Background: Increasing the complexity in modern radiotherapy techniques have increased the delivery time lowering consequently the treatment efficacy. Through simulating the delivery time delay encountered in such techniques, its’ effect on two cancer cell lines and the compensating doses given to prevent such effect was investigated. Materials and Methods: F10B16 and 4T1 cancer cell lines were exposed to simulated clinical fractionated radiotherapy procedures commonly used in complex techniques. The survival rate of the cells exposed to 2, 4, and 6 Gy of ionizing radiation with two equal subfractions given at various time intervals between the fractions (0.25-4 hours) were determined using the MTT assay. Then, relevant compensating doses were calculated and their efficacy in counterbalancing the time delay was assessed. Results: The cells’ survival was increased with prolonged treatment times in the fractionated groups being more significant at the lower time intervals (up to 2 hours) and for the higher radiosensitive cells (4T1). Giving the compensated doses decreased the survival of the cells. Conclusion: Delivering appropriate compensating doses to the prolonged fractionated groups can counterbalance the effect of time delays encountered in complex radiotherapy techniques.