The natural radioactive decay of radon (222Rn) gas gives rise to many products that release ionizing radiation, proven to be harmful to human health. Perhaps the best-known source of radiation that has been linked to thyroid cancer (TC) is the chronic inhalation of radon gas. Indoor 222Rn levels in Guam have been documented to be as high as 220 picocuries per liter (pCi/L), exponentially higher than the United States Environmental Protection Agency (USEPA) baseline standard of just 4 pCi/L. Thyroid cancer is the fifth common cancer in women on Guam and Filipinos had the highest thyroid cancer incidence rates compared to other ethnic groups in Guam. The objective of this study is to determine whether there is an association between thyroid cancer incidence, geographic location (village/zone), ethnicity, and amount of indoor radon exposure in Guam. The data sets used in this ecological study came from three major independent data collecting sources: Guam Environmental Protection Agency (GEPA) indoor 222Rn data (2002-2009), Guam Cancer Registry thyroid cancer incidence data (1998-2013), and Guam Census 2000 and 2010 demographic data. Simple linear regression analysis was performed to determine the relationship between mean indoor 222Rn levels and thyroid cancer incidence in Guam villages. Multiple regression analysis was conducted to account for the confounding effects of ethnicity (% Filipino), and zone. Results indicated that radon in Guam was found to be highest in Zone 1, the geographic villages situated atop a limestone plateau in the northern region where approximately 66% of Guam residents and over 90% of Filipinos live. In both univariate and ethnicity-adjusted regression analysis, a significant positive association was found between mean 222Rn level and thyroid cancer incidence (P = .024, and P = .048, respectively). When univariate and multiple regression analysis was conducted on the two GEPA 222Rn zone strata (Zone 1 and Zone 2/3), the relationship remained positive but not statistically significant. The positive correlation between mean 222Rn levels and thyroid cancer incidence suggested that more exposed individuals have increased susceptibility to cancer and possibly explained higher rate among Filipinos.

Citation Format: Candice S. Arceo, Grazyna Badowski, Gary R.W. Denton, Renata Bordallo. Indoor radon exposure and thyroid cancer incidence among Guam residents [abstract]. In: Proceedings of the Eleventh AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2018 Nov 2-5; New Orleans, LA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl):Abstract nr C075.