BACKGROUND AND PURPOSE: Compared to conventional external beam radiotherapy, IMRT requires significantly more time to deliver the dose. Prolonging dose delivery potentially increases DNA repair which would reduce the biological effect. We questioned whether retention of gammaH2AX, a measure of lack of repair of DNA damage, would decrease when dose delivery was protracted. MATERIALS AND METHODS: Exponentially growing SiHa cervical carcinoma cells were irradiated with 6 MV photons in a water tank using a VarianEX linear accelerator. Cells held at 37 degrees C received 2 Gy in 0.5 min and 4 Gy in 1 min. To evaluate effect of dose delivery prolongation, 2 and 4 Gy were delivered in 30 and 60 min. After 24 h recovery, cells were analyzed for clonogenic survival and for residual gammaH2AX as measured using flow cytometry. RESULTS: Increasing the dose delivery time from 0.5 or 1 min to 30 or 60 min produced a significant increase in cell survival from 0.45 to 0.48 after 2 Gy, and from 0.17 to 0.20 after 4 Gy. Expression of residual gammaH2AX decreased from 1.27 to 1.22 relative to background after 2 Gy and 1.46 to 1.39 relative to background after 4 Gy, but differences were not statistically significant. The relative differences in the slopes of residual gammaH2AX versus dose for acute versus prolonged irradiation bordered on significant (p = 0.055), and the magnitude of the change was consistent with the observed increase in surviving fraction. CONCLUSION: These results support the concept that DNA repair underlies the increase in survival observed when dose delivery is prolonged. They also help to establish the limits of sensitivity of residual gammaH2AX, as measured using flow cytometry, for detecting differences in response to irradiation.