This pilot clinical trial studies intensity-modulated accelerated partial breast irradiation (APBI) before surgery in treating older patients with estrogen receptor positive or progesterone receptor positive stage I breast cancer. APBI is a specialized type of radiation therapy that delivers a high dose of radiation directly to the tumor may kill more tumor cells and cause less damage to normal tissue. Giving radiation therapy before surgery may make the tumor smaller and reduce the amount of normal tissue that needs to be removed.

PRIMARY OBJECTIVES:

Establish the feasibility for preoperative APBI delivered with IMRT in the prone position using daily CT guidance for Stage I breast cancer patients who are intended to undergo a lumpectomy for breast conserving therapy.

SECONDARY OBJECTIVES:

1. To determine the acute and late surgical and radiation toxicity of preoperative APBI delivered with IMRT in the prone position.
2. To document the cosmetic appearance of the breast when treated with preoperative APBI IMRT in the prone position.
3. Establish the percentage of patients that can meet dosimetric goals and normal tissue constraints if treated with preoperative APBI IMRT in the prone position.
4. Evaluate molecular changes in breast cancer and/or stroma before and after preoperative APBI

OUTLINE:

Patients undergo image-guided intensity-modulated APBI twice daily (BID) in the prone position over a period of 5-10 days for a total of 10 treatment. Within 4-6 weeks post-APBI, patients undergo lumpectomy.

After completion of treatment, patients are followed up at 4 weeks and then every 6 months for 5 years.

Inclusion Criteria:

• The patient must consent to be in the study and must have signed an approved consent form conforming with federal and institutional guidelines.
• Patient must be ≥ 50 years
• Core biopsy demonstrating breast cancer and receptors that are ER or PR positive.
• Core tissue must have HER 2 negative followed by current ASCO/CAP guidelines.
• The patient must have clinical node negative, stage I breast cancer.
• The surgical treatment must be intended to be a lumpectomy
• The biopsy site must have been demarcated by a clip(s)
• Gross disease must be unifocal on Mammo/ MRI imaging
• Patients must have estrogen receptor (ER) and progesterone receptor (PR) analysis performed on core biopsy
• Patient must be able to tolerate lying in the prone position with arms extended forward.
• Must be able to tolerate MRI scan with contrast
• At the time of enrollment, patients must have had bilateral mammograms within 6 months.
• Patients must be willing to undergo breast cancer surgery minimally 4, maximally 6 weeks post APBI.
• Patients with a history of non-breast malignancies are eligible if they have been disease free for 5 or more years prior to enrollment and are deemed by their physicians to be at low risk for recurrence. Patients with the following cancers are eligible if diagnosed and treated within the past 5 years: carcinoma in situ of the cervix, carcinoma in situ of the colon, melanoma in situ, and basal cell and squamous cell carcinoma of the skin.

Exclusion Criteria:

• Age < 50 years
• Hormone unresponsive breast cancer
• T-2 ( > 3.0 cm), T-3, Stage III, or Stage IV breast cancer.
• N-1, N-2, or N-3 clinical axillary nodes
• Mastectomy intended
• Unwilling to undergo anti-endocrine therapy
• Suspicious microcalcification, densities, or palpable abnormalities ( in the ipsilateral or contralateral breast) unless biopsied and found to be benign.
• Non-epithelial breast malignancies such as sarcoma or lymphoma.
• Paget's disease of the nipple
• Proven multicentric carcinoma (invasive or DCIS) in more than one quadrant or separated by > 4 centimeters.
• Any prior treatment with radiation therapy, chemotherapy, biotherapy, or hormone therapy for the currently diagnosed breast cancer prior to study enrollment.
• Prior breast or thoracic RT for any condition.
• Psychiatric of addictive disorders or other condition that in the opinion of the investigator would preclude the patient from meeting the study requirements.