Thyroid Cancer

Background

The Canadian Cancer Society estimates that thyroid cancer will be the seventh most commonly diagnosed cancer overall in Canada in 2020, and the fifth most commonly diagnosed in females [1]. It is most commonly diagnosed among people aged 20 to 55.

Little is known about occupational risk factors for thyroid cancer. Exposure to ionizing radiation (from radiation therapy, diagnostic imaging or nuclear fallout) during childhood and adolescence is the strongest known risk factor for thyroid cancer. In the workplace, the most consistent associations have been seen for radiation-exposed workers and health care occupations, while some other studies have seen an increased risk in pesticide-exposed workers and agricultural occupations [2].

 
Known occupational risk factors
    • Ionizing radiation (exposure during adulthood) [3]
      • X- and gamma-radiation from performing diagnostic imaging
      • Administering radioactive iodine treatment
Possible occupational risk factors
    • Pesticides (such as organochlorines) [2,4]
Key Findings

Some previous studies have found increased risk of thyroid cancer among workers in agricultural workers, who may be exposed to pesticides. However, no increased risks were seen among these groups in the ODSS.


Healthcare

Medical workers may be exposed to low-dose ionizing radiation when they administer radiation therapy or perform diagnostic imaging. Exposure to ionizing radiation during adulthood is a possible risk factor for thyroid cancer.

    • Occupations in medicine and health: 1.11 times the risk
      • Denturists, dental hygienists, dental assistants and dental technicians: 2.35 times the risk
      • Radiological technicians: 1.33 times the risk
      • Nurses, graduate, except supervisors: 1.21 times the risk
    • Diagnostic and therapeutic services industry, not elsewhere specified*: 1.71 times the risk
    • Hospitals: 1.11 times the risk

*This industry includes laboratories (dental, medical, x-ray, etc.) providing analysis, diagnosis or treatment service to the medical or dental professions or to patients on prescription of a physician or dentist, and organizations such as the Victorian Order of Nurses, Red Cross Blood Transfusion Service, and St. John Ambulance.

Relative Risk by Industry and Occupation

Figure 1. Risk of thyroid cancer diagnosis among workers employed in each industry group relative to all others, Occupational Disease Surveillance System (ODSS), 1983-2016

The hazard ratio is an estimate of the average time to diagnosis among workers in each industry/occupation group divided by that in all others during the study period. Hazard ratios above 1.00 indicate a greater risk of disease in a given group compared to all others. Estimates are adjusted for birth year and sex. The width of the 95% Confidence Interval (CI) is based on the number of cases in each group (more cases narrows the interval).

 

Figure 2. Risk of thyroid cancer diagnosis among workers employed in each occupation group relative to all others, Occupational Disease Surveillance System (ODSS), 1983-2016 

The hazard ratio is an estimate of the average time to diagnosis among workers in each industry/occupation group divided by that in all others during the study period. Hazard ratios above 1.00 indicate a greater risk of disease in a given group compared to all others. Estimates are adjusted for birth year and sex. The width of the 95% Confidence Interval (CI) is based on the number of cases in each group (more cases narrows the interval).

Table of Results

Table 1. Surveillance of thyroid cancer: Number of cases, workers employed, and hazard ratios in each industry (SIC)

SIC Code * Industry Group Number of cases Number of workers employed Hazard Ratio
(95% CI) †
1 Agriculture 46 35,007 0.60
(0.45-0.80)
2/3 Forestry, Fishing and
Trapping		 16 10,699 0.92
(0.56-1.51)
4 Mines, Quarries and
Oil Wells		 30 23,203 0.83
(0.58-1.19)
5 Manufacturing 1,606 693,458 0.98
(0.93-1.04)
6 Construction 301 210,956 0.91
(0.80-1.02)
7 Transportation, Communication
and Other Utilities		 421 197,154 1.08
(0.97-1.19)
8 Trade 1,061 429,108 1.01
(0.95-1.08)
9 Finance, Insurance and
Real Estate		 75 23,974 1.15
(0.92-1.45)
10 Community, Business and
Personal Service		 2,049 599,126 1.06
(1.00-1.12)
11 Public Administration and
Defense		 524 190,795 1.04
(0.95-1.14)
         
* SIC: Standard Industrial Classification (1970)
† Hazard rate in each group relative to all others

 

Table 2. Surveillance of thyroid cancer: Number of cases, workers employed, and hazard ratios in each occupation (CCDO) group

CCDO Code * Occupation Group Number of cases Number of workers employed Hazard Ratio
(95% CI) †
11 Managerial, administrative
and related		 121 30,958 1.24
(1.04-1.49)
21 Natural sciences, engineering
and mathematics		 63 26,327 1.13
(0.88-1.45)
23 Social sciences and
related fields		 146 30,676 1.28
(1.09-1.51)
25 Religion 0 128 —
27 Teaching and related 215 48,396 1.09
(0.95-1.25)
31 Medicine and health 641 135,167 1.11
(1.02-1.21)
33 Artistic, literary,
recreational and related		 33 14,985 0.87
(0.61-1.22)
41 Clerical and related 649 197,206 1.03
(0.95-1.12)
51 Sales 467 148,030 1.06
(0.97-1.17)
61 Service 1,149 370,720 1.03
(0.96-1.10)
71 Farming, horticultural
and animal husbandry		 57 50,138 0.56
(0.43-0.72)
73 Fishing, hunting,
trapping and related		 0 558 —
75 Forestry and logging 13 10,684 0.76
(0.44-1.31)
77 Mining and quarrying,
including oil and gas field		 20 13,040 1.03
(0.66-1.59)
81 Processing
(mineral, metal, chemical)		 142 79,214 0.81
(0.68-0.95)
82 Processing
(food, wood, textile)		 232 99,223 0.91
(0.80-1.03)
83 Machining and related 351 189,405 0.96
(0.86-1.08)
85 Product fabricating,
assembling and repairing		 821 328,254 1.14
(1.06-1.23)
87 Construction trades 275 215,577 0.82
(0.73-0.93)
91 Transport equipment
operating		 296 168,082 1.01
(0.89-1.14)
93 Materials handling and related,
not elsewhere classified		 322 153,026 0.97
(0.87-1.09)
95 Other crafts and
equipment operating		 67 28,303 0.99
(0.78-1.26)
99 Other occupations not elsewhere classified 404 215,202 0.89
(0.81-0.99)
         
* CCDO: Canadian Classification Dictionary of Occupations (1971)
† Hazard rate in each group relative to all others

Please note that ODSS results shown here may differ from those previously published or presented. This may occur due to changes in case definitions, methodological approaches, and the ongoing nature of the surveillance cohort.

References

  1. Brenner DR, Weir HK, Demers AA, Ellison LF, Louzado C, Shaw A, Turner D, Woods RR, Smith LM. Projected estimates of cancer in Canada in 2020. Can Med Assoc J. 2020;192(9):E199–205.
  2. Aschebrook-Kilfoy B, Ward MH, Della Valle CT, Friesen MC. Occupation and thyroid cancer. Occup Environ Med. 2014;71(5):366–80.
  3. International Agency for Research on Cancer (IARC). List of Classifications by cancer sites with sufficient or limited evidence in humans, IARC Monographs Volumes 1-127 (2020).
  4. Han MA, Kim JH, Song HS. Persistent organic pollutants, pesticides, and the risk of thyroid cancer: Systematic review and meta-analysis. Eur J Cancer Prev. 2019;28(4):344–9.