Thyroid Cancer

Background

What is thyroid cancer?

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).

 

Possible occupational risk factors
    • Ionizing radiation (exposure during adulthood)
      • X- and gamma-radiation from performing diagnostic imaging
      • Administering radioactive iodine treatment
    • Pesticides
Key Findings

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.

    • Nurses, registered, graduate and nurses-in-training: 1.2 times the risk

    • Radiological technologists and technicians: 1.3 times the risk

    • Dental hygienists, assistants and technicians: 2.3 times the risk

    • Pharmacists: 4 times the risk

Textile industry

More research is needed to understand why workers in the textile industry have an increased risk of thyroid cancer. Workers in this industry may be exposed to hazards such as formaldehyde, benzene, dyes, bleaches, transfer agents, and cotton dust, but so far none of these exposures are established risk factors for thyroid cancer.

    • Patternmaking, marking and cutting occupations in textile, fur and leather products: 1.6 times the risk

    • Sewing machine operators, textile and similar materials: 1.5 times the risk

    • Textile weaving occupations: 1.2 times the risk

    • Miscellaneous textile industries: 1.7 times the risk

Industries associated with pesticide exposure

Some previous studies have found increased risk of thyroid cancer among workers in agriculture and the wood processing, pulp, and papermaking industry. However, no increased risk was seen among these groups in the ODSS.

    • Agriculture: 0.6 times the risk

    • Wood industries: 0.9 times the risk

    • Paper and allied industries: 0.8 times the risk

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 1606 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 1061 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
2049 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 1149 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. CMAJ 2020;192:E199-205.
  2. Aschebrook-Kilfoy B, Ward MH, Della Valle CT, Friesen MC. Occupation and thyroid cancer. Occup Environ Med 2014;71(5):366-380.