Chemotherapy cycles are generally not recommended to be extended, but some special cases, such as patients with heavy myelosuppressive reactions such as white blood cell decline, or heavy reactions of nausea and vomiting, which cannot tolerate chemotherapy, can only be extended, but not for too long, generally in about a week. If the time is too long, the effect will be discounted and the effect of chemotherapy will be weakened. Chemotherapy is a kind of systemic treatment, which can kill most of the cancer cells in the body by chemical treatment to reach the whole body. Of course, one chemotherapy treatment may only kill 70%-80% of cancer cells, and the remaining cancer cells will develop again within a certain period of time, so a second chemotherapy treatment is needed at a certain interval. The common ones are three-week regimen and four-week regimen, that is, 21 days between the second chemotherapy and 28 days between the second chemotherapy.