This is a multi-centre randomized phase II trial in women with invasive carcinoma of the breast with negative axillary nodes or Ductal Carcinoma In-situ (DCIS) treated by Breast Conserving Surgery (BCS). Eligible, consenting patients will be randomly allocated to receive radiotherapy of 3 Dimensional Conformal Radiation Therapy (3DCRT) Accelerated Partial Breast Irradiation (APBI) 30 Gray (Gy) in 5 daily fractions of 6Gy or 27.5Gy in 5 daily fractions of 5.5Gy over one week. Patients will be followed at 12, 24, 36 and 60 months post randomization. Cosmetic outcome will be measured using photographs and evaluated by a panel of trained radiation oncologists. Radiation toxicity will be assessed using National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE).

Women with invasive carcinoma of the breast with negative axillary nodes or DCIS, following BCS and/or chemotherapy, who meet the inclusion criteria will be screened and approached for eligibility assessment and potential study enrollment. All potentially eligible and consenting patients who do not meet the exclusion criteria will have a Computed Tomography (CT) simulation of the ipsilateral breast prior to randomization. All cases will be required to be planned prior to the randomization to ensure that dosimetric coverage and normal tissue restrictions can be met. If the seroma is not clearly visible or if the surgical clips do not adequately identify a previous surgical cavity, the patient will not be eligible for randomization.

Eligible, consenting patients will be randomly allocated to receive radiotherapy of 3DCRT APBI 30Gy in 5 daily fractions of 6Gy or 27.5Gy in 5 daily fractions of 5.5Gy over one week. Patients will be followed at 12, 24, 36 and 60 months post randomization. Cosmetic outcome will be measured using photographs and evaluated by a panel of trained radiation oncologists using the EORTC Breast Cosmetic Rating System. Radiation toxicity will be assessed using the NCI CTCAE version 4.02. The primary outcome is adverse cosmetic outcome at 2 years. The trial will be conducted in clinical centres within Canada.

Inclusion Criteria:

1. Females with a new histological diagnosis of DCIS or invasive carcinoma of the breast with no evidence of metastatic disease.
2. Treated by BCS with microscopically clear resection margins for invasive and non-invasive disease (or no residual disease on re-excision).
3. Negative axillary node involvement, including positive cells only identified by Immunohistochemistry (IHC) as determined either by sentinel node biopsy or axillary node dissection, or a clinical exam for patients with DCIS only.

Exclusion Criteria:

1. Age less than 50 years

2. Known to be Breast Cancer Type 1 Susceptibility Protein (BRCA 1) and/or Breast Cancer Type 2 Susceptibility Protein (BRCA 2) positive

3. Tumour size >3 cm in greatest diameter on pathological examination (including the invasive and non-invasive components)

4. Tumour invades the skin (i.e. T4 disease)

5. Tumour histology limited to lobular carcinoma only

6. Grade III invasive breast carcinoma or nuclear grade III for patients with DCIS alone

7. Triple negative invasive breast cancer

8. Her2+ve invasive breast cancer that will not receive Herceptin

9. Bilateral invasive malignancy of the breast (synchronous or metachronous)

10. More than one primary tumor in different quadrants of the same breast

11. Presence of an ipsilateral breast implant or pacemaker

12. Status for adjuvant systemic therapy not determined

13. Unable to commence radiation therapy:

    1. within 12 weeks of the last surgical procedure on the breast, or
    2. within 8 weeks of the last dose of chemotherapy

14. Previous or concurrent malignancies except:

    1. Non-melanoma skin cancer
    2. Carcinoma in-situ of the cervix
    3. Invasive carcinoma of the cervix, endometrium, colon and thyroid, treated 5 years prior to study entry with no evidence of disease

15. Serious non-malignant disease (e.g. cardiovascular, pulmonary, systemic lupus erythematosus (SLE), scleroderma) which would preclude definitive radiation treatment.

16. Current pregnancy or lactating.

17. Psychiatric or addictive disorders which would preclude obtaining informed consent or adherence to the protocol.

18. Geographic inaccessibility for follow-up.

19. Inability to localize surgical cavity on CT (i.e. no evidence of surgical clips or seroma)

20. Inability to adequately plan the patient for the experimental technique.