Megestrol Acetate for Fatigue Management in T-DXd Treated Breast Cancer

Inclusion Criteria:

1. Female, aged 18-75 years
2. Pathologically confirmed inoperable or metastatic breast cancer with complete ER, PR, and HER2 status
3. HER2-positive (IHC 3+ or IHC 2+ with FISH positive), HER2-low (IHC 1+ or IHC 2+ with FISH negative), or HER2-ultralow (IHC 0 with ≤10% weak incomplete membrane staining) \[most recent specimen used\]
4. Investigator-assessed indication for Trastuzumab Deruxtecan (T-DXd) therapy
5. No prior treatment with Trastuzumab Deruxtecan
6. Received ≤5 lines (including 5 lines) of prior chemotherapy
7. ECOG Performance Status 0-1
8. Estimated life expectancy ≥12 weeks
9. Adequate organ function
10. Reliable contraception or negative serum/urine pregnancy test within 7 days prior to enrollment; willing to use appropriate contraception during study and for 8 weeks after last dose
11. Voluntary participation with good compliance

Exclusion Criteria:

1. Severe underlying disease, comorbidities, active infection, or severe metabolic disorders
2. Clinically significant severe fatigue at baseline (FACIT-F score \<30)
3. Currently receiving other antitumor therapies
4. Pregnant or lactating patients
5. Poor compliance or unable to complete normal follow-up
6. History of allergy to megestrol acetate or other components of the formulation
7. History of thromboembolism (use with caution)
8. Other malignancies diagnosed within 5 years, except: resected non-melanoma skin cancer, adequately treated cervical carcinoma in situ, locally radically treated prostate cancer, surgically radically treated ductal carcinoma in situ, or malignancies diagnosed \>2 years ago with no current disease evidence and untreated for ≤2 years before randomization
9. Any condition judged by investigator that may affect study conduct or outcome assessment