M Khorramizadeh - Department of Medical Physics, Faculty of Medicine, Dezful University of Medical Sciences, Dezful, Iran
A Saberi - Department of Medical Genetics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran
M Tahmasebi–birgani - Department of Medical Physics, Faculty of Medicine, Dezful University of Medical Sciences, Dezful, Iran
P Shokrani - Department of Medical Physics and Medical Engineering, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran
A Amouhedari - Department of Radiation Oncology, Milad Hospital, Isfahan, Iran

Introduction: Stereotactic body radiotherapy delivers hypofractionated irradiation with high dose per fraction through complex treatment techniques. The increased complexity leads to longer dose delivery times for each fraction. The purpose of this study is to investigate the impact of prolonged fraction delivery time with high-dose hypofractionation on the killing of cultured ACHN cells.Methods and Materials: The radiobiological characteristics and repair half-time of human ACHN renal cell carcinoma cell line were studied with clonogenic assays. A total dose of ۲۰ Gy was administered in ۱, ۲ or ۳ fractions over ۱۵, ۳۰ or ۴۵ min to investigate the biological effectiveness of radiation delivery time and hypofractionation. Cell cycle and apoptosis analysis was performed after ۳-fraction irradiation over ۳۰ and ۴۵ min.Results: The α/β and repair half-time were ۵.۲ Gy and ۱۹ min, respectively. The surviving fractions increased with increase in the fraction delivery time and decreased more pronouncedly with increase in the fraction number over a treatment period of ۳۰ to ۴۵ min. With increase in the total radiation time to ۳۰ and ۴۵ min, it was found that with the same total dose, ۲- and ۳-fraction irradiation led to more cell killing than ۱-fraction irradiation. ۳-fraction radiation induced G۲/M arrest, and the percentage of apoptotic cells decreased when the fraction delivery time increased from ۳۰ min to ۴۵ min.Conclusion: Our findings revealed that sublethal damage repair and redistribution of the cell cycle were predominant factors affecting cell response in the prolonged and hypofractionated irradiation regimes, respectively.

Hypofractionation, Prolonged Fraction Delivery Time, Renal cell carcinoma, Stereotactic Body Radiotherapy, Sublethal Damage Repair