Partial Breast Irradiation Using Interstitial Permanent Palladium-103 Seed Implant

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,