Purpose: The efficacy of accelerated partial breast irradiation (APBI) utilizing brachytherapy or conventional external beam radiation has been studied in early stage breast cancer treated with breast conserving surgery. Data regarding stereotactic treatment approaches are emerging. The CyberKnife linear accelerator enables excellent dose conformality to target structures while adjusting for target and patient motion. We report our institutional experience on the technical feasibility and rationale for SAPBI delivery using the CyberKnife radiosurgery system.Methods: Ten patients completed CyberKnife SAPBI in 2013 at Georgetown University Hospital. Four gold fiducials were implanted around the lumpectomy cavity prior to treatment under ultrasound guidance. The synchrony system tracked intrafraction motion of the fiducials. The clinical target volume (CTV) was defined on contrast enhanced CT scans using surgical clips and post-operative changes. A 5 mm expansion was added to create the planning treatment volume (PTV). A total dose of 30 Gy was delivered to the PTV in 5 consecutive fractions. Target and critical structure doses were assessed as per the National Surgical Adjuvant Breast and Bowel Project B-39 study.Results: At least 3 fiducials were tracked in 100% of cases. The Mean treated PTV was 70 cm3 and the mean prescription isodose line was 80%. Mean dose to target volumes and constraints are as follows: 100% of the PTV received the prescription dose (PTV30). The volume of the ipsilateral breast receiving 30 Gy (V30) and above 15 Gy (V>15) was 14% and 31% respectively. The ipsilateral lung volume receiving 9 Gy (V9) was 3% and the contralateral lung volume receiving 1.5 Gy (V1.5) was 8%. For left sided breast cancers, the volume of heart receiving 1.5 Gy (V1.5) was 31%. Maximum skin dose was 36 Gy. At a median follow up of 1.3 years, all patients have experienced excellent/good breast cosmesis outcomes, and no breast events have been recorded.Conclusions: CyberKnife stereotactic accelerated partial breast irradiation is an appealing technique for partial breast irradiation offering improvements over existing APBI techniques. Our early findings indicate that CyberKnife SAPBI delivered in 5 daily fractions is feasible, well tolerated and is a reliable platform for delivering APBI
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