Physics Contribution: Investigating the Temporal Effects of Respiratory-Gated and Intensity-Modulated Radiotherapy Treatment Delivery on In Vitro Survival: An Experimental and Theoretical Study

Purpose: To experimentally and theoretically investigate the temporal effects of respiratory-gated and intensity-modulated radiotherapy (IMRT) treatment delivery on in vitro survival. Methods and Materials: Experiments were designed to isolate the effects of periodic irradiation (gating), partial tumor irradiation (IMRT), and extended treatment time (gating and IMRT). V79 Chinese hamster lung fibroblast cells were irradiated to 2 Gy with four delivery methods and a clonogenic assay performed. Theoretical incomplete repair model calculations were performed using the incomplete repair model. Results: Treatment times ranged from 1.67 min (conformal radiotherapy, CRT) to 15 min (gated IMRT). Survival fraction calculations ranged from 68.2% for CRT to 68.7% for gated IMRT. For the same treatment time (5 min), gated delivery alone and IMRT delivery alone both had a calculated survival fraction of 68.3%. The experimental values ranged from 65.7% ± 1.0% to 67.3% ± 1.3%, indicating no significant difference between the experimental observations and theoretical calculations. Conclusion: The theoretical results predicted that of the three temporal effects of radiation delivery caused by gating and IMRT, extended treatment time was the dominant effect. Care should be taken clinically to ensure that the use of gated IMRT does not significantly increase treatment times, by evaluating appropriate respiratory gating duty cycles and IMRT delivery complexity.