Inclusion Criteria:

          -  Patient consent and signature of approved consent form.

               -  Age greater than 55 years and postmenopausal.

               -  Life expectancy of at least ten years

               -  Stage 0 or I breast cancer. Tumour size < 2 cm.

               -  Invasive cancer must be pathologic grade 1 or 2

               -  Histology must be either Ductal Carcinoma In Situ or invasive ductal
                  adenocarcinoma of the breast.

               -  Tumour removed by lumpectomy with clear margins (DCIS and invasive).

               -  Unifocal disease

               -  For invasive breast cancer, axillary staging by either sentinel node biopsy or
                  axillary dissection (minimum of 6 axillary nodes). Not required for DCIS.

               -  Brachytherapy generally should be performed within 16 weeks of the last surgery
                  (lumpectomy, re-excision of margins, or axillary staging procedure). If > 16
                  weeks, will be assessed on case-by-case basis, based on seroma visibility and the
                  presence of surgical clips in the tumour bed.

               -  Tumour must be Estrogen Receptor positive.

               -  The lumpectomy cavity (seroma) must be clearly delineated by ultrasound and CT
                  and should be < 3 cm diameter (equivalent sphere) resulting in a maximum of 125
                  cc planning target volume (PTV).

               -  Breast seed implant must be considered technically deliverable by interstitial
                  brachytherapy by the attending radiation oncologist and must be a minimum 2-plane
                  implant.

               -  If prior non-breast malignancies, must have been disease-free for 5 or more
                  years. Time limit waived for carcinoma in-situ of the cervix or colon, melanoma
                  in-situ, and basal or squamous cell carcinoma of the skin.

        Exclusion Criteria:

          -  Stage T2 or higher breast cancer

               -  Histologically positive axillary nodes.

               -  High grade (grade 3) invasive ductal carcinoma

               -  Extensive DCIS

               -  Lymphatic or Vascular Invasion positive

               -  Significant persistent post surgical complications

               -  Palpable or radiographically suspicious ipsilateral or contralateral axillary, or
                  regional nodes, unless histologically confirmed negative.

               -  Suspicious microcalcifications, densities, or palpable abnormalities in the
                  ipsilateral or contralateral breast unless confirmed benign.

               -  Proven multi-centric carcinoma (invasive cancer or DCIS)

               -  Paget's disease of the nipple.

               -  Synchronous bilateral invasive or non-invasive breast cancer.

               -  History of previous ipsilateral invasive breast cancer or DCIS.

               -  Surgical margins that are positive or cannot be microscopically assessed.

               -  Clear delineation of the target lumpectomy cavity not possible.

               -  Breast implants.

               -  Prior ipsilateral breast or thoracic radiotherapy.

               -  Known genetic mutation in Breast Cancer Associated (BRCA 1 or 2)

               -  Collagen vascular disease,