Background: This study was undertaken to determine whether cancer of the brain and central nervous system (CNS) occurs together with other types of cancer more often or less often than would be expected due to chance.
Methods: The study was based on data collected by the Surveillance, Epidemiology, and End Results (SEER) Program for cancers diagnosed in the U.S. between 1973 and 1998. The standardized incidence ratio (SIR) for new malignancies was estimated as the number of patients diagnosed with a particular type of cancer (observed), divided by the number of diagnoses expected based on person-years of follow-up and incidence rates for SEER cancer registries.
Results: Among patients who were diagnosed first with cancer of the brain or CNS, statistically significant excesses were observed for cancers of bone (SIR = 14.4), soft tissue (SIR = 4.6), brain and CNS (SIR = 5.9), salivary gland (SIR = 5.1), and thyroid gland (SIR = 2.7) in addition to acute myelocytic leukemia (SIR = 4.1) and melanoma of the skin (SIR = 1.7). Excess risk of new malignancies was markedly greater after first cancers of the brain or CNS diagnosed in childhood compared with similar diagnoses in adulthood. In reverse associations, significant excesses of cancer of the brain and CNS were observed only after diagnoses of acute lymphocytic leukemia (SIR = 7.4) and cancer of the testis (SIR = 1.8) or the thyroid gland (SIR = 1.7).
Conclusions: Cancer treatment appears to have been the major factor underlying most of the positive associations between brain cancer and primary cancers of other types, with a probable lesser contribution from shared genetic susceptibility. Results provide little or no evidence that brain cancer shares important etiologic factors with the common cancers of adulthood.