Invasiveness and Metastatic Aggressiveness in Small Differentiated Thyroid Cancers : Demography of Small Papillary Thyroid Carcinomas in the Swedish Population

Background: The western world is seeing a rising incidence of thyroid cancer. Improved diagnostic methods do not entirely explain this increase. Papillary thyroid carcinoma (PTC) is the most common subtype of thyroid cancer. Small PTC (<= 20 mm) and especially papillary thyroid microcarcinomas (PTMC <= 10 mm) are considered to be low-risk tumors but some cases are considerably more aggressive. Sufficient understanding of these mechanisms is a long-term goal for more efficient and safer treatment of these tumors. Methods: We identified 959 cases of small PTCs in the validated Scandinavian Quality Register for Thyroid, Parathyroid and Adrenal Surgery, grouped according to lymph node metastasis. These were analyzed according to age, gender, tumor size and geographic region. Results: Patients with N1b disease (lateral lymph nodes metastases) had a smaller tumor size compared to patients with N1a disease (8.6 mm vs 10.1 mm respectively, p < 0.05). Patients and specifically females with N1b disease were younger than those with N0 or N1a disease. Patients with N1b disease had a lower proportion of females (60%) compared to N0 and N1a groups (81% and 78%, respectively). The incidence of operated small PTCs and of lymph node engagement differs between geographic regions in Sweden. Conclusions: Small PTC and especially PTMC seem to show different patterns of aggressiveness and demography regarding lateral lymph node metastases and 7% had N1b disease and tumor <1 cm, underscoring the importance of lymph node evaluation in PTMC patients. More understanding of predictive factors, mechanisms for metastatic disease and causes of regional differences, is needed