Sci‐Sat AM (1) General‐04: Quantitative impact of 3D inter fractional patient setup errors on TCP/NTCP

Patient setup errors in external beam radiation therapy are a cause for differences between planned dose distribution and the one delivered to the patient. A changed dose distribution results in a changed tissue response, expressed as tumor control probability (TCP) and normal tissue complication probability (NTCP). A method was developed to calculate the changes of TCP and NTCP resulting from measured patient setup errors. Patient position was measured with a 2D‐3D registration method using orthogonal EPID images of the patient in treatment position. Setup errors were expressed as 6 degrees of freedom (rigid body translations and rotations); non‐rigid transformations and internal organ motion was not considered. The measured setup errors were used to change beam angles and couch position in a copy of the original treatment plan, such that an equivalent beams‐eye‐view of the patient was achieved for each treatment beam. The dose distribution for this modified treatment plan was then calculated. This procedure was repeated in additional copies of the original treatment plan for each fraction in which patient position had been measured. Once the treatment was completed, dose distributions for all measured fractions were added and the dose‐volume‐histograms (DVHs) were calculated for each region of interest. The DVHs in turn served as a basis for the calculation of TCP and NTCP. This method was applied to two prostate patients, whose position was measured during six fractions of their treatment. The setup errors did not change the TCP, but NTCP of the rectum increased by 9.0% and 2.2% respectively.