The adjuvant radiotherapy (RT) of the early-stage breast cancer patients as local treatment aims to eliminate the potential microscopic residual disease in the surgery bed or satellites in its neighborhood. Nowadays accelerated partial breast irradiation (APBI) is recommended for highly selected patients. This prospective randomized study compares the targeted external beam APBI with commonly used accelerated whole-breast irradiation (WBI) in terms of feasibility, safety, tolerance, and cosmetic effects. It is designed as non-inferiority trial and its aim is to increase the level of evidence for establishment of external beam APBI in indicated patients into daily clinical practice.

Early-stage breast cancer patients after partial mastectomy are screened for eligibility. The inclusion criteria are age ˃ 50 years, non-lobular carcinoma histology, size ≤ 2 cm, negative margins ≥ 2 mm, L0, ER-positive, HER-2 negative. Enrolled patients are equally randomized into two arms according to radiotherapeutic regiment - external beam APBI (5× 6 Gy) and accelerated whole breast irradiation with the boost (15× 2,67Gy + 5× 2Gy). The follow-up visits are planned at the end of RT and 1, 3, 6, 9, and 12 months after radiation, in the second year every 4 months and then every 6 months. Cosmetics results and toxicity are evaluated using questionnaires, CTCAE criteria, and photodocumentation of the irradiated chest. The main objective of presented study is to evaluate the feasibility, safety, tolerance, and cosmetic effects of SBRT irradiation to the surgery bed in five fractions. In addition to standard clinical examination and evaluation of acute and late side effects, patients together with clinicians and nurses will independently complete a questionnaire on the impact of irradiation on the cosmetic effect. The technique of targeted external beam APBI should demonstrate better feasibility and less toxicity than the standard regimen in the adjuvant setting in treating early-stage breast cancer patients. Consequently, the presented study should increase the level of evidence for RT-indicated patients to the establishment of external APBI into daily clinical practice.

Patients referred to medical attention for adjuvant radiotherapy of early-stage breast cancer at the Department of Radiation Oncology, Masaryk Memorial Cancer Institute (MMCI) were screened for eligibility. If all inclusion/exclusion criteria will be met, they were invited to participate in the present study.

Inclusion Criteria:

1. Age ≥50 years
2. Karnofsky index > 70
3. Partial mastectomy (breast-conserving surgery)
4. DCIS G1/2 ≤ 2.5 cm with negative margins (≥ 3 mm) or invasive (non-lobular) luminal-like HER2 negative carcinoma ≤ 2 cm with negative margins (≥ 2 mm) without LVI
5. In the case of invasive carcinoma, performing of axillary dissection (≥6 negative lymph nodes) or negative sentinel node biopsy

Exclusion Criteria:

1. Prior to other chest or breast surgery (including breast reconstruction), the absence of surgical clips in the tumor bed
2. Prior ipsilateral chest or breast radiotherapy
3. Neoadjuvant systemic therapy
4. Adjuvant chemotherapy
5. Multifocal or multicentric involvement
6. BRCA 1 or 2 mutations or known mutations in other high penetrance genes
7. Any systemic illness (collagen vascular diseases) or unstable medical condition that might pose additional risks for the performance of radiotherapy including claustrophobia or jactation
8. Any other factors that, in the opinion of the site investigators, would interfere with adherence to study requirements
9. Pregnancy or breastfeeding
10. Inability or unwillingness of the subject to sign written informed consent