Incidence and survival of Swedish patients with differentiated thyroid cancer

Abstract

Key words: thyroid carcinoma; papillary; follicular; incidence; prognosis; excess mortalityAlthough thyroid cancers are rare in children and adolescents, they are still one of the most frequent cancers in these age groups. They constitute 1% of all malignancies worldwide 1 and are heterogeneous in terms of histology, clinical presentation, treatment response, and prognosis. Papillary (PTC) and follicular (FTC) thyroid carcinomas are the two entities referred to as differentiated non-medullary thyroid carcinomas of the follicular epithelium. Differentiated non-medullary thyroid cancer is more common among women, particularly during the fertile part of women's life.PTC is the most common of the 2 entities, comprising between 60 -90% of differentiated thyroid cancer. 2 The incidence of PTC for women rises to a peak at 55-65 years, whereas this pattern is less pronounced in men. 3 The incidence of FTC increases slowly with age and reaches its highest level around 60 years of age for both men and women.The only established risk factor for thyroid cancer in humans, besides age and gender, is ionizing radiation. 4 Studies of individuals living in the Chernobyl areas and of the survivors of the atomic bombs in Hiroshima and Nagasaki have shown an increased risk of particularly PTC, although only among those exposed as children. 4 Sex hormones, iodine deficiency and other factors have been proposed as risk factors for thyroid cancer but findings are inconsistent. 5

Long-Term Follow-Up After Complete Resection of Well-Differentiated Cancer Confined to the Thyroid Gland

“…This discrepancy could be the result of etiologic differences that correlate with geography, as reported 10-year and 20-year DSS from Sweden, Germany, and Austria, where goiter is endemic or prevented by iodide substitution, were similar or worse compared with this study. 3,9,38,39 Whereas tumor size (median 50 mm) was slightly higher, the age distribution, which was a major risk factor, was similar in our cohort (median age, 41.3 years in the PTC group) with those reported from countries without endemic goiter (United States, Asia). Studies from regions with endemic goiter, show that the proportion of FTC in patients with WDTC is higher in these regions.…”

“…The fact that people grow to older ages and that the incidence of WDTC peaks in the sixth decade adds further weight to the importance of correct initial treatment. 3 An important finding of this study is that no patients died of their disease more than 18 years after complete resection of the thyroid cancer. Although this is likely the result of the very good prognosis for young patients and the risk of death from cancer independent causes (competing causes) in the older population, it is noteworthy.…”

“…Analyzing the Kaplan-Meier survival curves of other long-term studies, it becomes evident that, indeed, death resulting from WDTC is unlikely more than 15 years after resection. 3,7,36,37,[39][40][41] This finding has direct implications for further studies evaluating risk factors and prognostic parameters. Follow-up periods of 10 to 15 years are sufficient for a clinically significant evaluation of WDTC.…”

“…Most WDTC display a relatively benign disease course with a reported 10-year disease-specific survival (DSS) of up to 96% for papillary thyroid carcinoma (PTC) and 90% for follicular thyroid carcinoma (FTC). 3,4 The low tumorigenicity of this cancer renders the determination of optimal treatment strategies through prospective clinical trials with meaningful follow-up periods nearly impossible. Therefore, an evaluation of factors that influence overall survival (OS) and DSS is dependent on retrospective studies based either on large study populations or on longterm follow-up.…”

Occupational exposure to ionizing radiation and electromagnetic fields in relation to the risk of thyroid cancer in Sweden

“…First, it was impossible to distinguish between the different histological types of thyroid cancer. It would have been interesting to ascertain what proportion of cases corresponded to papillary carcinoma; this tumor is of the type that has been most frequently associated with exposure to ionizing radiation, and its incidence appears to have risen notably in Sweden in the last 50 years, a phenomenon that has been attributed to the widespread use of radiotherapy for treating benign diseases of the neck, although other hypotheses cannot be ruled out (29).…”