WHO (World Health Organization) defines papillary thyroid microcarcinoma (PTMC) as papillary thyroid carcinoma with a maximum lesion diameter of no more than 10 mm without local lymph node and/or distant organ metastases and extrathyroidal invasion. The actual prevalence of PTMC is not known, as the vast majority of PTMCs are lifelong asymptomatic. The vast majority of PTMC seen clinically are incidental findings on examination. According to multiple autopsy studies, PTMC is a fairly common incidental finding, with a maximum detection rate of 36% in a Finnish study and multiple studies in Japan finding an autopsy detection rate of PTMC in the Japanese population of around 10%. Meta-analyses have found that the high rate of PTMC detection at autopsy correlates with the degree of care taken in pathological sectioning and the degree of care taken by the examiner, so the actual prevalence may be higher. In contrast, detailed pathological examination of a group of thyroid tissues removed for non-thyroidal disorders revealed that PTMC was detected in about 20% of the thyroid tissues. The prevalence of thyroid cancer in routine autopsies is 100-1000 times higher than that of clinically detected thyroid cancer. In contrast to the clinical type of thyroid cancer, microscopic thyroid cancers are very common, independent of iodine intake and gender, and are almost always papillary. Given this high incidence, the treatment of PTMC has been controversial and surgical intervention in every case of PTMC is clearly an unachievable task. It is certain that the vast majority of PTMCs remain “silent” for long periods of time – that is, even without intervention, these cancers may not have an impact on the patient’s life expectancy. The greatest impact of PTMC is not the lesion itself, but rather the psychological impact of the fear of the term “cancer”. Therefore, at the Porto Thyroid Cancer Conference in Spain, it was advocated that the term “carcinoma” be avoided and replaced by “micro papillary tumor” for micro papillary carcinoma occurring in adults. Lin et al. found that the type of surgery (total, subtotal or unilateral resection) and the use of radiometabolic therapy did not affect the prognosis of PTMC after a 10-15 year follow-up study, and concluded that limited treatment was sufficient for PTMC without metastasis and that resection of one lobe alone was a wise choice. A summary of 40,000 cases by Bilimoria et al. also supports these conclusions. The prognosis of PTMC is extremely good. It can be said to be indistinguishable from the general population. None of the PTMC detected at autopsy died of thyroid cancer-related disease. Papillary thyroid carcinoma is highly susceptible to lymph node metastasis, and about 1/4 of clinical cases with local lymph node metastasis may even be the first clinical manifestation of the disease. Grebe SK et al. showed that the prognosis of papillary carcinoma with lymph node metastasis is not worse than that of those without lymph node metastasis. The incidence of thyroid cancer has increased significantly in all countries of the world in the last two decades when ultrasound diagnosis and FNA techniques have been widely used for the diagnosis of thyroid nodules. The vast majority of scholars believe that this increasing trend is mainly due to advances in screening techniques. Almost all of these elevated thyroid cancers are papillary or follicular papillary cancers with excellent prognosis, while the incidence of other types of thyroid cancer with poorer prognosis has remained virtually unchanged. Despite the large number of early-stage carcinomas detected, the mortality rate of nail cancer in the population has remained stable, and the idea that these screening techniques (mainly high-resolution ultrasonography) provide the best timing for treatment of nail cancer is nothing more than self-defeating. A 5-year follow-up of a group of untreated micro papillary thyroid carcinomas found that only 6.7% of patients had an increase in lesion size, and there were no cases of distant metastases or deaths from thyroid cancer. The authors concluded that it is not too late to wait until the lesion progresses to the point of clinical symptoms before surgical treatment for micro papillary carcinoma of less than 10 mm. Pathologically, PTMC does not differ from clinical papillary carcinoma. More research is now focused on the molecular level in an attempt to uncover the molecular mechanisms by which PTMC develops into clinical papillary carcinoma. For example, CyclinD1 is one of the most important molecular markers, which is overexpressed in 93.3% of papillary carcinomas with clinical manifestations and in only 12.5% of PTMCs. Histopathology 2005;47:248-56. Perhaps one day we will be able to determine which thyroid cancer requires aggressive intervention and which one does not. (Clinical thyroid cancer is defined as thyroid cancer with clinically palpable nodules, with lymph node metastases and/or distant metastases, and extra-thyroidal invasion)