The goal of this clinical trial is to evaluate the efficacy and safety of Viscum album extract when used in combination with pembrolizumab as adjuvant chemotherapy in adult patients (aged 19 and older) who have been diagnosed with malignant neoplasm of unilateral breast and have completed surgery.

The main questions it aims to answer are:

Does the addition of Viscum album improve clinical outcomes when combined with pembrolizumab as adjuvant chemotherapy? Is Viscum album safe and tolerable for use in this patient population?

The goal of this clinical trial is to evaluate the efficacy and safety of Viscum album extract when used in combination with pembrolizumab as adjuvant chemotherapy in adult patients (aged 19 and older) who have been diagnosed with malignant neoplasm of unilateral breast and have completed surgery.

This clinical trial aims to recruit 40 adult participants aged 19 years or older who have been diagnosed with malignant neoplasm of the unilateral breast and require adjuvant chemotherapy, including immune checkpoint inhibitors, after completing surgical tumor resection and radiotherapy.

Potential participants will receive a full explanation of the study's purpose and procedures and will voluntarily provide written informed consent. After screening for eligibility based on inclusion and exclusion criteria, only those who meet the criteria will be enrolled and proceed with the study.

Participants in the treatment group will receive Viscum album concurrently with pembrolizumab as part of their adjuvant chemotherapy regimen for 18 weeks in order to evaluate the efficacy and safety of Viscum album. Participants in the waitlist control group will undergo scheduled assessments without any additional intervention for the first 18 weeks, followed by a 4-week intervention phase starting from week 19.

Researchers will compare the treatment group receiving pembrolizumab plus 18 weeks of Viscum album to the waitlist control group, who will not receive any intervention for the first 18 weeks, to see if early administration of Viscum album leads to better outcomes compared to delayed intervention.

Participants will:

Sign informed consent after receiving full explanation of the study Be screened for eligibility based on inclusion and exclusion criteria If eligible, participants will be assigned to either the treatment or control group Receive subcutaneous injections of Viscum album three times per week for 18 weeks (treatment group only) Undergo scheduled clinical assessments throughout the study period

Inclusion Criteria:

1. Adults aged 19 years or older

2. Histologically confirmed diagnosis of triple-negative breast cancer

3. ECOG Performance Status of 0 or 1

4. Completed surgery and radiotherapy for localized disease, and either (a) requires adjuvant chemotherapy including immune checkpoint inhibitors, or (b) has initiated such therapy within 3 cycles

5. Laboratory values meeting the following criteria:

   Absolute Neutrophil Count (ANC) ≥ 1,500/μL Platelet count (PLT) ≥ 100,000/μL Hemoglobin (Hb) ≥ 9.0 g/dL Albumin ≥ 3.0 g/dL Acceptable levels of creatinine, total bilirubin, AST, and ALT ≤ 3 × upper limit of normal (ULN)

6. Able to understand the study information and voluntarily provide written informed consent

Exclusion Criteria:

1. Presence of distant metastasis
2. Uncontrolled pleural effusion, ascites, or pericardial effusion
3. Other non-cancer comorbidities (e.g., dementia, cerebrovascular disease, end-stage renal disease) that are expected to significantly impair physical function
4. Pregnant or breastfeeding women
5. Estimated life expectancy of less than 3 months, making adequate follow-up unlikely
6. Any other medical condition that may interfere with study results or any situation in which, in the investigator's judgment, participation is not appropriate

At this time, there is no plan to share individual participant data due to institutional policy and/or the sensitive nature of the clinical data collected. Future sharing may be considered on a case-by-case basis pending appropriate approvals.