• Age between 18 and 80 years, with an Eastern Cooperative Oncology Group (ECOG) performance status of 0-2.

• Primary diagnosis of breast cancer confirmed by preoperative pathological examination.

• Serum 25(OH)D levels less than 20 ng/ml (50 nmol/L).

• Patients who have not previously received chemotherapy and who plan to undergo at least 4 cycles of

• Patients with adjuvant chemotherapy or combined targeted therapy.

• Life expectancy of at least 6 months.

• No other uncontrolled benign diseases at the time of recruitment.

• All patients must have complete clinical medical records.

• Willingness to voluntarily sign an informed consent form.