SU-E-T-09: Empirical Evidence for Decreased Cell Survival in Flattening Filter Free Beams

To present empirical evidence for a decrease in cell survival with the introduction of markedly increased average dose rates in flattening filter free beams.

Data of (Bewes et al., 2008) are used to calculate a modification factor for cell survival curves, using the linear quadratic model, that is inversely proportional to the reciprocal of the average dose rate.

The survival curve becomes steeper as the average dose rate increases above a ∼600 cGy/min threshold. For average dose rates below this threshold, no significant effect or even an increase in cell survival was observed (as explained by the Lea-Catcheside dose protraction factor). For a maximum assumed dose modification factor of 1.2 a decrease in cell survival of 3.5% for a dose of 2Gy and 36% for a dose of 8 Gy is predicted for a MM576 cell line. For a maximum assumed dose modification factor of 1.4 a decrease in cell survival of 60% for a dose of 2Gy and 1 order of magnitude for a dose of 8 Gy is predicted for a NCI-H460 cell line. For a dose modification factor of 2.65 with a dose rate of 24 Gy/min, a decrease in cell survival of 1 order of magnitude at 2Gy and 5 orders of magnitude at 8 Gy is predicted for a NCI-H460 cell line if it is assumed that the inverse proportionality is true.

Although no effects on cell survival was seen previously with clinically achievable dose rates (except for enhanced cell survival through repair for increased treatment times), it might become more evident with the increase of the average dose rate in flattening filter free beams. This will necessitate revision of fractionation schemes and maximum tolerable doses for organs at risk for stereotactic body radiation therapy.