As more and more tumors are starting concurrent radiotherapy, the toxic side effects of concurrent radiotherapy are drawing our attention more and more while the efficacy is improving, especially bone marrow suppression. Clinically, it is often observed that patients’ leukocytes are still low at the time of chemotherapy expiration due to the myelotoxicity of radiotherapy, given that granulocyte colony-stimulating factor (GCSF) can rise up. However, the question that arises is: Can chemotherapy be administered immediately? Some people say that we should rest for 48 hours to review and if it is normal, chemotherapy can be administered. However, after 48 hours, many patients’ white blood cells have dropped again, so they need to raise the white blood cells again, and chemotherapy is postponed again and again. This article introduces the timing of GCSF use, dosing schedule and precautions, and hopefully will give clinicians some advice. Prophylactic use of GCSF ASCO guidelines: Prophylactic use of GCSF in non-primary patients should only be used in patients who develop granulocyte deficiency comorbidities (e.g., fever) after the previous course (no prophylactic use of GCFS) and whose dose reduction may affect the efficacy of the treatment. NCCN guidelines: Patients at >20% risk of developing granulocyte deficiency are at high risk and require prophylaxis with GCFS. 10%-20% risk is considered at intermediate risk. See NCCN guidelines for specific evaluation criteria. The recurrence rate is 50-60% after the next chemotherapy for patients who have had granulocyte deficiency comorbidities. The use of GCFS prophylaxis may reduce the risk by 50%. In summary, GCFS is not prophylactic in patients with solid tumors in general, without previous granulocyte comorbidities and without risk factors. Timing of GCFS for treatment It is well documented that GCFS is best started 24-48 hours after chemotherapy. Discontinue before the next chemotherapy and try not to use it on the day of chemotherapy. The increase in leukocytes after GCFS is bimodal: the first peak is 2-3 days after administration, then drops to the lowest level 5-6 days later, then increases again and reaches the second peak 8-9 days later. The first peak is the result of GCFS promoting the peripheral release of mature granulocytes already present in the bone marrow blood pool. The second peak is the result of GCFS stimulating the proliferation and differentiation of bone marrow granulopoietic progenitor cells to maturity and release into the peripheral blood. The duration of drug administration should not be less than 3 days. If the duration is too short, only the first peak can be induced, and the leukocytes can easily drop to the lowest point and become infected and febrile when the drug is stopped. Toxic side effects Local pain, weakness, fever, muscle aches and pains. Patient education and explanation should be done in advance with patients. Why can’t I use it on the day of chemotherapy? The neutrophils produced by stimulation will be destroyed by chemotherapeutic drugs, which will aggravate their damage to bone marrow reserve function and increase the risk of moderate bone marrow suppression.