The higher the differentiation of the tumor, the lower the degree of malignancy. The degree of differentiation of the tumor is divided into four types: undifferentiated, low-differentiated, moderately-differentiated and highly-differentiated, with undifferentiated tumors having the highest degree of malignancy and highly-differentiated tumors having the lowest degree of malignancy. The lower the degree of differentiation, the greater the difference with normal human cells, the faster the growth of this type of tumor cells, the more likely to have distant metastasis. Especially for undifferentiated and poorly differentiated tumors, even if they are detected at an early stage clinically, it cannot be ruled out that distant metastasis has already occurred, only that it cannot be detected with the current ultrasound or CT examination methods.