The effect of dose rate on the real-time change in optical density (DeltaOD) of a GAFCHROMIC EBT film is quantified using a previously reported optical readout approach. A range of doses (5-1000 cGy) and dose rates (16-520 cGy min(-1)) are used, and a statistically significant difference between DeltaOD of films exposed at different dose rates occurs within approximately one order of magnitude change in the dose rate. A small increase in per cent standard deviation of measured DeltaOD values is also observed when the entire dose rate range was used, but in all cases combining all DeltaOD values produces per cent standard deviation of <4.5%. Thus, whether the dose rate effect is clinically significant depends on the specific application of EBT and the desired accuracy.