The Radiobiological Effect of Flattening Filter Free (FFF) Beam Delivery Parameters on A549 and MCF-7 Cell Survival: An In Vitro Study

Background: Recent improvements, such as flattening filter-free (FFF) beams, enable the delivery of treatments at higher dose rates and in shorter times. However, the biological influence still remains uncertain. Here, we examined how dose rate and dose per pulse influence responses to FFF beams compared with flattened beams in cancer cells with different radiosensitivities. Methods: A549 (lung, high α/β) and MCF-7 (breast, low α/β) cells were irradiated using VMAT (6 MV FF, 6 and 10 MV FFF; 600–2400 MU/min; 5–20 Gy). Cell viability was assessed by the WST-1 assay, and reproductive survival by clonogenic assay. Results: The responses differed by cell type and dose, with A549 exhibiting restricted sensitivity, demonstrating more pronounced effects at 10 Gy, while MCF-7 displayed greater responsiveness at 5–10 Gy under elevated dose rates. At 20 Gy, differences were reduced. Metabolic readouts did not consistently match clonogenic survival. Conclusions: Overall, FFF effects appear cell-specific, and the suggestion is that clinical translation should be guided by tumor-specific radiosensitivity rather than an assumption of universal benefit.