Bias in Evaluating Trends of Thyroid Cancer Incidence between Rural and Urban Populations

Abstract Background Diagnosis of thyroid cancer is based on fine needle aspiration cytology of thyroid nodules, based on nodules’ sonographic appearance and size. The American Thyroid Association published new guidelines for management of thyroid nodules in 2009 and 2015, where in some cases, only follow-up of thyroid nodules without any other procedure is recommended. This may lead to a decrease in diagnosed thyroid cancer cases. We studied thyroid cancer trends in Israel between the Arab population who are more likely to be living in rural areas, and the Jewish population who predominantly living in urban areas, and assesses whether these guidelines were reflected in the trends. Methods Data on new thyroid cancer cases were retrieved for Jews and Arabs separately from the Israel National Cancer Registry. Trends in age standardized incidence rates between 1996-2019 were evaluated for the two population groups using joinpoint regression model. Population group was included in the regression models to compare the trends between the two groups. We also studied trends in treatment of thyroid cancer at Rambam Health Care Campus. Results In both populations, age standardized thyroid cancer incidence rate increased between 1996 and 2013. However, between 2013-2019, Annual Percentage Change continuously increased in Arabs, but decreased in Jews. In the hospital study, we found that the Arab patients were 30% more likely to have surgery for small tumors. Conclusions The decrease in thyroid cancer incidence in the Jewish population compared with the Arab population may be related to differences in implementing the 2009 guidelines, which do not recommend fine needle aspiration cytology for very small tumors. We have found evidence that these guidelines were not followed to the same extent among Arab community clinics. This may explain the persisting increasing trend among Arabs, and could be one of the possible explanations for the differences in trends between the two groups. Key messages • Updated Guidelines regarding thyroid cancer diagnosis may be differently implemented by rural and urban populations. • Trends assessment may be biased due to differences in guidelines implementation.