Purpose: The risk of developing bilateral kidney cancer has not been adequately defined in any large, population-based study with long-term follow-up to our knowledge.
Patients and methods: We estimated the risk of metachronous bilateral renal cell cancer in patients diagnosed with unilateral kidney cancer, as recorded in the nationwide cancer registries of Norway and Sweden. Altogether 28,642 patients were followed for an average of 4.4 years. The standardized incidence ratio--the ratio of the observed number of bilateral cancers to the number expected on the basis of the incidence in the Norwegian and Swedish population at large--was used as a measure of relative risk. We used multivariate Poisson regression to separate the effects of the explanatory variables.
Results: A synchronous bilateral renal cell cancer was reported in 86 patients. A total of 112 metachronous bilateral cancers were recorded during 126,493 person-years of follow-up compared with 35.8 expected, yielding an overall relative risk (RR) of 3.1 (95% CI, 2.6 to 3.8) and a cumulative incidence of 0.8% after 20 or more years of follow-up. In the multivariate analyses, risk increased monotonically with younger age at first diagnosis (P for trend < .001); compared with patients who were 60 years or older, those younger than 40 years were at a 17-fold higher risk (RR = 17.4; 95% CI, 10.1 to 29.8). We also found a modest but statistically significant decreasing trend with increasing duration of follow-up.
Conclusion: The risk of metachronous bilateral renal cell cancer is drastically higher among patients first affected at a young age, suggesting a subset of early onset renal cell cancer with a strong genetic component.