Background: Five-year survival estimates in standard cancer reports provide a general description of disease outcome that is useful for surveillance and comparison purposes. However, for cancer survivors these overall survival rates may be discouraging, and the relevant question regarding an individual is this: Once he or she has survived for a specified period of time, what is the probability of survival over the next period of time?
Methods: To address this, conditional survival rates by histology for malignant brain tumor survivors were estimated using the SEER public use data and the Portable Survival System, with 19,105 brain and other nervous system patients diagnosed between 1979 and 1993. Given that the survival curve declines more rapidly in the first 2 years than in subsequent years, conditional probabilities of surviving 5 years given survival to 2 years and 95% confidence intervals (CIs)were calculated. As age is a strong prognostic factor for these tumors, conditional probabilities were also estimated by categories of age.
Results: Estimated 2- and 5-year relative survival rates for patients with malignant brain and other CNS tumors were 36.2% and 27.6%; however, the conditional probability of surviving to 5 years, given survival to 2 years, reaches 76.2% (95% CI: 74.8-77.6). Conditional probabilities varied by histology and age at diagnosis. The conditional probability of surviving 5 years after surviving 2 years was 67.8% (95% CI: 62.6-73.1) for patients with anaplastic astrocytomas, 36.4% (95% CI: 31.9-41.6) for patients with glioblastomas, and 79.8% (95% CI:75.3-84.1) for patients with medulloblastomas.
Conclusions: Conditional probabilities provide important and encouraging information for those who are brain tumor survivors. The utility of these estimates for other time intervals and other cancers or diseases should be considered.