MEASUREMENT OF PATIENT POSITIONING ERRORS IN 3-DIMENSIONAL CONFORMAL RADIOTHERAPY OF THE PROSTATE

Purpose/Objective: To determine the spatial distribution of setup erro rs for patients treated with six-field, three-dimensional (3D) conform al radiation therapy for prostate cancer. Methods and Materials: Port films for 50 patients were analyzed retrospectively. The port films we re digitized and compared, using image registration software, to simul ator films (representing the ideal treatment position). Patient positi oning uncertainty for a given setup was determined using port films fr om three projections, two obliques, and one lateral. A total of 1239 p ort films and 300 simulator films were analyzed for the study. Patient position was analyzed for out-of-plane rotations and time trends over the course of treatment. Results: The distribution of systematic setu p errors for the 50 patients, defined as the mean patient displacement for the treatment course, had a mean and standard deviation (SD) of ( -0.1 +/- 1.9) mm, (0.4 +/- 1.4) mm, and (-0.3 +/- 1.3) mm in the medio lateral (ML), superior-inferior (SI) and anterior-posterior (AP) direc tions, and (-0.1 +/- 0.2) for rotational errors. The distribution of r andom setup errors about the mean approximated a normal distribution a nd the standard deviations for the population of patients in the Mi,, SI, and AP directions, were 2.0 mm, 1.7 mm, and 1.9 mm, respectively. The distribution of out-of-plane rotations had 1 SD of 0.9 degrees and 0.6 degrees about the SI and AP axes. Ten of the 50 patients demonstr ated a statistically significant time trend in their setup position re sulting in shifts ranging from 2 to 7 mm. Conclusions: The setup verif ication protocol appears to minimize systematic setup errors to a leve l that approaches the sensitivity of the image registration technique. The random day to day fluctuations, represented by the average values of the standard deviations, are minor in comparison to the currently used margins, which further emphasizes the effectiveness of this proto col in conjunction with the use of the immobilization device. (C) 1997 Elsevier Science Inc.