Carcinoma in situ is a type of malignant tumor and belongs to the earlier stage of cancer. Generally, in situ cancer and malignant tumor can be distinguished by their local pathology, clinical manifestations and disease prognosis, etc. 1. Local pathology: in situ cancer generally only manifests in the tissue or organ where it first develops, has not yet broken through the skin or submucosal basement membrane, is confined to the lesion site and has not invaded the surrounding tissues. Malignant tumor can show invasion of surrounding tissues and organs, and can also show metastasis of distant tissues and organs; 2. Clinical manifestations: Carcinoma in situ can show lumps and nodules at the lesion site, with slower development and less obvious pain. Malignant tumor grows faster, and when metastasis occurs, it can show symptoms of metastatic sites, such as local pain, bleeding, ulcers, etc., which can lead to death in serious cases; 3. Prognosis of disease: Generally speaking, after surgical excision, in situ cancer has a better prognosis, and the possibility of recurrence and malignant transformation is low. Malignant tumors, due to their severity, can generally only relieve patients’ symptoms through surgery, radiotherapy and chemotherapy, and it is difficult to control the progress of the disease. Therefore, once diagnosed with carcinoma in situ, patients should seek timely medical consultation and adopt surgical resection and other methods to treat the disease as soon as possible to avoid deterioration and development into malignant tumor. Patients after surgery should maintain healthy living habits and regular review to prevent recurrence.