Stereotactic Accelerated Partial Breast Irradiation Using CyberKnife with Non-Invasive Skin Fiducial Marker Tracking in Early-Stage Breast Cancer: A Retrospective Study of Feasibility, Dosimetry, and Early Outcomes.

Purpose: This study aimed to evaluate the feasibility, dosimetric characteristics, and early clinical outcomes of CyberKnife (CK)-based accelerated partial breast irradiation (APBI) using non-invasive skin fiducial marker tracking in an Asian population. Materials and Methods: We retrospectively analyzed 74 female patients diagnosed with early-stage breast cancer who underwent APBI using the CK system between May 2019 and December 2024. Patient selection was based on the modified 2017 American Society for Radiation Oncology (ASTRO) consensus criteria. The total tumor doses were 30 Gy in 5 consecutive daily fractions. Non-invasive skin fiducial markers were used for respiratory motion tracking. Dosimetric parameters were recorded according to European Society for Radiotherapy and Oncology (ESTRO) and Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC) recommendations. Primary clinical outcomes, acute and chronic toxicities were evaluated during a median follow-up period of 26.5 months. Predictive factors for toxicity were assessed using receiver operating characteristic (ROC) curve analysis. Results: A total of 74 patients having a median age of 56 years were included in the study, with a median follow-up period of 26.5 months. Non-invasive skin fiducial markers demonstrated a strong correlation with internal surgical clips, validating their accuracy for motion tracking. The median conformity and homogeneity indexes were 1.15 and 1.20, respectively. Median mean heart doses were 1 Gy (left-sided) and 0.5 Gy (right-sided), while the ipsilateral lung mean dose was 2.34 Gy. Two patients (2.7%) developed ipsilateral breast tumor recurrence. There were no grade ≥2 toxicities or cardiopulmonary events observed. Radiation dermatitis represented the most common acute toxicity (48.6%), whereas breast fibrosis was the most frequent late toxicity (12.2%). Skin D0.03cc >29.45 Gy and PTV-to-breast volume ratio >14.5% were associated with grade 1 dermatitis, while a breast volume <455.2 cm Conclusion: By retrospective reviewing, APBI using the CyberKnife system with non-invasive skin fiducial marker tracking is a safe, precise, and effective treatment option for early-stage breast cancer. Although this retrospective study with limited follow-up demonstrated favorable dosimetric outcomes and minimal acute toxicity, further prospective studies with larger cohorts and longer observation are needed to validate these findings.