: TO measure the patient positioning and prostate motion variability and to estimate its influence on the calculated 3D dose distribution in 3D conformal radiotherapy of patients with localized prostate carcinoma.
: Patient positioning variability was determined retrospectively by comparing weas determined by 107 computed tomography (CT) examinations with a CT simulator in 28 patients during radiotherapy.
: In each observed direction, the patient positioning variability and prostate motion showed a normal distribution. This observation enabled the calculation of a combined error of both components. The standard deviation (1 SD) of the patient positioning error in three directions ranged 3.1 to 5.4 mm; the prostate motion variability was significantly greater in the anterior-posterior direction (1 SD = 2.8 mm) than in the mediolateral direction (1 SD = 1.4 mm). The 1 SD of the estimated combined error was in the anterior-posterior direction 6.1 mm and in mediolateral direction 3.6 mm.
: The range of patient positioning variability and prostate motion statistically predictable under the patient setup cnditions used. Dose-volume histograms demonstrating the influence of the combined error of both components on the calculated dose distribution are presented.
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