There is no recognized fastest deteriorating cancer, but certain cancers are significantly more malignant than others and progress very rapidly, including small cell lung cancer, pancreatic cancer, undifferentiated thyroid cancer and so on. 1. Small cell lung cancer: small cell lung cancer is a malignant tumor of neuroendocrine origin with extremely high degree of malignancy, characterized by rapid progression and early metastasis, and will be resistant to chemotherapy very soon despite its relative sensitivity to chemotherapy. Most patients die within 1 year of diagnosis. 2. Pancreatic cancer: pancreatic cancer is usually insidious, but the development is very rapid, the 5-year survival rate is less than 10%, which is recognized as one of the cancers with the worst prognosis. 3. Undifferentiated thyroid cancer: Undifferentiated thyroid cancer is mostly seen in the elderly, with strong aggressiveness and lymph node metastasis in early stage, so the prognosis of patients is extremely poor. In addition to the above cancers, liver cancer, bile duct cancer, gastric cancer, and seminomas are all highly malignant. The most crucial factor affecting the prognosis of cancer patients lies in early detection and early treatment, and usually the earlier the clinical stage, the better the prognosis. It is recommended that high-risk groups of cancer patients should undergo regular medical checkups to find out the tendency of cancerous changes as early as possible, and then intervene in a timely manner for treatment.