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Biological Effect of Motion On Fractionated Dose Delivery: Statistical Analysis and Simulation

D Ruan

D Ruan1*, (1) UCLA School of Medicine, Los Angeles, CA

SU-E-T-294 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To analyze and validate with simulation with radiobiological effect of motion on dose delivery. To reveal the impact of motion pattern, dose modulation level and fractionation pattern on this effect.

Methods:Cell survival fraction was modeled according to the LQ. We derived the contribution from instantaneously received dose caused by movement by tracking the trajectory of each cell subvolume, and utilized a first order perturbation approach to analyze its integral effect. The local instantaneous dose was modeled as a nonstandard Gaussian process by taking interfraction setup as fractionwise constant and intrafractional motion to be normally distributed. The analysis was performed as a function of spatial location to account for variations in radiobiological property and dose pattern. We derived the expected value to the biological effect and revealed the factors and their impact on the motion-induced discrepancy from expected biological effect. Numerical simulation was performed to verify the analysis.

Results:The discrepancy between the motion-affected and expected radiobiological effect (in terms of biological effective dose) was revealed to scale quadratically with the local dose gradient, the standard deviation of the intra-fraction motion. The interfraction motion contributes to such discrepancy via both a linear bias and a quadratic noncentrality, indicting a persistent effect even if daily setup were random and averaged out in first order in physical dose. Late responding tissues with a small alpha/beta ratio are affected more significantly than early responding tissues. Simulation results verified these observations.

Conclusion:This analysis suggests that motion-induced discrepancy in biological effect (from expectation at planning) is most significant for late responding tissues located in the neighborhood of high dose gradient, when treated with a small number of fractions. This may explain the relatively high toxicity for treatment with SBRT and rationalizes the importance of localization accuracy and placement stability for such treatment.

Funding Support, Disclosures, and Conflict of Interest: funded in part by AACR and TRDRP.

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