As a rectal cancer patient, a simple understanding of the situation related to preoperative and postoperative chemotherapy will help you to have a bottom in your heart and more confidence to adhere to the treatment. For the differences of preoperative and postoperative chemotherapy, there are mainly the following points. The purpose of chemotherapy is different: the main purpose of preoperative chemotherapy is to shrink tumor lesions, reduce tumor stage, create conditions for surgery and improve patients’ anal preservation rate; while the purpose of postoperative chemotherapy is to kill tumor cells (micrometastases) that cannot be removed by surgery and reduce the chance of recurrence and metastasis after surgery. The cycles of chemotherapy are not consistent: the cycles of preoperative chemotherapy will generally be shorter than the cycles of postoperative chemotherapy. Pre-operative chemotherapy generally does not exceed 3 months, because during chemotherapy, on the one hand, the patient’s liver and kidney function and physical status are damaged, and too long chemotherapy may increase the complications of surgery; on the other hand, the patient’s tumor may still be progressing, and once the disease progresses to the point of inoperability to operate, it will be more than worth the loss. Pre-operative and post-operative chemotherapy for rectal cancer are interrelated. The duration of conventional adjuvant chemotherapy for rectal cancer is 6 months, and the number of chemotherapy sessions varies depending on the patient’s treatment plan. Patients with a 2-week regimen of chemotherapy will require 12 sessions of chemotherapy; patients with a 3-week regimen will require 8 sessions of chemotherapy; this 6-month period of chemotherapy includes both preoperative and postoperative chemotherapy. If a patient has had chemotherapy for 3 months prior to surgery, only 3 more months of chemotherapy are needed after surgery. Patients who have not had chemotherapy prior to surgery will need to have the full 6 months of chemotherapy after surgery. The dose of chemotherapy drugs is basically the same: preoperative chemotherapy and postoperative chemotherapy for rectal cancer use basically the same drugs and the same dose (there may be dose adjustment), but preoperative chemotherapy will be more stringent. Pre-operative chemotherapy has a guiding effect on the dosing of post-operative chemotherapy. This is because the effect of preoperative chemotherapy can be used to determine the patient’s sensitivity and tolerance to the chemotherapy drug. If the drug is effective for the patient and the side effects are acceptable, this chemotherapy drug can be continued after surgery. However, since preoperative chemotherapy is to make the tumor lesion reach the requirement of surgery as soon as possible, the standard of chemotherapy will be more stringent and the dose should be more accurate. After the surgery, the tumor lesions are removed cleanly and the chemotherapy drugs may be adjusted or even simply taking oral drugs will be sufficient. Therefore, the dose of post-operative chemotherapy will be less than the pre-operative one in some cases. Patients should not worry about the different doses of pre-operative and post-operative chemotherapy, it does not mean that there is any problem with the disease treatment. The necessity of pre-operative and post-operative chemotherapy: In recent years, pre-operative chemotherapy has been more and more widely used because it can bring more benefits to rectal cancer patients, so its importance has been recognized. In contrast, post-operative chemotherapy seems to be “less important”. The key to rectal cancer treatment is surgery, which is a big step towards successful treatment if the tumor is removed cleanly and no tumor cells remain. If the risk of metastasis recurrence is low or the patient cannot tolerate chemotherapy after surgery, it is not necessary to do postoperative chemotherapy.