Trial to Compare MRI-guided Precision Prone Irradiation (PPI) Versus CT-guided Breast Irradiation

The Investigators are investigating the potential role of MRI-guided Radiation Therapy for patients receiving breast radiotherapy in the prone position with a MRI-Linac radiotherapy system, the Precision Prone Irradiation (PPI) technique. Hypothesis: The investigators would like to hypothesize that breast radiation therapy using MRI-guided system including the MRI-Linac (Arm 2) is comparable to the current standard of treatment using conventional CT-based system (Arm 1), in terms of local control of the disease at 2 year time point.

Patients ≥ 50 yrs with histologically confirmed unilateral DCIS or Stage I invasive breast cancer; received breast conserving surgery (pN0 or pNx) with tumor pathology ≤ 2 cm and excision margin ≥ 1 mm Inclusion Criteria: * Women status post segmental mastectomy * If unilateral, pT1 breast cancer excised with negative margins AND/OR pTis excised with negative margins. * If bilateral, pT1 breast cancer excised with negative margins AND/OR pTis excised with negative margins. * Clinically N0 or pN0 or sentinel node negative * Ductal carcinoma in situ DCIS with negative margins (no DCIS on inked margins). Exclusion Criteria: * Previous radiation therapy to the ipsilateral breast. * \> 90 days from last surgery, unless s/p adjuvant chemotherapy * \> 60 days from last chemotherapy * Patients unable to tolerate MRI or are otherwise unsuitable for MRI-based RT treatment * Metastatic disease