This phase II trial studies how well deep inspiratory breath hold or prone breast radiation therapy works in reducing cardiac dose in patients with breast cancer or ductal carcinoma in situ. Deep inspiratory breath hold is a technique that involves a patient holding their breath during radiation to move the heart away from the breast to minimize radiation to the heart. Prone breast radiation therapy is a technique in which a patient receives treatment "face-down" to minimize the radiation to the rest of the body, including the heart and lungs. Comparing deep inspiratory breath hold to prone breast radiation therapy may help doctors improve radiation to the target while reducing radiation to healthy tissues surrounding the target.

Patients attend a simulation visit and undergo two CT scans, one in the prone position and one in the supine position with DIBH. Within 1 week after the simulation visit, patients undergo radiation therapy either in the supine position with DIBH or in the prone position daily for 15-30 consecutive days as per physician's prescription

Inclusion Criteria:

• Any woman with biopsy proven left breast DCIS or invasive cancer who has undergone a lumpectomy and who requires whole breast irradiation to the breast alone (and not to any nodal regions) as per the treating radiation oncologist

Exclusion Criteria:

• Previous radiation to the ipsilateral breast
• All pre-menopausal women will require a urine qualitative pregnancy test to exclude pregnancy; pregnant women will be excluded from the study