Women diagnosed with DCIS will often hear about risk or chance. Risk information is complex and can be presented in various ways. This can make it difficult to use risk to help make medical decisions. Understanding some of the basic principles of risk can make it easier to think about it when discussing decisions with providers or doing your own research.
Risk (or chance) is very closely related to probability. Human health is a complicated system and determining risks are complex, but there are familiar systems that can illustrate what risk is which are much simpler; for example, tossing a coin.
A “fair” coin is one that is not weighted to favor one side or the other. If you imagine dropping a fair coin one time you cannot predict what will happen. It cannot be known for sure whether it will land heads up or tails up. But if you drop 100 fair coins a pattern will became apparent. About half the coins (50) will land heads up and half will land tails up. Probability describes the pattern that is seen after many repetitions. Experts study these patterns (probabilities) to describe the risk or chance of an event happening. In the case of the fair coin, many repetitions (over several centuries) have shown that the chance of it landing face up is about one half (50%). You still cannot predict what will happen to any one coin, but you CAN say a coin is just as likely to land heads up as heads down.
Determining risk for health conditions such as DCIS is far more complicated, but it is a very similar process. If we look at just one patient we cannot see a pattern, but if we see what happens to many patients with the same condition, a pattern may emerge. Studying this pattern may help us to determine the risks or chances that something may happen, for example a future breast cancer.
It may seem obvious that a fair coin would land face up half the time. But in science and medicine it is always necessary to have evidence. Many trials have been done with large numbers of coins over the last three centuries. In one trial, a coin was tossed 24,000 times! It landed heads up 12,012 times about 50.05% of the time.
In a similar way, researchers study many patients with the same condition in as many different groups as they can to be sure they are seeing a true pattern. Just like the coins, once they are confident in that pattern, they use it to determine risk or chance. In medicine, these studies are conducted in different ways, but one of the most important types of study is a clinical trial. There is a section on this website that describes clinical trials in more detail. It also includes a video.
As you talk with your providers or do your own reading, you may see or hear about different types of risk. It is important to know what type of risk is being described. Two of the most common types are relative risk and absolute risk.
RELATIVE RISK compares two or more GROUPS. For example, we can compare a bag of one hundred marbles that contains ONE blue marble with a second bag that contains TWO blue marbles. The risk of drawing a blue marble is twice as high for the second bag as the first. The relative risk of drawing a blue marble from the second bag is 200% compared to (relative to) the first bag. This does not tell us anything about the risk for any one marble drawn from either bag alone.
ABSOLUTE RISK tells what is more or less likely to happen for ANY given member of a group being studied. In the example above, the absolute risk of drawing a blue marble from the first bag is only 1 out of 100 (1%). The absolute risk of drawing a blue marble from the second bag is 2 out of 100 (2%). Even though RELATIVE RISK is twice as high for the second bag compared to the first bag, the ABSOLUTE risk is still very small for the second bag.
Relative risk and absolute risk are also used when discussing groups of patients. When you are using risk assessment to help make a medical decision, it is always best to use absolute risk. It is describing what is more likely to happen to any individual in the group being studied.
INDIVIDUAL RISK tells us what is more or less likely to happen to a SPECIFIC individual. It can be difficult to estimate the risk for an individual person (or even an individual coin). This is because there are many factors that must be considered. For a woman diagnosed with DCIS, her age, environment, genetics, family history, diet/exercise habits, and grade/size of DCIS, can all influence her INDIVIDUAL risk.
It is easy to think that the risks you hear about are YOUR risks, but this is rarely the case. There are some computer programs that can give you a more personalized risk, but they still cannot predict exactly what will happen to YOU — only what is more likely to happen to someone in your situation. It is important that you fully understand different types of risk so you can make the decision that is best for you.
DCIS is far more complex than coins or marbles. It has only recently become a more common diagnosis and researchers have not yet had the opportunity to study patients in large enough groups to fully understand all the patterns (probabilities). Because of this, experts disagree about what the risks are and how they can be reduced. Medical researchers cannot accurately predict which DCIS will lead to invasive breast cancer. Women may receive different advice about treatment options. Studies have shown that women diagnosed with DCIS often overestimate their breast cancer risk. For example, they may believe the chance of DCIS returning and dying of cancer is higher than it actually is.