Accelerated Partial Breast Irradiation (APBI) Using Stereotactic Body Radiation Therapy (SBRT)

This study is testing accelerated partial breast irradiation (APBI) — a shorter, more targeted form of radiation that focuses only on the area around the lumpectomy site rather than the whole breast — using a technique called stereotactic body radiation therapy (SBRT) in women with early-stage breast cancer. **You may be eligible if...** - You are a woman aged 50 or older - You have early-stage breast cancer (low-to-intermediate grade DCIS or invasive ductal carcinoma) - Your tumor is less than 2 cm (or less than 2.5 cm for DCIS) - You have had a lumpectomy with clear margins and no lymph node involvement - Your cancer is hormone receptor-positive, HER2-negative, and BRCA-negative **You may NOT be eligible if...** - You have lobular breast cancer or cancer in multiple spots - Your cancer has spread to lymph nodes or distant organs - You received chemotherapy before surgery (neoadjuvant) - You have an ipsilateral (same-side) pacemaker or breast implant - You are pregnant or have severe heart, lung, or liver disease Talk to your doctor to see if this trial is right for you.