Carcinoma in situ is a special disease, primary cancer and invasive cancer may be known and heard of by many people in our life, but for carcinoma in situ, it is not so common, but it is certain that if patients do not get treatment in time, it can endanger their own life and health, so patients should pay attention to it. What are the differences between diseases, let me introduce next. Carcinoma in situ refers to carcinoma that is only found in the epithelial layer of mucous membrane or in the superficial layer of skin, often affecting the whole epithelial layer and not yet infiltrating into the submucous layer or dermis. There are also intrafollicular carcinomas in the breast that do not infiltrate outside the lobules, which are also called intrafollicular carcinomas in situ. If the cancer tissue develops secondary to this, it may become an invasive carcinoma. It is generally believed that if in situ cancer can be treated actively, it is possible to be cured. Primary carcinoma Primary carcinoma is the gradual transformation of normal cells in normal tissues and organs into cancer cells under the long-term effect of various internal and external carcinogenic factors, and then into cancer cell masses, i.e. “primary carcinoma”. Generally speaking, “primary” refers to the place of origin of cancer. Primary cancer can occur in all organs and tissues of human body except finger (toe) nail and hair, and if cancer cells are transferred from the primary site to other parts of the body through blood and lymphatic vessels, it is called metastatic cancer. For example, if lung metastasis occurs in liver cancer, it is medically diagnosed as: primary liver cancer and metastatic lung cancer. It should be said that primary cancer includes all stages of cancer development, i.e.: in situ cancer → infiltrating cancer → spreading and metastasis to the whole body. In most cases, primary cancer belongs to early stage cancer, which can be recovered with early detection, early diagnosis and early treatment and systematic treatment. Infiltrating cancer will develop into infiltrating cancer after infiltrating cancer cells break through the basement membrane, and infiltrating cancer is eligible for major disease insurance regardless of whether metastasis occurs. The occurrence of cancer is a gradual process. Since in situ cancer has only a small number of cells with limited cancerous changes, which is difficult to be detected by conventional examination means, most of the cancers found in clinical practice are infiltrating cancers, and pathological reports of in situ cancer are rare. Metastatic cancer Metastatic cancer mainly refers to the metastasis of cancer cells from other tissues or organs. Metastatic cancer is cancer cells invading into blood vessels, lymphatic vessels or body cavity from the primary site, moving with blood or body fluid, and forming the same type of cancer as the primary cancer in distant sites or organs. Metastatic cancer must meet two conditions, one is that the site of occurrence must be the distant part of the primary cancer; the other is that the nature of cancer must be the same as the primary cancer. Only malignant tumors can metastasize, and metastasis prompts the spread of malignant tumors, causing greater and more widespread harm to the organism, as well as making cancer treatment very difficult. Extensive metastasis of cancer is often the main reason why advanced cancer cannot be cured surgically. Since cancer is prone to metastasis, sometimes metastatic cancer is found first and the primary cancer is identified later.