The goal of this observational study is to learn how the lymphatic system changes before and after radiotherapy in female patients with breast cancer. The study aims to understand early lymphatic changes that may lead to breast-cancer-related lymphedema (BCRL) and to help identify patients who might benefit from early preventive rehabilitation.

The main questions it aims to answer are:

How does lymphatic flow and vessel function change after radiotherapy, as measured by indocyanine green (ICG) lymphography?

Can early imaging changes on ICG lymphography predict later swelling or lymphedema symptoms in the arm?

There is no comparison or treatment group, since all participants will receive radiotherapy as part of their standard breast cancer care.

Participants will:

Undergo ICG lymphography before radiotherapy and again within 4-6 weeks after completing radiotherapy

Receive a small injection of ICG dye under the skin and have near-infrared imaging to visualize lymphatic flow

Complete follow-up assessments (for some participants) at 3, 6, or 12 months to monitor long-term lymphatic changes

Continue their usual standard medical and rehabilitation care throughout the study

This study will include 40 female participants, aged 18 years or older, who have been diagnosed with stage I-III breast cancer and are scheduled for postoperative radiotherapy. Participants with a history of lymphedema, prior radiotherapy, severe organ disease, or known ICG/iodine allergy will not be included.

The information collected will include imaging findings from ICG lymphography, arm circumference and volume measurements, and patient-reported outcomes on arm discomfort or swelling. These data will help researchers identify early imaging biomarkers of radiation-induced lymphatic dysfunction.

The study does not involve any experimental treatment or random assignment. All procedures are routine diagnostic or rehabilitation methods that are already approved for clinical use. The ICG test uses a very small amount of dye and is considered safe, with allergic reactions being rare.

The findings of this study may provide new insights into how radiotherapy affects the lymphatic system and may contribute to developing personalized strategies to prevent or minimize lymphedema after breast cancer treatment.

Inclusion Criteria:

• Female, ≥18 years old Pathologically confirmed stage I-III breast cancer Scheduled to receive postoperative radiotherapy (RT)

Exclusion Criteria:

• Previous or current lymphedema Prior radiotherapy to the ipsilateral breast or axilla History of neoadjuvant chemotherapy Severe systemic diseases (heart, renal, hepatic failure) Active infection Pregnancy or lactation Allergy to indocyanine green (ICG) or iodine-based contrast agents

Female breast cancer survivors (stage I-III) scheduled for adjuvant radiotherapy after surgery. Participants are recruited consecutively from the Department of Rehabilitation Medicine, Radiation Oncology and General Surgery at Hallym University Dongtan Sacred Heart Hospital. The study population represents patients at risk for radiation-induced lymphatic dysfunction or breast-cancer-related lymphedema.