Patients who experienced neutropenia after receiving pegfilgrastim on day 1 of the first round are randomly assigned to either the control or experimental arm. Patients in the control arm continue pegfilgrastim injection on day 1 while patients in the experimental arm will receive pegfilgrastim injection on day 3 to see if changing the pegfilgrastim administration date from day 1 to day 3 starting from the second round of chemotherapy could reduce the incidence of neutropenia

The use of pegfilgrastim, which has a long-lasting effect in the human body, after myelosuppressive chemotherapy for solid tumors, including breast cancer, and blood cancers has led to a decrease in the incidence of neutropenia, including febrile neutropenia, and a reduction in medical costs by shortening hospitalization periods related to side effects of chemotherapy. During the early stages of drug development, there was controversy over the timing of pegfilgrastim administration, and attempts were made to administer it at various times, from the day of chemotherapy to the day of neutropenia began. However, based on several studies, it is now known that patients who receive pegfilgrastim on the day of chemotherapy or after 4 days of chemotherapy have a higher incidence of febrile neutropenia. Therefore, administering pegfilgrastim on day 1-3 after chemotherapy is ideal, but in reality, it is often difficult for patients to visit the hospital multiple times after chemotherapy, so many patients are discharged after receiving pegfilgrastim on day 1. The FDA and National Comprehensive Cancer Network (NCCN) guidelines also recommend administration on day 1. It is known that patients who experience neutropenia after the first round of chemotherapy are more likely to experience it again in subsequent rounds. The investigators aimed to see whether changing the pegfilgrastim administration date from day 1 to day 3 starting from the second round of chemotherapy for patients who experienced neutropenia after receiving pegfilgrastim on day 1 of the first round can reduce the incidence of neutropenia.

Inclusion Criteria:

• Women aged 18 to 75 years old as of the date of study registration.
• Patients with histologically confirmed invasive adenocarcinoma.
• Patients with confirmed estrogen receptor, progesterone receptor, and Her2 receptor status.
• Patients with Eastern Cooperative Oncology Group (ECOG) performance status 0-1.
• Patients with a left ventricular ejection fraction (LVEF) ≥55%.
• Patients who have agreed to participate in this trial and have provided written consent.

Exclusion Criteria:

• Patients with a history of breast cancer treatment
• Patients with a history of chemotherapy, radiation therapy, immunotherapy, or biotherapy for malignancies other than breast cancer
• Patients with infectious diseases
• Patients with serious illnesses that may affect this clinical trial: cardiovascular disease, kidney disease, liver disease, endocrine disease, tumors, or diabetes
• Other individuals deemed by the clinical trial investigators to be unable to participate in the trial.