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Treatment simulation is usually performed with a conventional simulator using kV X-rays or with a computed tomography(CT) simulator before the treatment course begins. The purpose is to verify patient setup under the same conditions as for treatment planning. Systematic (preparation) setup errors can be introduced by this process. The purpose of this study is to characterize the setup errors using electronic portal image (EPI) analyses and to propose a method to reduce the systematic component by performing simulation and patient preparation on the treatment machine. In this study, the first four or five days EPIs were analyzed from a total of 533 prostate cancer patients who were simulated on conventional simulators. We characterized setup errors using four parameters: { M ( μ i ) , Σ ( μ i ) , RMS ( σ i ) , Σ ( σ i ) } , where μ i and σ i are individual patient mean and standard deviation, M, Σ , and RMS are the mean, standard deviation, and root-mean-square of underlying variables ( μ i and σ i ). We have performed a simulation of removing systematic components by correcting the first day setup error. As a comparison, we also carried out a similar analyses for patients simulated on a CT simulator and patients treated on a linac with an on-board kV CTimaging system, although a limited number of patients were available in these two samples. We found that Σ ( μ i ) = ( 2.6 , 3.4 , 2.4 ) mm , and RMS ( σ i ) = ( 1.5 , 1.9 , 1.0 ) mm in lateral, anterior/posterior, and cranial/caudal directions, indicating that systematic errors are much larger than random errors. Strong correlations were found between measurement on the first day and μ i , implying the first day’s measurement is a good predictor for μ i . The same parameters were also computed for days 2–4, with and without the first day correction. Without correction, M ( μ i ) 2 – 4 = ( 0.7 , 1.6 , − 1.0 ) mm , and Σ ( μ i ) 2 – 4 = ( 2.6 , 3.5 , 2.4 ) mm . With correction, M ( μ i ) 2 − 4 = ( 0.0 , 0.4 , 0.4 ) mm , much closer to zero, and Σ ( μ i ) 2 – 4 = ( 1.8 , 2.2 , 1.2 ) mm , also much smaller. While the use of a CT simulator can reduce the systematic errors, the benefits of first day correction can still be observed, although at a smaller magnitude. Therefore, the systematic setup error can be significantly reduced if the patient is marked and fields are verified on the treatment machine on the first fraction, preferably with an on-board kV imaging system.

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.