Phase II Protocol of Proton Therapy for Partial Breast Irradiation in Early Stage Breast Cancer

This study tests whether proton beam radiation — a very precise type of radiation therapy — can safely treat only the part of the breast where cancer was removed (called partial breast irradiation), reducing side effects compared to treating the whole breast. **You may be eligible if...** - You are a woman aged 50 or older with early-stage breast cancer (Stage 0, I, or II) - You have had a lumpectomy (surgery to remove the tumor but not the whole breast) with clean margins - Your cancer is invasive ductal carcinoma or ductal carcinoma in situ (DCIS); it must be estrogen receptor positive if invasive - Your lymph nodes tested negative for cancer spread - Your tumor was 3 cm or smaller **You may NOT be eligible if...** - You have had previous breast cancer treatment on the same breast - Your cancer has spread to lymph nodes - You have the BRCA1 or BRCA2 genetic mutation - You have lobular (not ductal) breast cancer - You have HIV, lupus, scleroderma, or other collagen vascular diseases - You are pregnant or breastfeeding - Your tumor showed signs of spreading into blood or lymph vessels (lymphovascular invasion) Talk to your doctor to see if this trial is right for you.