Inclusion Criteria:  stage 0-II breast cancer Invasive ductal carcinoma (NOS, NST, other subtypes), invasive lobular carcinoma, ductal carcinoma in situ (DCIS) Invasive carcinoma/DCIS, which is unifocal and unicentric pT1-2, maximal tumor size < 3 cm pN0 (no metastases to the nodes) LVI(-) - no lymphovascular invasion M0 (no distant metastases) Surgical margins free of cancer (no tumor on ink) in each direction; in the case of DCIS, the minimum margin of 2 mm in each direction Lack of indications to chemotherapy/immunotherapy pre- or postsurgically (e.g., triple-negative, HER2-positive breast cancer) Treatment start not later than 12 weeks after surgery (optimally 4-8 weeks, after wound healing) Informed handwritten signed patient's consent  Exclusion Criteria:  stage III-IV breast cancer pT2-4, maximal tumor size ≥ 3 cm surgical margins cannot be properly microscopically assessed EIC(+) - presence of extensive intraductal component Paget's disease or microscopically assessed skin involvement pN1-3M1 (presence of nodal or distant metastases) pre- or postsurgical indications for chemotherapy/immunotherapy other cancer (less than five years at time of recruitment) except skin cancer or cured FIGO 0-I cervical cancer time of pregnancy or lactation collagen disorders (congenital or acquired) psychiatric disorder disabling patient's compliance breast appearance or postsurgical status disabling safe interstitial multicatheter implantation lack of informed handwritten signed patient's consent