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Cancer-related reductions in survival: extent and duration evaluated using a large cohort study of twins, 1943-2011.

BACKGROUND
The time during which there is an increased risk of death for cancer survivors was evaluated in a large twin study, which allows for matching on shared components such as age, genes, and socioeconomic factors in childhood.


METHODS
By use of data from Danish registers, time to death from initial cancer was studied prospectively in twins in two different settings. The twins were diagnosed with at least one cancer in the period 1943-2011. Setting I included 5,680 same-sex twin pairs aged six and over, while Setting II included 3,218 twin individuals from age 70 and over. The study provides comparisons within twin pairs and across birth cohorts, age at diagnoses, and time at diagnosis.


RESULTS
In 2001-2011, the five-year mortality risk for a twin surviving cancer after the age of 70 was twofold that of the co-twin, regardless of sex and zygosity, and it was 1.5-fold if the twin survived the initial nine months. After five to six years, the mortality risk corresponded to that of the co-twin. In previous decades, the excess hazard risk was considerably higher for both older and younger cohorts. There were no indications of change in relative survival across old birth cohorts.


CONCLUSIONS
This large twin study suggested that for a cancer-treatment survivor diagnosed at age 70 or later, the additional mortality risk was largely absent five years later, by which time the survival relative to the co-twin was 60%.


IMPACT
Elevated mortality risk after cancer is offset after five-six years.