Survival statistics are very general. Although they may be used to help predict prognosisprognosis1. The expected outcome or course of a disease. 2. The chance of recovery or recurrence., they must be interpreted very carefully and cannot be used alone to predict what will happen to a particular person. Researchers sometimes use survival statistics to evaluate the effectiveness of treatments they are studying.
There are many different ways of measuring and reporting cancer survival statistics. Most statistics are reported for a specific time period, such as 1, 3, 5 or 10 years.
Observed survival is the proportion of people with a particular cancer who are alive at a certain point in time after their diagnosis.
For example, an observed survival of 70% at 5 years means that a person has, on average, a 7 in 10 chance of being alive 5 years after 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 is a ratio that compares the observed survival for a group of people with cancer to the expected survival of people in the general population who share the same characteristics as the people with cancer (such as age, sex, place of residence), but who are assumed to be free of cancer.
For example, a 5-year relative survival of 62% means that, on average, people diagnosed with cancer are 62% as likely to live for at least 5 years after their diagnosis compared to people in the general population.
Unlike observed survival, which considers all causes of death, relative survival measures survival from cancer only. Estimates of a relative survival ratio can be greater than 100%. This indicates that the observed survival of the people with cancer is better than that expected from the general population.
Relative survival has become the preferred way of analyzing survival for people with cancer. Five-year relative survival ratios are commonly used.
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 beyond the median survival and the other half are not. For example, if 50% of people with 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 (are disease-free) for a defined 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.