General methods for the GBD 2015 study have been published previously. Herein, we present methods pertaining to the liver cancer estimation. Descriptions of the estimation process are available in the eAppendix in Supplement 1 (link, no link found) (eFigure 1, eFigure 2, and eTable 1).
The estimation process starts with liver cancer mortality, which we estimated using vital registration system data and cancer registry incidence data that were transformed to mortality estimates using separately modeled mortality-to-incidence ratios. Data were processed to adjust for aggregated causes, age groups, or uninformative causes of death. Liver cancer mortality was modeled by developing a large set of plausible models using different model types and combinations of covariates, that were tested using out-of-sample predictive validity (eTable 3 and eTable 4 in Supplement 1 (link, link, no link found)). The 2.5% and 97.5% quantiles from 1000 draws of the posterior distribution were used to generate 95% uncertainty intervals (UI). Liver cancer mortality was scaled with other causes of death to sum to 100% of the demographic estimates of all-cause mortality. Years of life lost were calculated by multiplying each death by the standard life expectancy. To generate mortality estimates for 4 liver cancer etiologies, proportions of liver cancer due to different causes were identified in a systematic review (eTable 5 in Supplement 1 (link, link, link, no link found)). Cases were attributed to HBV, HCV, alcohol, and other causes, which include remaining etiologies like liver flukes, nonalcoholic steatohepatitis, and aflatoxins. To estimate proportions for all locations, by sex, and over time, models were generated using DisMod-MR 2.1, a Bayesian meta-regression model (eAppendix in Supplement 1). Liver cancer mortality estimates were split into etiologies using the modeled proportions. Liver cancer incidence was estimated by dividing mortality by mortality-to-incidence ratios. Survival was estimated based on a theoretical best and worst liver cancer survival and a scaling factor derived from age-standardized mortality-to-incidence ratios. Prevalence was calculated using incidence and survival estimates and divided into 4 phases reflecting changing disability during: (1) diagnosis and treatment; (2) remission; (3) disseminated; and (4) terminal phase. Prevalence for each phase was multiplied by distinct disability weights to generate years lived with disability (eTable 6 in Supplement 1 (no link found, link, no link found)). The sum of years of life lost and years lived with disability represents DALYs. One DALY can be interpreted as 1 lost year of “healthy life.”
To group countries with similar development status, a Sociodemographic Index (SDI) was used, which combines total fertility rate, average educational attainment in the population over age 15, and measures of income per capita (eFigure 3 and eTable 7 in the Supplement 1).To assess the contribution of demographic vs epidemiological changes, we decomposed trends into 3 components—population aging, growth, and change in age-specific rates. Rates are reported as mean per 100 000 person-years with 95% UI in parentheses. Age-standardized rates were computed using the GBD population standard.
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