Bone cancer is rare, comprising less than 1% of all cancers; however, it is the seventh most common type of cancer in adolescents, representing 5% of all cancers in those aged 0 - 19 years [1] . Because the incidence of bone cancer is clearly age-dependent, an analysis of its incidence by age groups would be more meaningful [2] [3] . Bone cancer imposes the risk of permanent restrictions in mobility, reduced function or even the loss of a limb, and possible physical disfigurement [4] . Reported or proposed risk factors for bone cancer include age, male sex, education, occupation, marital status, and area of residence [4] [5] . There is marked geographic variation observed in the prevalence of Paget’s disease, with a high prevalence noted in the UK, Australia, and North America and a lower prevalence observed in Asia and the Middle East [6] [7] .

Hence, our study was designed to assess bone cancer mortality rates and the associated social factors in the Inner Mongolia region of China. The data covered a large population and reflected the most recent status in Inner Mongolia.

During the period 2008-2012, the crude bone cancer mortality rate was 1.12/100000 (95% CI = 1.02 - 1.21). The mortality rate by sex was 2.24 in men and 1.25 in women, resulting in a male-female ratio of 1.8.

Figure 1 shows that by sex, the crude bone cancer mortality rate was higher among men than among women between 2008 and 2012, although the male-female ratio decreased over time. The ratio of the bone cancer mortality rate between the sexes was 2.9 in 2008, versus 1.4 in 2012. Meanwhile, no significant between-year difference in the mortality rate was observed between 2008 and 2012 (men: χ² = 4.65, P = 0.325; women: χ² = 2.21, P = 0.698).

Figure 2 presents the age-related mortality rates for bone cancer by sex at five monitoring points from 2008

to 2012. In general, mortality increased with increasing age. There was a sharp upward trend for men and women aged 50 years old and older. Meanwhile, the mortality rate peaked in women aged 40 - 50 years, who exhibited a higher rate than their male counterparts.

Table 1 shows the results of multivariate analyses of socio-demographic characteristics and the risk of bone cancer for men and women separately. Men and women with a lower education level exhibited an increased risk of bone cancer. Workers in fields involving extensive manual labor displayed a lower risk of bone cancer mortality, although the OR was higher for men than women (0.6 vs. 0.45). Unmarried people had a higher risk of bone cancer than married people. There was no statistically significant relationship between living in a rural or urban area and the risk of bone cancer.

The bone cancer mortality rate in Inner Mongolia was higher than that in Western countries [2] [11] but lower

Abbreviations: OR―odds ratio; CI―confidence interval.

than that in some Asian countries [3] [12] . In our study, the bone cancer mortality rate was consistently higher for men than for women from 2008 to 2012 (2.23 vs. 1.79). However, the rate was lower than the world average (2.27 - 2.39 and 3.3 - 4.6 per million individuals for men and women, respectively) [3] .

This study found that the risk of bone cancer was greater in women than in men younger than 40 - 50 years, whereas in men and women older than 50 years, a sharp upward trend in bone cancer risk was observed. Some previous findings supported our result [2] [3] . This suggests that the etiology of bone cancer may be hormonally driven, leading to differences in skeletal growth between men and women, especially among young people [13] . Older people have a higher risk for bone cancer death. Vitamin D deficiency is recognized as pandemic, and the elderly may be particularly affected [14] . It is theoretically possible that vitamin D deficiency could contribute to the risk of bone cancer, as vitamin D is required for bone development and vitamin D deficiency has been associated with bone and other cancers [15] . Different environmental and/or genetic risk factors could also be involved in the development of bone cancer in the elderly [3] .

Bone cancer rates were highest among the lower educated segment of the population. This finding is consistent with most existing studies [16] [17] . People with a lower education level experience a higher burden of cancer [17] . By contrast, a high level of education may be linked to increased knowledge of health or greater physical activity. Physical activity may help to maintain bone health [18] . Our result also illustrated that people employed in fields involving extensive manual labor had a lower risk of bone cancer mortality. Physical activity among people employed in fields involving extensive manual labor may result in greater bone density [19] .

The findings of our study illustrated that unmarried people had a higher risk of bone cancer than married people. Reports on health and mortality by marital status have consistently identified that unmarried individuals generally report poorer health, and they have greater risks of cancer mortality than their married counterparts. A study reported that unmarried patients are at significantly higher risk of presentation with metastatic cancer [20] . Caring and helpful spousal behaviors contribute to physical health [21] .

In conclusion, we reported the characteristics of bone cancer in Inner Mongolia from 2008 to 2012. The mortality rate of bone cancer was not significantly difference between men and women during this period. The risk of bone cancer increased with increasing age in both men and women. Unmarried people and those with a low education status had a greater risk of bone cancer. Meanwhile, employment in a field involving extensive manual labor appeared protective against bone cancer. Further studies are needed to assess more recent trends in bone cancer mortality.

This work was supported by Inner Mongolia Autonomous Region Colleges and Universities of Science and Technology Research Projects (No. NJZY13415), the Inner Mongolia Medical University Technology million project (No. NY2011BW006), and the Natural Science Foundation of Inner Mongolia in China (2013MS1124).