We cannot ignore the real component of the rise in thyroid cancer incidence

Abstract

Recently, Bernier et al published a study in Cancer that demonstrated a distinct rise in the incidence of thyroid cancer in the pediatric population. This study reported an increase of 4.43% yearly for the 16 years studied. The authors conceded that overdiagnosis may play a role, yet stated that it is unlikely to entirely explain this increase. They asserted this finding based on the fact that they found a rise in larger and later stage thyroid cancers as well as smaller and earlier stage thyroid cancers. They further called for an investigation into the real rise in thyroid cancer in this population. It is interesting to note that this is not the first group to point out the rise in thyroid cancer occurring in the pediatric population. In a counterpoint editorial by Chen et al, the authors discounted the idea that a true increase in thyroid cancer incidence is substantial. Overdiagnosis and overtreatment occur for multiple reasons: technological developments, commercial interests, looming medicolegal fears, and perhaps even deliberate profiteering. However, patient demands also contribute to this phenomenon. In one study, approximately 87% of adults were reported to believe that routine cancer screening is a good idea and 74% believed that finding cancer early saves lives. Although many authors are quick to explain the rise in thyroid cancer as an artifact of the overdiagnosis of clinically insignificant thyroid cancers, multiple groups all over the world have shown that this is not sufficient to explain the rise in thyroid cancer. Chen et al cited the existence of a disease reservoir of detectable cancer that occurs in the pediatric population. They based this on a growing number of studies of the population exposed during the Fukushima Daiichi nuclear power plant accident. However, others have proposed that these so-called subclinical cancers may be in a growth arrest pattern and recommend careful long-term monitoring considering the long life expectancy of the individuals. Chen et al further stated that it is likely that the increase in the use of computed tomography scans in children would invariably demonstrate a rise in thyroid nodules. However, studies have indicated that the majority of incedentalomas are detected by ultrasound and not by neck computed tomography. Increased scrutiny does not fully explain the rise in thyroid cancer, and we need to ask ourselves whether changes in environmental factors or lifestyle changes are driving part of this increase. Although these factors are not clear at this time, there are several potential possibilities that have been put forward. The prevalence of obesity has risen substantially in many countries in recent years, and has consistently been associated in epidemiological studies with an increased risk of thyroid cancer. Smoking is associated with a reduced risk of thyroid cancer, and thus a declining smoking prevalence also may be a contributing factor toward thyroid cancer incidence. Exogenous factors such as iodine deficiency and improved iodine nutrition also have been suggested as potential risk factors for thyroid cancer and may have contributed to the increase. Reproductive issues have long been suspected because thyroid cancer is overwhelmingly a women’s disease. However, to my knowledge, few studies to date have conclusively shown a strong correlation. In a recent study using the Harvard Nurses’ Health Study II cohort, we demonstrated that a later age at the time of first birth, a later age at menopause, and a higher number of reproductive years were significantly associated with an increased risk of thyroid cancer. Conversely, we also found that a longer duration of oral contraceptive use was associated with a