Analysis and reduction of 3D systematic and random setup errors during thesimulation and treatment of lung cancer patients with CT-based external beam radiotherapy dose planning

Purpose: To determine the magnitude of the errors made in (a) the setup of patients with lung cancer on the simulator relative to their intended setup with respect to the planned treatment beams and (b) in the setup of these patients on the treatment unit, To investigate how the systematic component of the latter errors can be reduced with an off-line derision protocol for setup corrections. Methods and Materials: For 39 patients with CT planning, digitally-reconstr ucted radiographs (DRRs) were calculated for anterior-posterior and lateral beams. Retrospectively, the position of the visible anatomy relative to th e planned isocenter was compared with the corresponding position on the dig itized simulator radiographs using contour match software. The setup accura cy at the treatment unit relative to the simulator setup was measured for 4 0 patients for at least 5 fractions per patient in 2 orthogonal beams with the aid of an electronic portal imaging device (EPID), Setup corrections we re applied, based on an off-line decision protocol, with parameters derived from knowledge of the random setup errors in the studied patient group. Results: The standard deviations (SD) of the simulator setup errors relativ e to the CT planning setup in the lateral, longitudinal, and anterior-poste rior directions were 4.0, 2.8, and 2.5 mm, respectively. The SD of rotation s around the anterior-posterior axis was 1.6 degrees and around the left-ri ght axis 1.3 degrees. The setup error at the treatment unit had a small ran dom component in all three directions (1 SD = 2 mm), The systematic compone nts were larger, particularly in the longitudinal direction (1 SD = 3.6 mm) , but were reduced with the decision protocol to 1 SD < 2 mm with, on avera ge, 0.6 setup correction per patient. Conclusion: Setup errors at the simulator, which become systematic errors i f the simulation defines the reference setup, were comparable to the system atic setup errors at the treatment unit in case no off-line protocol would have been applied. Hence, the omission of a separate simulation step can re duce systematic errors as efficiently as the application of an off-line cor rection protocol during treatment. The random errors were sufficiently smal l to make an off-line protocol feasible, (C) 2001 Elsevier Science Inc.