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Preliminary Analysis of Patient Setup Uncertainty and Intra-Fraction Motion at UT Southwestern Medical Center

F Kalantari*, M Yang , UT Southwestern Medical Center, Dallas, Texas


(Sunday, 7/29/2018) 4:00 PM - 4:30 PM

Room: Exhibit Hall | Forum 5

Purpose: Patient setup during radiation treatment is an important source of uncertainty that should be incorporated as a margin into treatment planning for optimal outcome. There are different methods of defining margins based on the random and systematic errors calculated from patient setup data. In UT Southwestern, therapists have been recording the shifts made from initial setup based on image guidance (kV-kV or CBCT). The purpose of this study is to revisit this large database for clinical insights and verify current margin used at UT Southwestern is sufficient.

Methods: The setup shifts of 1200 patients (240 SBRT and 960 non-SBRT) were analyzed. For each patient the shift values along all three directions were recorded for each fraction. For SBRT patients the intra-fraction shift values were also analyzed. The setup random and systematic errors in each direction were calculated based on the method as defined by Van Herk. Proper margins were calculated according to the Van Herk formula for each type of data.

Results: Our preliminary analysis showed the accuracy and reproducibility of initial setup based on patient skin marks and fiducials is relatively low for both non-SBRT and SBRT setups. Both the systematic and random errors along all three directions are in the range of 2 to 3mm, which resulted in a margin of 1 – 1.5cm. Surprisingly, the SBRT frame used in SBRT cases did not achieve better setup reproducibility when compared to typical IMRT setups. For SBRT cases, the intra-fraction motion resulted in less than 3mm margin in all directions which falls within our current margin of 3-5mm for SBRT patients.

Conclusion: Our results highlighted the crucial role of image guidance for patient setup in minimizing the associated uncertainty. In future studies we plan to evaluate other factors that can affect setup reproducibility such as user registration bias.


Systematic Errors, Setup Errors


Not Applicable / None Entered.

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