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Background: The incidence of papillary thyroid cancer (PTC) has been increasing over the recent decades, especially that of small papillary thyroid cancers (sPTCs) (≤ 20mm in size). sPTCs are generally classified as low risk cancers with a very favourable diagnosis, yet some of these cancers still cause locoregional and distant metastasis, recurrence and even death.

Aims: To investigate the role of tumour stromal, environmental and demographical factors affecting the metastatic aggressiveness of sPTCs in Sweden.

Material & Methods: Selected tumour stromal proteins (Types I (Col1) and IV (Col4) collagens, alpha smooth muscle actin (a-SMA) and matrix metallopeptidase 9 (MMP-9)) were analysed for their role in metastatic disease (Paper I). Demographic and clinicopathological differences regarding recurrence between metastasized vs. non-metastasized sPTCs in Sweden were studied in 2 registry-based retrospective observational cohort studies (Papers II & III). The geographic distribution of patients with sPTC in Sweden was pinpointed and layered with maps of gamma radiation deposits of radionuclides Caesium-137 (Cs-137), Thorium-232 (Th-232), Uranium-238 (U-238) and Potassium-40 (K-40) using different spatial analysis methods (Paper IV).

Results: Col1 and Col4 were significantly more expressed in the non-metastatic tumours compared with metastatic ones. Patients with N1b disease were younger, had a smaller tumour size and higher recurrence rates compared to patients with N0 and N1a disease. The mean number of metastatic LNs at initial surgery was higher in the N1b group than the N1a group and correlated with more recurrent disease. The prevalence of metastatic sPTC was associated with significantly higher levels of gamma radiation from Th-232, U-238 and K-40.

Conclusions: The higher expression of Col1 and Col4 in the non-metastasized tumours indicates a potential protective role in tumour progression. LN stage N1b at diagnosis, and having five or more metastatic nodes, are strong risk factors for cancer recurrence and decreased disease-free survival in sPTC. Environmental factors such as gamma radiation are believed to play a major role in the pathogenesis of the PTC. These findings altogether underscore the importance of LN evaluation, tumour biological as well as environmental factors in sPTC patients, suggesting that the management of these patients should be based on an individual risk stratification instead of a “one size fits all” approach.