cancer treatments, called targeted therapies, not only improve quality of life but prolong life merely by “freezing” or stabilizing the growth of cancer (without necessarily shrinking tumors); this has led to a new mindset about the goals of therapy. Especially for cancers that are not considered curable, prolonged stabilization of the cancer can be as worthy a goal as obtaining remission.
Oncologists (physicians with advanced training and certification in the medical care of people with cancer) have the same concerns as patients, but with a focus on how the health of their patients is or will be affected by cancer growth. For each patient, oncologists weigh several factors to assess and anticipate the growth potential of a cancer.
These include:
(1) Examining the pathology report, which can indicate the aggressiveness of the cancer and its potential to return after treatment;
(2) Determining how rapidly any symptoms caused by the disease have developed; and
(3) Assessing the extent of the cancer as determined by imaging tests (CT scans, MRIs, bone scans, and PET scans) and blood tests.
Some types of cancer generate a protein, called a “tumor marker,” that is released into the bloodstream and can be measured through a simple blood test. Although very elevated tumor marker levels often indicate an aggressive cancer, these tests are conducted primarily to track the progress of treatment (as a cancer is successfully treated, its tumor marker will fall).
The main tumor markers are:
Major cancer tumor markers (blood tests)
Tumor marker - Cancer
AFP, HCG - testicular, liver (AFP only)
CEA - colorectal
CA 15-3, CA 27-29 - breast
CA 19-9 - pancreatic, biliary tract
CA-125 - ovarian
PSA - prostate
M-protein; free light chains multiple - myeloma
LDH - lymphoma
Beta-2 - microglobulin myeloma, lymphoma
Oncologists process all this information to determine if a cancer is fast or slow growing and if it has a high or low potential to spread to other organs. Oncologists must see the full cancer landscape for each patient, that which the affected person could not possibly see.
Following these assessments, the oncologist makes recommendations as to whether treatment should be started urgently (the same day) or in the near term (in a few days or weeks), or whether treatment can be deferred based on the future behavior of the cancer (that is, no treatment is necessary at present). For example, a person who experiences sudden back pain and is found to have a rapidly growing tumor that is pressing on the spinal cord requires urgent treatment to alleviate pain and prevent paralysis.
In contrast, a seventy-five-year-old man with a slow-growing prostate cancer that is not causing any symptoms may never need the cancer treated. All of these clinical lines of thought revolve around the growth properties of the cancer in question. For each patient, the growth assessment of the cancer is best understood through discussions with the oncologist.
Cancer researchers are also focused on growth as they work to discover new and better ways of treating cancer.
Scientists study the molecules inside cancer cells that stimulate them to multiply and grow. By understanding how these important molecules work, researchers can develop drugs that will block them from functioning. The hope is that interfering with these critical targets will cause the cancer cells to die.
These growth targets and the drugs designed to block them are discussed later.
We’ve established that growth is central to thinking about cancer. But what does it mean for cancer to grow, and to grow in an unlimited way? What actually is growing?
The answer is the number of cancer cells. All cancers start with one cell, and that cell multiplies to form the tumors that are ultimately detected. One cell becomes two cells. These two cells then duplicate themselves to become four cells, which multiply to eight cells, and so on, until there is an entire population of cells.
It is generally thought that one billion cancer cells need to have formed before a cancer can be detected. This is the number of cells present in a one-centimeter tumor (nearly a third of an inch).
The ability to detect cancer when far fewer cells are present is a high priority of cancer research. While the growth of cancer cells is certainly a bad thing, the growth of healthy cells is of course, necessary for our bodies to function properly.
The difference between normal cell growth and cancer cell grow this that normal growth is always precisely timed and controlled. For example, when a human fetus is developing, cell growth is explosive because one fertilized egg must give rise to the trillions of cells that ultimately compose a body. Yet the process of making the heart, brain, or any other organ is tightly regulated: cells stop growing once the correct organ pattern is laid down. In fact, when an organ reaches maturity, most of its cells lose the capacity to multiply.
This is why our heart cannot replace damaged cardiac muscle after a heart attack and why our bodies cannot heal a spinal cord injury by making new nerve tissue.
Mature adult organs have a limited capacity to regenerate, with the exception of the liver, the inner lining of the intestines, and the bone marrow.
Fetal tissue, on the other hand, has the full capacity to form new cells, which is why fetal stem cells (the cells with the greatest regenerative capacity) are being studied as a way to help victims of numerous illnesses and injuries, such as Parkinson’s disease and spinal cord damage.
The hope is that if fetal stem cells are implanted in an environment of nerves, for example, they will sprout new nerve cells to replace the damaged ones.
The major exception to the rule that adult cells do not multiply freely is cancer.
Cancer cells derive from the cells of our fully formed organs, but they have found a way, through genetic mutation, to bypass the natural brakes on cell growth. By sustaining alterations to DNA elements that control growth, cancer cells acquire a limitless ability to multiply. In addition, they become impervious to the checks and balances that our bodies have developed to restrain rebellious cells.
Fortunately, other factors limit the size of any tumor (cancers do not just grow and grow). Yet because of this powerful growth engine, cancer must be fought with strong treatments, such as chemotherapy and radiation that attempt to stop this growth in its tracks.
The differences in growth between normal cells and cancer cells can be exploited by chemotherapy and radiation, which preferentially attack the actively dividing cancer cells.
Can we determine the exact growth rate of a cancerous tumor? No.
At this time, there is no precise way for doctors to assess the growth rate of a particular cancer. The technology has not yet been developed. Moreover, such a measure would be a complicated affair, because cancers change their growth patterns as they increase in size (the rate of growth slows as they get bigger) and as they are exposed to treatments (which attempt to slow the rate of growth considerably). But if such a test were available, it would undoubtedly show that no two cancers grow at exactly the same rate.
Across the vast spectrum of cancer and its many different types is an even greater range of growth rates.
Some cancers grow fast, and some grow slowly. This rate relates mainly to the specific constellation of molecules that define each cancer (no two cancers are exactly alike). For most tumors, changes in size are a balance between factors that promote growth and others that limit growth, such as the available supply of blood and nutrients. In fact, some cancers grow so slowly that they remain the same size from month to month or even from year to year.
Although cancer is commonly thought of as a disease caused by cells “growing out of control” or “running amok,” this simple conception of cancer is inaccurate. I have been caring for B. seventy-five-year-old man who recently received his first treatments for a non-Hodgkin’s lymphoma (a cancer of an immune cell called a lymphocyte) that was diagnosed when he was fifty years old. Throughout much of the intervening twenty-five years, B. experienced what we call “stable disease.” He carried on with his life with his cancer untreated.
Stable disease is when a person and his or her cancer live in peaceful coexistence - a period when the cancer is not growing much and the body’s natural defenses can keep it in check. It also implies that the cancer is not causing the patient any symptoms, so he or she is not ill. In the past year, however, Don’s lymphoma grew more rapidly, a situation termed “progressive disease,” which necessitated his treatment. Yet despite the diagnosis of cancer many years ago, Don has lived a full and active life before, during, and after its treatment.
The message of B. story is that many cancers do not grow like wildfire. For sure, some do develop rapidly, such as the blood cell cancers acute leukemia and high-grade lymphoma and aggressive forms of the more common organ-derived cancers. But for others, the dominant problem with cancer cells is not that they are growing out of control and forming large tumors but that they just won’t die once they are formed.
This leads us to the second essential property of cancer.
