Radiobiological effects of altering dose rate in filter-free photon beams.

To validate that altering radiotherapy dose rate through either changing pulse repetition frequency or instantaneous dose rate does not have an effect on cell survival, two human carcinoma and a hamster lung cell line were irradiated with various beam settings. Varian TrueBeam linac with a flattening filter free mode of operation was used for all experiments. The results obtained indicate that either method of changing dose rate has no effect on cell survival in the three cell lines studied. Filtered and filter free modes were also compared in treatments with protracted dose delivery which significantly increases overall treatment time. Cell survival indicated no difference between filter and filter free beam delivery in any of the protraction schemes. An increase in survival was seen in both modes upon protracting dose delivery to 15, 30 or 60 min rather than delivering acutely. Further, analysis of induced DNA double-strand breaks via the γH2AX assay showed no difference between filtered and unfiltered beams. The following study suggests that increasing dose rate is an acceptable manner of decreasing radiotherapy treatment time that does not have any detrimental effects on in vitro cell eradication.