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The Effects of Radiobiological Parameter Uncertainty On Theoptimal Radiotherapy Schedule

Y Watanabe

Y Watanabe1*, H Bardri2 , K Leder3 , (1) University of Minnesota, Minneapolis, MN, (2) Target Corporation, Minneapolis, MN, (3) University of Minnesota, Minneapolis, MN


WE-RAM3-GePD-TT-3 (Wednesday, August 2, 2017) 10:30 AM - 11:00 AM Room: Therapy ePoster Theater

Purpose: To study the effect of the uncertainty in the radiobiological parameters of the L-Q model on the total tumor dose and the total number of fractions by using a novel stochastic optimization algorithm of the treatment schedule.

Methods: An optimization algorithm of radiotherapy treatment schedule was developed by considering the uncertainty of the radiobiological parameters in the L-Q model, i.e., the alpha and beta values of both the tumor and the normal structures as well as the sparing fraction of normal structures. The objective function was given by the biologically effective dose (BED) with the effect of the cell repopulation. The dose constraints of the normal structures were such that BEDs of the optimized schedule were smaller than known tolerance BED for a 2Gy fraction (EQD2) with the probability p. For the radiotherapy of the head and neck, lung, and prostate, we studied the effect of the magnitude of the parameter uncertainty and the confidence level of the achievability of the optimization goal on the optimal treatment schedule.

Results: For the head and neck, we considered six healthy structures (spinal cord, brainstem, skin, mandible, larynx, and parotids). The data of the radiobiological parameters were taken from existing literature. As the probability p decreased, the total dose, D_total, and the number of fractions, N, decreased. As the standard deviations of the model parameters decreased, D_total and N also decreased.

Conclusion: By reducing the uncertainty of the radiobiological parameters, we can offer a treatment with a lower total dose, the smaller number of fractions, and a larger fraction size without compromising the tumor local control whilst maintaining the acceptable level of the normal organ toxicity. Furthermore, the total dose and the number of fractions can be decreased by accepting a higher probability of not meeting the normal structure dose constraints.

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