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To report the implementation, dosimetric results and early experiences of stereotactic accelerated partial breast irradiation (SAPBI) following breast conserving surgery (BCS) for low-risk early stage invasive breast cancer. 

Between November 2018 and October 2021, 64 patients with low risk early invasive (St I-II) breast cancer underwent BCS were enrolled in our phase II prospective study. SAPBI was performed with Cyber-Knife (CK) M6 machine, to a total dose of 25 Gy in 4 daily fractions of 6.25 Gy. Respiratory movements were followed with implanted gold markers and Synchrony system. Corrections for patient displacement and respiratory movement during treatment were performed with the robotic arm. Side effects, cosmetic results and dosimetric parameters were assessed. 

At a median follow up to 24 months (range: 1-35) no locoregional or distant metastases was observed. The average volume of the surgical cavity, clinical target volume (CTV) and planning target volume (PTV_EVAL) was 7.7 cm³ (range: 1.75-27.3 cm³), 54.3 cm³ (range: 26.2-103.5 cm³) and 74.6 cm³ (range: 40-135.4 cm³) respectively. The mean value of the PTV/whole breast volume ratio was 0.09 (range: 0.04-0.19). No grade 2 or worst acute and late side-effect was detected. Acute side effects included Grade 1 (G1) erythema occurred in 13 (20.3%) patients, while G1 oedema was observed in 7 (10.9%) cases. G1 pain was reported by 3 (4.6 %) patients. Late side effects included G1 skin toxicity in 2 (3%), G1 and G2 fibrosis in 2 (3%) and 1 (1.5%) patient respectively. G1 pain in 3 (4.7%) cases. Cosmetic outcome was excellent in 41 (64 %) and good in 23 (36 %) patients. 

SAPBI with CK is a reproducible and feasible technique for the delivery of external beam APBI following BCS for the treatment of low-risk, early-stage invasive breast carcinoma. Our early findings are promising, CK-SAPBI delivered with four daily fractions is well tolerated by the patients.