Organ/patient geometric variation in external beam radiotherapy and its effects.

Treatment variation in positioning of the organ/patient with respect to the radiation beams causes a temporal dose variation in critical normal tissues adjacent to the treatment target. This temporal variation induces uncertainties in understanding the normal tissue dose response, thereby limiting reliable treatment evaluation and optimization. The aim of this study is to model and analyze the temporal variation of organ dose distribution, and its effect on the biological effective dose. The study mainly focuses on the temporal dose variation caused by intertreatment organ motion/ deformation and daily setup error. Sensitivity of the biological effective dose to organ/patient geometric variation, dose distribution, and treatment fractionation will be investigated. Significant deviation of the biological effective dose could be expected in a critical normal structure, even if the cumulative dose deviation in this structure is negligible. Patients with similar geometric variation characteristics can experience significantly different biological effective dose, and the differences are sensitive to the dose distribution and the total number of treatment fractions.