Chapter 4: Cancer incidence in Canada: trends and projections (1983-2032) – Thyroid cancer - HPCDP: Volume 35, Supplement 1, Spring 2015

Chapter 4: Projections by Cancer Site

19. Thyroid cancer

The average annual number of thyroid cancer in 2003–2007 was 795 in males and 2810 in females, representing 1.0% and 3.8% of all new Canadian male and female cancer cases, respectively (Tables 4.19.1 and 4.19.2).

Whereas incidence rates for all cancers combined and for most specific types of cancer in Canada are stable or decreasing, thyroid cancer rates are rising significantly in both sexes. Though relatively uncommon in Canada, thyroid cancer is the most rapidly increasing of all cancers in both sexes. This increase has been particularly rapid in females over the past 25 years, such that thyroid cancer was the sixth most common cancer in females in Canada during the last observation period (2003–2007) (Table 4.19.3 and 4.19.4).

Over the entire observation period, Ontario experienced the fastest growth in rates, British Columbia had the slowest linear increase, and other regions had similar moderate increases in between (Figure 4.19.1).

One in 223 males and 1 in 71 females can expect to be diagnosed with thyroid cancer in their lifetime, and 1 in 1937 males and 1 in 1374 females can expect to die from it.^{Endnote 36} Thyroid cancer has the highest 5-year relative survival rate of all cancers in Canada, at 98% in 2006–2008.^{Endnote 1}

Joinpoint analysis (data not shown, but the similar annual percent changes for males and the annual percent changes for females in their respective most recent observation periods are shown in Figure 3.1 or 3.2) further showed that the ASIRs of thyroid cancer in males in Canada increased 2.7% per year from 1986 to 1997 and then rose more quickly at 6.7% per year through 2007. The rates for females increased 4.4% per year during 1986 through 1998, accelerated to an 11.4% annual increase from 1998 to 2002 and thereafter rose more slowly by 6.9% annually. Cases are generally diagnosed at younger ages relative to other cancers. In 2003–2007, approximately 44% of new thyroid cancer cases in females were diagnosed in those aged under 45, 25% in those aged 45 to 54, 17% in those aged 55 to 64 and 15% in those aged 65 or older. For males, the corresponding percentages for the same age groups were 32%, 24%, 22% and 21% (Tables 4.19.1 and 4.19.2).

Through the entire observation period, the ASIRs were higher in females than in males in all age groups. The female-to-male ratio was highest in the youngest age group (<45), decreased steadily with age up to age 74 and increased slightly for the older age groups (Figure 4.19.2). Furthermore, the female-to-male ratio increased with time for each age group, with the most pronounced increases in those younger than 55. In the last observation period (2003–2007), the rates in the youngest females (under age 45) were nearly 5 times higher than those in males of the same age.

The observed increasing pattern of thyroid cancer ASIRs continued into the prediction periods in each age group in both sexes. However, the rates for both sexes are expected to reach their peak after 10 years in the youngest age group and after 20 years in 45–54 year age group (Figure 4.19.2). The rates are projected to increase in all regions, with the most prominent increase in Ontario and the least evident one in British Columbia (Figure 4.19.1). Internal ranking of ASIRs according to region is projected to be similar for males and females, with the highest rates in Ontario and the lowest in British Columbia.

From 2003–2007 to 2028–2032, the ASIRs of thyroid cancer for Canada are projected to increase by 55% in males, from 4.5 to 7.0 per 100 000, and by 65% in females, from 16.1 to 26.5 per 100 000 (Tables 4.19.3 and 4.19.4). The annual number of new cases is estimated to increase by 139% in males, from 795 to 1895, and by 146% in females from 2810 to 6910 (Tables 4.19.1 and 4.19.2).

Comments

The significantly increasing thyroid cancer incidence rates in Canada are similar to trends observed in other developed nations.^{Endnote 257, Endnote 258, Endnote 259} Increased diagnostic scrutiny may have resulted in this observed increase. Advanced diagnostic technologies (e.g. ultrasonography, computed tomography scan, magnetic resonance imaging and fine-needle aspiration biopsy) have facilitated the detection of small, subclinical thyroid carcinomas.^{Endnote 260} More frequent use of these technologies to diagnose benign thyroid diseases, which affect more females than males, may account for the more rapid increase of thyroid cancer in females.^{Endnote 260, Endnote 261}

Although US evidence supports the theory that the increase in thyroid cancer incidence reflects the increased detection of subclinical disease,^{Endnote 258} some investigators suggest that this increase is not based simply on greater detection, and that the changing prevalence of a known or an emerging risk factor may also explain this rise.^{Endnote 259} Risk of thyroid cancer has been linked to environmental sources of ionizing radiation, insufficient or excess iodine consumption, and heredity.^{Endnote 111, Endnote 262, Endnote 263} Increasing population exposure to diagnostic ionizing radiation may have increased the risk of thyroid cancer. Ionizing radiation was used to treat benign conditions in children and adolescents between 1930 and 1960.^{Endnote 264, Endnote 265} This use may explain some of the increase in thyroid cancer incidence in older women. A recent systematic review also supported a positive association of BMI with thyroid cancer risk.^{Endnote 266} Obesity rates have been increasing over the last 30 years in Canada,^{Endnote 51, Endnote 72, Endnote 73} and the increasing trend in obesity in the general population may be an important contributing factor in the rising thyroid cancer incidence.