Treatment strategy for differentiated thyroid cancer based on Japanese guidelines Differentiated thyroid cancer can be classified into papillary carcinoma and follicular carcinoma according to the pathological type. In the ATA guidelines, the treatment strategies for these two types are unified in the differentiated thyroid cancer treatment strategy, indicating that there is no significant difference in the treatment of the two types. However, in terms of biological behavior, they are very different: papillary carcinomas tend to metastasize to lymph nodes, while follicular carcinomas have a predominance of distant metastases. Moreover, papillary carcinoma can be diagnosed preoperatively in most cases by imaging or fine needle aspiration biopsy; follicular carcinoma is often diagnosed postoperatively only by pathological examination. Therefore, in the Japanese guidelines, the treatment of both is described separately. Treatment of papillary carcinoma: Can microscopic papillary carcinoma (papillary carcinoma less than 1 cm in diameter) be observed clinically first without immediate surgery? If lymph node metastasis, distant metastasis or extrathyroidal invasion are detected by palpation or imaging, papillary carcinoma must be treated surgically. Patients who do not have the above mentioned conditions can voluntarily choose clinical observation with adequate explanation and informed consent. With the availability of ultrasound and ultrasound-guided fine-needle aspiration techniques, the detection of microscopic cancers, including low-risk microscopic papillary carcinomas, has increased dramatically, and Davis observed a 2.4-fold increase in the incidence of papillary thyroid cancer from 1973 to 2002, but mortality remained stable over the same period. They reasoned that the increase in incidence reflected an increase in the detection of subclinical cases, such as microscopic and low-risk tumors. In Japan, incidental cases detected during population screening are reported to be 1000 times more common than clinically apparent tumors. Therefore, clinical observation of incidentally detected low-risk microscopic papillary carcinomas is already underway at relevant research institutions in Japan, and preliminary optimistic results have been obtained. Although studies need to include more cases and longer follow-up before definitive conclusions can be drawn, our current recommendation for clinical observation of low-risk microscopic papillary carcinoma is an optional option.