Partial Breast Irradiation (PBI) is a targeted radiation approach commonly administered post-lumpectomy, specifically targeting the tumour bed. This targeted therapy reduces the exposure to other nearby tissues such as lungs, heart, and chest wall. However, traditional PBI treatment involves lengthy multiple fraction courses which presents a burden to patients from rural and remote communities, who must travel long distances to receive high quality cancer care. The purpose of this study is to compare single fraction (SF) PBI vs. multiple fraction (MF) PBI.

Radiation can be delivered in multiple fractions, or doses, and can take up to several weeks or months of treatment depending on the type of cancer. Radiation can also be offered in a single fraction. Both techniques have evidence for use in clinical care. Multiple fraction is offered to reduce the amount of radiation given at a single time that could reduce late toxicities. However, single fraction radiotherapy is more cost-effective and saves patient time. With this trial, we will compare single fraction vs. multiple fraction PBI in regards to their impact on quality of life, rates of provider and participant reported toxicities, and local control.

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 (clinical breast) 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.
• 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 medical comorbidities precluding radiotherapy (e.g., connective tissue disorders such as lupus or scleroderma)
• Pregnant or breastfeeding

Eligible participants must be biologically female.

This will is an open-label randomized controlled study design, however, outcome assessors and data analysts will be blinded to the identity of each treatment arm.