The Use of Deep Inspiration Breath Hold and Prone Irradiation to Decrease Cardiac Radiation Exposure

Inclusion Criteria: * Patients must have a pathologically proven diagnosis of left-sided invasive breast cancer or ductal carcinoma in situ receiving breast/chestwall irradiation +/- regional nodal irradiation, or a right-sided invasive ductal cancer status post-mastectomy or lumpectomy who will be receiving breast/chest wall and nodal irradiation. * Patients must have undergone breast conserving surgery (i.e. lumpectomy, partial mastectomy) or mastectomy prior to enrollment. * Patients must be 18 years of age or older. * Study entry must be prior to CT simulation. * Per Medical College of Wisconsin Department of Radiation Oncology Standard of Care, women of childbearing potential must be non-pregnant and non-lactating and willing to use medically acceptable forms of contraception during radiation therapy. Patients under age 50 with a uterus and ovaries must have a negative serum pregnancy test within one week of planned CT simulation per Radiation Oncology Departmental guidelines prior to CT simulation. * Patients must provide study specific informed consent prior to study entry. * Patients must be able to hold their breath long enough to undergo CT simulation in DIBH (ten to fifteen seconds). The patient should also affirm they can lie in the prone position. This will be determined by the treating radiation oncologist, as per current standard of care. Exclusion Criteria: * Patients who are unable to hold their breath long enough to undergo CT simulation in DIBH. While this may affect patients with chronic obstructive pulmonary disease (COPD) and/or other respiratory conditions, the presence of these comorbidities alone will not exclude patients from enrollment so long as they can maintain breath hold long enough to perform DIBH.