Node Negative, Invasive Breast Cancer Single Fraction IOERT

See points VIII inclusion/exclusion criteria Operation: * Lumpectomy, steninal node biopsy * IOeRT After Surgery: * Meet with a medical oncologist and discuss if chemotherapy and/or hormone therapy is recommended as part of your treatment course. In some cases, pathology results may indicate the need for removal of additional lymph nodes. The doctor will review pathology and discuss if additional surgery is required. * Chemotherapy (if indicated) * Hormone therapy (if indicated) Follow-Up * Visits will be completed with the surgeon, radiation oncologist and research staff as the protocol indicates. * Assessements of acute toxicity according to CTC-toxicity scoring system * Assessments of late toxicity according to LENT-SOMA scoring systems * Assessment of cosmetic outcome according to scoring system and photo documentation in standardized positions.