Читать книгу Cancer is a Word, Not a Sentence - Miriam Stoppard - Страница 15

One patient compared staging tests to ‘a patrol of the premises by security guards—they usually don’t find anything, but they know how to sound the alert if there’s trouble.’

The point is that some cancers can invade to a greater extent locally than is apparent when the doctor examines you, and some can spread to distant areas of the body without causing any symptoms or noticeable trouble. If either of these things has happened, the treatment plan will have to be modified accordingly. So the screening tests are done in order to find out if there is anything unexpected going on. And that means that a very large number of people will be having tests which turn out not to show anything unexpected. It’s a nuisance, but it’s important.

The staging tests are selected on two basic and simple principles which we can best express as the answers to these two important questions:

If this particular cancer were to spread, where in the body it is most likely to spread to?

Which tests both have a high likelihood of detecting something wrong at an early stage and do not usually produce a false-alarm or false-positive? That means they don’t give the appearance of a serious abnormality when there is actually nothing wrong.

I can best illustrate these two principles with two tests in breast cancer—a bone scan, and a blood test called the carcino embryonic antigen (CEA).

The bone scan is actually quite a useful—and subtle—test. A small dose of a harmless radioactive isotope called technetium is given to you by intravenous injection. When the technetium circulates in the body, it is taken up almost exclusively by the cells in the bone that actually make the bone tissue. These cells are called osteoblasts, and where they take up the isotope the bone scan will show a fine pattern of tiny black dots.

In many cancers, the cancer cells settle in the bone and start destroying the bits of bone around them. This provokes a reaction by the defence team, the osteoblasts.

This reaction is almost always provoked if a group of breast cancer cells lodges in the bone. With other cancers, that reaction doesn’t always happen. But with breast cancer if there is even a relatively small group of cancer cells spreading to and settling in a bone—such as the spine, or the long bones of the arms or legs—the bone scan is highly likely to show them as a larger than average black splodge, or hot spot as it is called.

Now it also happens that other problems—particularly in the joints, such as arthritis—can also produce hot spots on the bone scan, but arthritis and most noncancerous problems usually look different (and appear in different patterns and places) from cancer metastases. So in the great majority of cases, an experienced radiologist can look at the bone scan and state with considerable certainty whether there are any areas that might be secondaries or not. In some cases, the bone scan itself cannot distinguish between a probably benign appearance such as arthritis and a probably metastatic appearance. Then, x-rays of the area or CT scans of the area will be required.

So even if the patient has no symptoms related to that area—no pain or discomfort—the bone scan will probably pick up an early secondary or metastasis. That’s what makes it so useful as a staging test, and that’s why it’s worth having one, when recommended, even though you may have no symptoms or problems in your bones.

Because breast cancer has a high predilection for spreading to the bones, in any situation where the breast cancer has demonstrated a higher than average risk of spreading—if the lymph nodes are postive, for example—a bone scan is worth doing.

It’s a different story for the carcino embryonic antigen (CEA). CEA is a substance secreted by several different cancer cells, including colo-rectal, breast, and some lung cancers. In breast cancer, however, the levels of CEA are usually normal when the breast cancer has not spread. The levels rise above normal only when there is a high total amount of cancer. Furthermore, the CEA is often raised in other conditions. In fact, people who smoke heavily can have a high CEA level even when there is nothing wrong with them. So, given the low rate of success in detecting small amounts of metastases, and given also the false-positive rate, the CEA is not worth doing as a staging test for breast cancer.

By and large these two principles are used in deciding which staging tests should be done for each type of cancer after diagnosis.