Clinical Outcomes of Robotic Stereotactic Accelerated Partial Breast Irradiation: A Structured Literature Review

Accelerated partial breast irradiation (APBI) has emerged as an evidence-based alternative to whole-breast irradiation (WBI) in appropriately selected patients with early-stage breast cancer. Within this landscape, robotic stereotactic APBI delivered with the CyberKnife (CK) system (Accuray Incorporated, Madison, WI) represents a technologically refined external-beam implementation, integrating non-isocentric beam arrangements, high-frequency image guidance, and optional fiducial-based respiratory tracking that allow steep dose gradients and reduced geometric margins. This structured literature review synthesizes the available clinical, dosimetric, and technical evidence on CK-APBI to define its current role among contemporary APBI modalities. A structured review of peer-reviewed studies published between 2011 and 2025 reporting clinical or dosimetric outcomes of CK-based APBI in early-stage breast cancer was performed. Eligible studies included pilot feasibility reports, phase I/II prospective trials, retrospective cohorts, comparative analyses, and dosimetric evaluations. Endpoints of interest were ipsilateral breast tumor recurrence (IBTR), acute and late toxicity, cosmetic outcomes, and dose to organs at risk (heart, lungs, uninvolved breast). Patient selection, treatment technique (fiducial placement, simulation, target delineation, motion management, dose-fractionation), and CyberKnife platform evolution (Iris, InCise multileaf collimator) were also evaluated. Across cohorts treated predominantly with 30 Gy in five consecutive daily fractions, CK-APBI achieved low IBTR rates (~0-2%) at intermediate and five-year follow-up, with predominantly grade ≤1 toxicity and excellent or good cosmesis in 80-96% of patients. Phase I dose escalation suggested that late grade 3 fat necrosis correlated with larger planning target volume, reinforcing the importance of strict volume control. Recent data, including a 74-patient cohort using non-invasive skin-fiducial tracking and a long-term cohort with five-year follow-up, confirmed durable local control and preserved cosmesis. Dosimetric analyses demonstrated favorable sparing of uninvolved breast tissue (V50 ~18-25%, V100 <10%), low ipsilateral lung dose, and reduced cardiac exposure in left-sided tumors compared with interstitial brachytherapy. Multileaf collimator-based delivery improved efficiency relative to Iris collimation. Fiducial-based image guidance and respiratory tracking enabled the clinical target volume to planning target volume margin reduction to ~2-3 mm, translating into smaller treated volumes and a plausible mechanistic basis for the favorable toxicity and cosmetic profiles observed. CK-APBI is a clinically mature and technically refined APBI option for appropriately selected early-stage breast cancer patients, with oncologic and cosmetic outcomes broadly concordant with randomized APBI evidence. Margin reduction enabled by image guidance and respiratory tracking provides a distinct technical advantage in limiting intermediate-dose exposure to uninvolved breast tissue and critical organs. However, the evidence base remains predominantly single-institution and non-randomized, with heterogeneous contouring and reporting and limited follow-up beyond five years. Prospective multicenter studies with standardized endpoints are needed to define the relative role of robotic stereotactic APBI among contemporary partial-breast options.