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Phys Med Biol. 2013 Mar 7;58(5):N83-94. doi: 10.1088/0031-9155/58/5/N83. Epub 2013 Feb 11.

An in vitro study of the radiobiological effects of flattening filter free radiotherapy treatments.

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  • 1Radiotherapy Physics, Northern Ireland Cancer Centre, Belfast Health and Social Care Trust, BT9 7AB, UK.


Flattening filter free (FFF) linear accelerators allow for an increase in instantaneous dose-rate of the x-ray pulses by a factor of 2-6 over the conventional flattened output. As a result, radiobiological investigations are being carried out to determine the effect of these higher dose-rates on cell response. The studies reported thus far have presented conflicting results, highlighting the need for further investigation. To determine the radiobiological impact of the increased dose-rates from FFF exposures a Varian Truebeam medical linear accelerator was used to irradiate two human cancer cell lines in vitro, DU-145 prostate and H460 non-small cell lung, with both flattened and FFF 6 MV beams. The fluence profile of the FFF beam was modified using a custom-designed Nylon compensator to produce a similar dose profile to the flattened beam (6X) at the cell surface but at a higher instantaneous dose-rate. For both cell lines there appeared to be no significant change in cell survival. Curve fitting coefficients for DU145 cells irradiated with constant average dose-rates were 6X: α = 0.09 ± 0.03, β = 0.03 ± 0.01 and 6FFF: α = 0.14 ± 0.13, β = 0.03 ± 0.02 with a significance of p = 0.75. For H460 cells irradiated with the same instantaneous dose-rate but different average dose-rate the fit coefficients were 6FFF (low dose-rate): α = 0.21 ± 0.11, 0.07 ± 0.02 and 6FFF (high dose-rate): α = 0.21 ± 0.16, 0.07 ± 0.03, with p = 0.79. The results indicate that collective damage behaviour does not occur at the instantaneous dose-rates investigated here and that the use of either modality should result in the same clinical outcome, however this will require further validation in vivo.

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