Inclusion Criteria:
1. Isolated ipsilateral unifocal breast lesions;
2. Histologically confirmed invasive breast carcinoma or carcinoma in situ;
3. Limited size (\< 2 cm) without evidence of skin involvement;
4. Negative histologic margins of resection;
5. Negative axillary lymph nodes;
6. No synchronous distant metastases;
7. Bilateral breast mammogram or MRI within 120 days prior to study entry;
8. For invasive in-breast recurrence, no more than 120 days since whole-body (positron emission tomography) PET-CT scan OR CT scan of the chest, abdomen, and pelvis, and bone scan (if clinically relevant);
9. ≥ 24 months interval between initial breast conserving therapy (surgery and whole breast radiotherapy) and recurrence;
10. Female, aged \>18 years;
11. Life expectancy of greater than 12 months;
12. ECOG performance status \<2;
13. Female participants of child bearing potential must be willing to ensure that they or their partner use effective contraception during the study and for 4 months thereafter;
14. Participant is willing and able to give informed consent for participation in the study;
Exclusion Criteria:
1. Regional recurrences (axillary, supraclavicular);
2. Positive histologic margins at resection;
3. Metastatic disease;
4. Previous breast RT performed with IORT, brachytherapy or previous partial breast treatment;
5. Known pathogenic mutation of BRCA1, BRCA2 or TP53 gene;
6. Patients who had chemotherapy within 2 weeks prior to study RT;
7. Participation in another clinical trial with any investigational agents within 30 days prior to study screening;
8. Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements;
9. Significant comorbidity precluding RT for breast cancer (cardiovascular or pulmonary disease, sclerodermia, systemic lupus erythematosus);
10. Other known malignant neoplastic diseases in the patient's medical history with a disease-free interval of less than 5 years (except for previously treated basal cell carcinoma of the skin and in situ carcinoma of the uterine cervix, endometrium or colon);
11. Inaccessibility for follow-up;
