The primary objective of this study is to determine in women with node negative BC ≤3cm in size, if PBI compared to WBI, both given once-a-day over 1 week following BCS, is non-inferior for LR and reduces adverse cosmesis. The primary outcomes are LR and patient-assessed cosmesis at 3 years post randomization.

This is a randomized, two-arm, single blinded trial comparing two radiation treatment modalities, PBI and WBI. Following BCS or on the completion of additional adjuvant chemotherapy, eligible and consenting patients with newly diagnosed and histologically confirmed invasive carcinoma of the breast (without evidence of metastatic disease); with microscopically clear resection margins of 1mm (or no residual disease on re-excision) and negative axillary node involvement will be randomized in a 1:1 fashion to receive either PBI (experimental group) or WBI (control group). Study participants will receive 26Gy in 5 fractions in both treatment arms, treated once per day, for a period of 5-7 days. Study participants will not be made aware of treatment allocation to prevent any potential bias in their assessment of cosmesis. Stratification factors include tumour size, estrogen receptor (ER) status, and clinical centre.

Inclusion Criteria:

For inclusion in this study, patients must fulfill all of the following criteria:

1. Female with a new histological diagnosis of invasive carcinoma of the breast with no evidence of metastatic disease (see AJCC TNM Cancer Staging, Appendix II).
2. Treated by BCS with microscopically clear resection margins >= 1mm for invasive and non-invasive disease or no residual disease on re-excision.
3. Negative axillary node involvement as determined by either sentinel lymph node biopsy or axillary node dissection or clinical assessment with a negative axillary ultrasound and/or biopsy, for women with unifocal tumours <= 2cm, histologic grade 1 or 2, ER or PR+ and HER2-ve that are being planned for endocrine therapy

Exclusion Criteria:

Patients who satisfy any of the following exclusion criteria are NOT eligible for this study:

1. Age less than 50 years.
2. Known to be BRCA 1 and/or BRCA 2 positive.
3. Tumour size >3cm in greatest diameter on pathological examination.
4. Evidence of extensive intraductal component (EIC) (defined as an invasive tumour with a ductal carcinoma in situ (DCIS) component comprising at least 25% and extending beyond the invasive component to surrounding normal breast tissue) with the following exception: smaller tumours with EIC where the combined size (of the invasive and DCIS components) are <= 3cm remain eligible
5. Evidence of a DCIS component > 3cm
6. Lobular carcinoma only.
7. More than one primary tumour in different quadrants of the same breast (patients with multifocal breast cancer are eligible).
8. Synchronous or previous contralateral breast cancer (patients with contralateral DCIS or LCIS are eligible).
9. History of non-breast malignancy within the last 5 years other than treated non-melanoma skin cancer or treated in-situ carcinoma.
10. Known pregnancy or currently lactating.
11. Inability to localize tumour bed on CT planning (no evidence of surgical clips or seroma).
12. Inability to plan the patient for the experimental technique.

Study participants will be made unaware of treatment allocation to prevent any potential bias in their assessment of cosmesis.