In many developed countries, the incidence of thyroid cancer, especially of small papillary carcinoma, increased without increase in thyroid cancer mortality during the recent three decades. Thus, how to treat small papillary carcinoma has become a very big clinical issue. Based on more than 1000 times difference between the reported incidences of latent thyroid cancer in autopsy studies and of thyroid cancer detected with screening studies and the prevalence of clinical thyroid cancer, I had a hypothesis that most papillary microcarcinoma (PMC) of the thyroid remain small and that doing surgery for all PMCs will result in more harm than good. In 1993 I proposed observation without immediate surgery for low-risk PMC at Kuma Hospital, which was approved and the trial started in the same year. The trial is now called active surveillance (AS).
　　At 10 years of AS only 8.0% of patients showed enlargement of ≥3 mm while only 3.8% showed nodal metastasis. None of the patients, including those who underwent rescue surgery after the detection of progression, showed life-threatening recurrence and none died of thyroid carcinoma. Adverse events were significantly more frequent in patients who underwent immediate surgery than in those who had AS, while medical costs were significantly higher. Therefore, AS ought to be the primary method of low-risk PTMC management.
　　On the kinetic analyses of tumor growth during AS, our novel doubling rate calculation showed only 3% of 169 tumors exhibited rather rapid growth, while 22%, 57%, and 17% exhibited slow growth, stable disease, and shrinkage, respectively. The estimated doubling rates before presentation were significantly higher than the doubling rates during AS, suggesting that there should have been a rapid growth period before presentation.