REPRODUCIBILITY OF FIELD ALIGNMENT IN DIFFICULT PATIENT POSITIONING

Purpose: Quantitative assessment of the accuracy of field alignment in a homogeneous group of patients with difficult positioning (postopera tive irradiation after total hip replacement). Methods and Materials: In 95 patients linear and rotational discrepancies were measured betwe en the simulation and first check him and between five consecutive ver ification films. Results: For the total group of patients, all deviati ons were normally distributed with mean values of approximately zero a nd standard deviations of 4.0-8.0 mm (linear discrepancies) and 3.5-5 degrees (rotational discrepancies). Deviations were similar for the tr ansition from simulator to the treatment machine and for subsequent tr eatment delivery, with 50% and 95% of absolute differences being less than 5 mm and 15 mm, respectively. Conclusions: Our analysis indicates that statistical fluctuations are considerably more important than er rors introduced at start of treatment. Therefore, a first check film s eems to be inadequate to predict the expected inaccuracies for the who le course of treatment. In addition, our results should help to prescr ibe appropriate safety margins for patients with difficult positioning .