Incidental detection, imaging modalities and temporal trends of differentiated thyroid cancer in Ontario: a population-based retrospective cohort study

Abstract

T here is debate and concern about a worldwide epidemic of thyroid cancer overdiagnosis. 1,2 Several factors suggest that a large reservoir of indolent thyroid cancers is being detected by sensitive imaging techniques. Incidence rates of thyroid cancer are increasing worldwide in every continent except Africa, whereas mortality rates from thyroid cancer are low and relatively stable. [3][4][5] Large increases in the registrations of differentiated thyroid cancers (≥ 90% of thyroid cancer cases 6 ) and subclinical microcarcinomas (≤ 1.0 cm in diameter; 50% of thyroid cancers in America 7 ) have paralleled increases in the availability and use of sensitive imaging techniques, concomitant with increased incidence rates. [8][9][10] Further, autopsy studies show a large potential reservoir of occult thyroid cancers awaiting diagnostic scrutiny. 11 Nonetheless, increases in thyroid cancer may also be related to aging populations, 12,13 use of ionizing radiation (a risk factor for thyroid cancer) to treat benign conditions between the 1930s and 1960s 14 and changes in environmental or lifestyle factors. 15,16 Increases in larger, clinically detectable thyroid cancers 17,18 and recent increases in thyroid cancer mortality rates 15,19 support such hypotheses. Such factors that counter the prevailing overdiagnosis hypothesis exist in Canada. 13,18,20 Diagnostic imaging may lead to thyroid cancer diagnosis during investigations for conditions unrelated to the thyroid

Mortality among papillary thyroid cancer patients by detection route: a hospital-based retrospective cohort study

“…In our study, the proportion of incidentally detected thyroid cancer increased from 36% in 2011 to 82.3% in 2018. This phenomenon has also been reported in several other countries, such as Japan, Canada, and the USA ( 8 , 9 , 17 ), which suggests that increased imaging and diagnostic scrutiny may have contributed significantly to the upsurge of thyroid cancer incidence observed ( 18 ).…”

“…Overall, our finding on the proportion of non-incidentally detected PTC is consistent with a multicenter study involving four countries (the USA, Canada, Denmark, and South Africa), which showed that 30% of the thyroid cancer cases were identified through symptoms ( 15 ). But the large variation across centers – for example, incidentally detected cases accounted for only 18% male and 11% female cases during 2013–2017 in Ontario, Canada ( 8 ) but as high as 62% of all cases during 2013–2016 in Queensland, Australia ( 16 ) – highlights the impact of local health-care system and medical practice. With the widespread use of diagnostic imaging and fine-needle aspiration biopsy over the past years, a growing number of thyroid cancers had been discovered incidentally.…”

“…Incidentally detected cancers, i.e. the detection of cancers on imaging for reasons unrelated to the thyroid ( 6 , 7 ), have become a large and increasing component of the observed PTC incidence ( 8 , 9 ). A few studies have shown that incidentally detected thyroid cancers are more frequently low risk and have better progression-free survival ( 6 ), but evidence is still lacking on whether the clinical strategies of PTC should also consider the route of detection.…”