Increased incidence of thyroid cancer in World Trade Center-exposed emergency workers is predominantly due to increased detection of asymptomatic thyroid cancer during medical surveillance, according to study results published in JAMA Internal Medicine.
Exposure to debris during the terrorist attacks on the World Trade Centers on September 11, 2001, may represent an environmental exposure that has contributed to an increased risk for thyroid cancer in first responders. However, rescue and recovery workers exposed to the World Trade Center site have also been provided with annual screening and treatment, which may have increased the frequency of asymptomatic cases that are detected.
Researchers compared the incidence of thyroid cancer in World Trade Center-exposed firefighters and emergency medical workers with the incidence in demographically similar individuals from the Rochester Epidemiology Project (Olmstead County, Minnesota) between 2001 and 2018 to determine whether the observed increased incidence is associated with World Trade Center exposure or increased medical surveillance.
A total of 14,987 firefighters and emergency service personnel (100% men) who worked on the World Trade Center recovery between the morning of September 11, 2001, and July 25, 2002, were included in the study. Women were excluded from the analysis because of a small sample size (n=491). Method of detection (asymptomatic or symptomatic) was obtained from medical records. Results were also compared against the United States male thyroid cancer rates determined by the Surveillance, Epidemiology, and End Results-21 (SEER-21) program.
A total of 72 cases of thyroid cancer were confirmed in the firefighter/medical worker cohort. The median age at diagnosis was 50.0 years and the median time to diagnosis was 11.5 years after exposure. None of the men experienced thyroid cancer-specific mortality and none developed metastatic disease. Of the 65 cases for which the detection method was noted, 81.5% were asymptomatic. Asymptomatic cases were almost exclusively papillary thyroid cancer, with a single incident of follicular carcinoma.
In the Rochester Epidemiology cohort, there were 99 instances of thyroid cancer in the same time period, of which 54 (54.5%) were asymptomatic. The median age at diagnosis was 50.0 years. Most cases (92.9%) were papillary cancers.
The age-standardized incidence of thyroid cancer was higher in the firefighter/medical worker cohort (24.7 per 100,000 person-years; 95% CI, 17.4-52.3) compared with the Rochester (10.4 per 100,000 person-years; 95% CI, 8.5-12.7) and SEER-21 cohorts (9.1 per 100,000 person-years; 95% CI, 9.0-9.1). Asymptomatic cases were responsible for the observed difference in incidence.
The age-adjusted relative incidence rate of thyroid cancer was also higher in the firefighter and emergency medical responder cohort compared with the Rochester cohort (relative rate [RR], 2.3; 95% CI, 1.7-3.2). This difference was most pronounced in the second half of the analysis period (after 2010; RR, 2.5; 95% CI, 1.6-3.8). There was no significant difference in the rate of symptomatic cases between the 2 cohorts (RR, 0.8; 95% CI, 0.4-1.5), but there was a significant increase in the rate of asymptomatic cases in emergency personnel (RR, 3.1; 95% CI, 2.1-4.7).
The investigators noted that the lack of a firefighter cohort that was not exposed to the World Trade Center site represented a limitation of the study. In addition, data regarding the size or staging of thyroid cancer cases were not available in the firefighter cohort.
“The high incidence of thyroid cancer post-9/11 appears to be attributable to the increased diagnosis of occult lesions owing to the detailed and frequent medical testing of [World Trade Center]-exposed individuals,” the study authors concluded. “There seems to be no incidence of disease-specific thyroid cancer mortality, and carcinoma discoveries were largely of the least-aggressive subtypes, suggesting that the observed RRs are due to a greater detection of subclinical cancers in Fire Department rescue/recovery workers than in the comparison population.”
Colbeth HL, Genere N, Hall CB, et al. Evaluation of medical surveillance and incidence of post-September 11, 2001, thyroid cancer in World Trade Center-exposed firefighters and emergency medical service workers [published online April 20, 2020]. JAMA Intern Med. doi:10.1001/jamainternmed.2020.0950