Types of cancer survival statistics
Survival statistics are based on population averages. They must be interpreted very carefully and cannot be used alone to predict what will happen to a particular person (a prognosis). Researchers sometimes use survival statistics to measure how effective treatments are.
There are many different ways to measure and report cancer survival statistics. Most statistics are reported for a specific time period, such as 1, 3, 5 or 10 years. Usually, 5-year survival statistics are used.
Observed survival is the percentage of people with a particular cancer who are alive at a certain point in time after their diagnosis.
For example, a 5-year observed survival of 70% means that, on average, people have a 7 in 10 chance of being alive 5 years after their diagnosis. Observed survival does not consider the cause of death, so the people who are not alive 5 years after diagnosis could have died from cancer or from another cause.
Relative survival compares the survival for a group of people with cancer to the survival expected for a group of people in the general population who share the same characteristics as the people with cancer (such as age, sex and place of residence). Ideally, the group of people used in the general population would not include people with cancer, but this estimate can be difficult to obtain. As a result, relative survival can sometimes be overestimated.
Unlike observed survival, which considers all causes of death, relative survival measures survival from cancer only.
For example, a 5-year relative survival of 63% means that, on average, people diagnosed with cancer are 63% as likely to live for at least 5 years after their diagnosis compared to people in the general population. Estimates of relative survival can be greater than 100%. This means that the observed survival of the people with cancer is better than the expected survival from the general population.
Net survival represents the probability of surviving cancer in the absence of other causes of death. It is used to give an estimate of the percentage of people who will survive their cancer.
For example, a 5-year net survival of 50% means that, on average, about 50% of people will survive their cancer for at least 5 years.
Researchers are starting to use net survival more often now, rather than relative survival, because it provides a more useful measure for tracking survival over time and comparing survival between populations.
Median means the middle value, or midpoint. Median survival is the length of time after diagnosis or the start of treatment at which half of the people with cancer are still alive. In other words, half of the people are expected to live at or beyond the median survival and the other half are not. For example, if 50% of people with a cancer are still alive 12 months after their diagnosis, then the median survival is 12 months.
Disease-free survival is the percentage of people with cancer who are alive without detectable disease for a specific period of time. For example, if cancer treatment results in a 70% disease-free survival over 5 years, then 7 out of every 10 people did not have any detectable disease for 5 years after treatment.