Optical Surface Monitoring Technology-Guided Large-Segment Radiotherapy for Breast Cancer

This study is testing an optical surface monitoring system — a camera-based technology — to help guide and verify deep-breath-hold radiation therapy (DIBH) for left-sided breast cancer, with the goal of protecting the heart from radiation exposure. **You may be eligible if:** - You have had surgery for left-sided breast cancer (breast-conserving surgery or mastectomy) - You are planned to receive radiation therapy to the whole breast or chest wall - You can hold your breath for more than 35 seconds and repeat it at least 6 times in a row - You have not received neoadjuvant chemotherapy or had breast reconstruction - You have no active heart disease **You may NOT be eligible if:** - You have bilateral (both-sided) breast cancer - Cancer has spread to distant parts of the body - You have had chest, abdominal, or pelvic radiation in the past - You have a prior or second primary cancer - You have serious heart problems (heart failure, recent heart attack, unstable arrhythmia) Talk to your doctor to see if this trial is right for you.