(a) Papillary carcinoma is the most common type of thyroid cancer, accounting for about 70% of cases. It varies in size. It is generally well differentiated and has low malignancy. The cancer tissue is brittle, soft and fragile with dark red color; however, papillary carcinoma in elderly patients is generally harder and paler. The center of papillary carcinoma often has cystic changes and the capsule is filled with bloody fluid. Sometimes the cancer tissue can be calcified and the cut surface is sandy. The above cystic changes and calcification are not related to the malignancy of the carcinoma and its prognosis. Microscopically, the carcinoma is seen to consist of columnar epithelial papillae, which can sometimes be mixed with follicle-like structures, and even papillary to follicle-like metaplasia is found. The lobe of papillary adenocarcinoma has an intact envelope, but it can also break through the envelope and invade the surrounding tissues in the later stages. The main route of dissemination is through the lymphatic tract, and the most common route is through the cervical lymph nodes, which can be found in about 80% of children and 2% of adult patients, followed by blood metastasis to the lung or bone. （Follicular adenocarcinoma is less common than papillary adenocarcinoma, accounting for about 20% of thyroid adenocarcinoma and ranking second. The cancer is soft, elastic, or rubbery, round, oval, or lobulated nodule-shaped. The cut surface is reddish-brown, and foci of fibrosis, calcification, hemorrhage, and necrosis are visible. Well-differentiated follicular adenocarcinoma is microscopically similar to normal thyroid, but with invasion of the envelope, blood vessels and lymphatic vessels; poorly differentiated follicular adenocarcinoma has irregular structures with dense clusters or cords of cells, rarely forming follicles. Although lymphatic metastasis is possible, the main route of dissemination is through the bloodstream to the lungs, bones and liver. Some follicular adenocarcinomas may recur long after surgical resection, but their prognosis is not as good as that of papillary adenocarcinoma. （Undifferentiated thyroid carcinoma accounts for 5% of thyroid carcinoma and mainly occurs in middle-aged or older patients, mostly in men. The mass is hard and irregular, fixed, and grows rapidly, and soon involves the thyroid gland diffusely, usually infiltrating the trachea, muscles, nerves and blood vessels within a short time, causing difficulty in swallowing and breathing. The tumor may be localized with tenderness. Microscopically, the cancer tissue is mainly composed of poorly differentiated epithelial cells with pleomorphic cells and common nuclear division phase. Lymph node enlargement may be seen in the neck, and there are also lung metastases. The disease has a poor prognosis and is ineffective for radioactive iodine treatment, and external irradiation only controls local symptoms.