The reasons why tumor is difficult to be treated are as follows: 1. tumor cells come from normal cells and are hidden in normal cells, which are more difficult to be identified; 2. it is not easy to be found early, it is impossible to predict the time of appearance of tumor cells and the time of metastasis or even recurrence, which is an important feature of tumor cells that are more difficult to be found; 3. tumor cells are changeable, just like a bad person will change some of his previous characteristics and biological behaviors, so it is not easy to find the coming and going time in time, and the changeability is an important reason why tumor is difficult to be treated.