The purpose of this study is to determine if partial breast proton therapy is effective treatment for early stage breast cancer following lumpectomy.

Radiation therapy is considered standard treatment for most women with early stage breast cancer following lumpectomy. Post-lumpectomy radiotherapy is a proven treatment that reduces cancer recurrence in the breast and improves survival. When standard whole breast radiation techniques are utilized, portions of the chest wall, lung and heart may also receive significant doses of radiation which can lead to radiation induced complications. Radiation techniques that limit the treatment area to the portion of the breast where the cancer arose can minimize and even eliminate radiation dose to the chest wall, heart and lung. This is called partial breast radiotherapy. This study is designed to evaluate the use of proton beam radiotherapy to deliver partial breast radiotherapy in women with early stage breast cancer.

Inclusion Criteria:

• Invasive ductal, medullary, papillary, colloid or tubular histologies
• Stages T1 or T2 (tumors < or = 3 cm) from lumpectomy specimen.
• No more then 3 positive nodes on axillary dissection or negative sentinel node.
• Negative surgical margins (>2mm)

Exclusion Criteria:

• Invasive lobular histology
• Previous chemotherapy for breast cancer
• Extensive intraductal component
• Collagen vascular disease
• Prior malignancy unless disease-free for 5 years
• Patients who are pregnant or lactating

Females with invasive nonlobular carcinoma greater or less than 3 cm

Change between baseline pre-treatment and 5 years post-treatment completion.