The Role of Medical Surveillance in Increased Post-9/11 Thyroid Cancer in World Trade Center-Exposed Workers

A new study examined whether there is a correlation between medical surveillance and incidence of post-Sept. 11, 2001, thyroid cancer among World Trade Center (WTC)-exposed firefighters and emergency medical service workers. They also explored differences between asymptomatic and symptomatic cases.

“The large quantity and variety of toxicants released into the environment following the World Trade Center (WTC) terrorist attacks on September 11, 2001 (9/11), may represent an environmental exposure contributing to the 2- to 3-fold greater risk of thyroid cancer among WTC-exposed populations compared with the general population,” the study authors wrote. They cited two recent studies that found an increase in WTC-related thyroid cancers among WTC responders versus matched controls.

“While radiation exposure was not reported to be elevated at the WTC site, other known carcinogens, such as polybrominated diphenyl ethers, were present at high levels and may disrupt mechanisms of thyroid hormone production. To our knowledge, no chemical substance has been consistently associated with thyroid cancer in humans. Alternatively, since 9/11, WTC-exposed rescue/recovery workers are provided with annual medical monitoring and treatment visits, which often include chest computed tomographic scans. The elevated thyroid cancer rate may therefore be an artifact of such frequent and detailed medical surveillance,” they noted.

The present study was a closed-cohort study that assessed how thyroid cancers were detected (asymptomatic and symptomatic) in 14,987 men who were monitored as part of the Fire Department–WTC Health Program and were from Sept. 12, 2001, to Dec. 31, 2018. The researchers collected age-, sex-, and histologic-specific Fire Department incidence rates and compared them to similar men from the Rochester Epidemiology Project. The primary exposure was WTC exposure, defined as rescue/recovery work at the site between Sept. 11, 2001, and July 25, 2002. The primary outcomes were:

• Number of incident thyroid cancers and their detection method categorizations in the Fire Department and Rochester Epidemiology Project cohorts
• Fire Department, Rochester Epidemiology Project, and Surveillance, Epidemiology, and End Results-21 age-standardized incidence rates of thyroid cancer
• Relative rates (RRs) comparing Fire Department and Rochester Epidemiology Project overall and by detection method categorization

Asymptomatic and Symptomatic Cases Present Different Outcomes

In the Fire Department cohort, 72 post-9/11 thyroid cancer cases were identified, of which 65 (90.3%) had a categorized detection method. Most cases were asymptomatic (n=53, 81.5%); 18.5% (n=12) were symptomatic. In asymptomatic cases, the median age at diagnosis was 50.2, compared to 46.6 in symptomatic cases. The overall age-standardized incidence of Fire Department thyroid cancers per 100,000 person-years—which the researchers noted was largely associated with asymptomatic cancers—was 24.7 (95% confidence interval [CI], 17.4 to 52.3), which was much higher than the Rochester Epidemiology Project (10.4; 95% CI, 8.5 to 12.7) and Surveillance, Epidemiology, and End Results-21 (9.1; 95% CI, 9.0-9.1). among symptomatic Fire Department cases, the RR did not significantly differ compared to the Rochester Epidemiology Project (0.8; 95% CI, 0.4 to 1.5), but the RR for asymptomatic cancers was more than three times that of the Rochester Epidemiology Project rate (3.1; 95% CI, 2.1 to 4.7).

The study was published in JAMA Internal Medicine.

“The results of this study suggest that a greater number of asymptomatic thyroid cancers have been diagnosed in Fire Department WTC-exposed rescue/recovery workers,” concluded the researchers, adding, “Our results have important implications for how thyroid cancer incidence rates are interpreted and how cases should be managed once diagnosed, not only for WTC-exposed cohorts, but for the general population as well.”