Table 1 presents the numbers of study subjects aged 20 or over from the database that was used to derive the screening, incidence, postoperative complication, and mortality rates of TC. The number of study subjects in the KCHS data was relatively similar over the years: 225,116 in 2010, 225,188 in 2012, and 224,994 in 2014. The respondents who had received TC screening in the most recent 2 years gradually increased. The number of study subjects in the NHID between 2006 and 2015 has consistently increased, from 35,500,836 in 2006 to 39,696,326 in 2015. From 2006 to 2015, the number of patients with incident TC increased from 15,943 in 2006 to 44,174 in 2012, but then decreased to 25,165 in 2015. The number of patients with postoperative complications of TC surgery increased continually between 2006 and 2010, decreased in 2011, and then increased to its peak of 7,694 in 2013. Later, the number of patients gradually decreased, along with TC incidence. In contrast to the incidence and postoperative complication rate of TC, the mortality of TC remained between 300 and 400 every year, without any significant fluctuations between 2006 and 2015. The magnitudes of the screening, incident, postoperative complications, and deaths cases of TC were much higher for women than men. However, the trends were similar to both women and men. The numbers of patients with the negative control outcomes by year are presented in Additional file 1: Table S3.
Fig. 1. presents the age-standardized incidence and mortality rates of TC in Korea between 2006 and 2015. TC incidence in men and women combined soared from 46.6 to 115.0 per 100,000 from 2006 to 2012, but then plummeted through 2015. TC incidence among women in 2012 was 188.2 per 100,000, reflecting a nearly 2.4-fold increase from 79.1 in 2006. The incidence of TC in men was much lower than in women, but it increased by 3.2 times, from 13.9 per 100,000 in 2006 to 44.0 in 2013. The increasing trend in TC incidence nosedived starting in 2012 among women and starting in 2013 among men. However, the mortality rate remained very low, at approximately 1.0 per 100,000 regardless of sex, and it showed no significant fluctuations between 2006 and 2015 (Additional file 1: Tables S4 and S5).
Table 2 presents the age-standardized screening, incidence, postoperative complication, and mortality rates of TC by calendar year and income level. In 2010, 2012, and 2014, from 12,562.0 to 18,717.5 adults aged 20 or over per 100,000 had undergone TC screening in the most recent 2 years. Between 2006 and 2015, TC incidence ranged from 46.6 to 115.0 per 100,000, and the postoperative complication rate was 8.9 to 19.7per 100,000. This indicates that postoperative complications occurred in 14.1% to 20.6% of incident TC cases every year. In contrast, the mortality of TC was stable over time, ranging only from 1.0 per 100,000 between 2006 and 2010 and 0.8 per 100,000 between 2011 and 2015. Every year between 2006 and 2015, the highest income quintile showed high screening and incidence rates of TC. The highest income quintile also showed high rates of postoperative complications associated with TC in most years. However, the mortality rate showed an opposite pattern, as the highest mortality rate occurred in the lowest income quintile.
As shown in Fig. 2., every indicator, except the mortality rate, showed a higher age-standardized rate as income increased (Fig. 2a, b, and c). However, the mortality rate showed the opposite trend (Fig. 2d). The mortality rate of the lowest income quintile was higher than that of the other income quintiles. Furthermore, postoperative complications of TC showed a positive association with income between 2006 and 2013. In 2011, the complication rate decreased in every income group, and then it increased in 2012. The incidence rate began to decrease in 2013, and the complication rate of the highest income quintile nosedived, indicating that the highest-income participants responded more quickly to the decrease in TC incidence than other income quintiles (Fig. 2c).
Fig. 3. presents the age-standardized incidence and mortality rates of lung cancer and stroke by year and income level. Both lung cancer and stroke showed different patterns of incidence and mortality rates from TC. The incidence and mortality rates of lung cancer and stroke continually decreased. During the observation period, TC showed a rapid increase and decrease, but no similar fluctuation was observed for lung cancer (Fig. 3a) and stroke (Fig. 3b). The incidence of lung cancer showed no clear pattern by calendar year, although the mortality rate of lung cancer decreased (Fig. 3a). The incidence and mortality rates of stroke showed a consistent decrease between 2006 and 2015 (Fig. 3b). Every income group also presented a similar pattern. Furthermore, the patterns in incidence rates by income quintiles showed an opposite pattern from that of TC. Whereas TC had a high incidence and low mortality rate in the highest income quintile, for lung cancer and stroke, the lowest income quintile showed the highest rates of incidence and mortality (Fig. 3c, d, e, and f).