Single vs Hypofractionated Irradiation For Timely Access to Partial Breast Radiotherapy

Inclusion Criteria: * Female participants age 40 or older * Able to provide informed consent * pTis-2 pN0 cM0 breast cancer, with tumor size \<3 cm as per provincial guidelines * Able to complete electronic or paper entry of participant reported outcomes independently or with assistance from caregiver/family/friend/research staff * Eastern Cooperative Oncology Group (ECOG) performance status 0 - 2 * A history and physical examination, including ECOG performance status, performed within 8 weeks prior to enrollment. * Participant is judged able to: * Maintain a stable position during therapy * Tolerate immobilization device(s) that may be required to deliver PBI safely * Negative pregnancy test for People of Child-Bearing Potential (POCBP) within 4 weeks of RT start date Exclusion Criteria: * History of non-breast malignancies except adequately treated non-melanoma skin cancers, in situ cancers treated by local excision or other cancers curatively treated with no evidence of disease for ≥ 5 years. * Uncontrolled concurrent malignant cancer * Seroma not visible * Ipsilateral implanted cardiac device * Prior radiotherapy requiring summation for planning. * Inability to meet mandatory planning constraints. * Requirement for a radiation boost (as determined by the treating investigator) * Positive surgical margins * Surgical cavities lacking clear delineation (surgical clips are not required but may assist in target delineation) * Known germline BRCA1/2 mutation. * Serious …