Tangential breast irradiation: comparison of patient positioning and beam parameters after planning investigation at a CT scanner and a simulator with CT option.

Purpose: A prerequisite for the exact planning of a radiotherapy is the precise accordance of the patient positioning during the planning CT and the therapy simulation. How far possible differences between patient set-up during the planning CT at a therapy simulator with CT option or a conventional CT influence the planned dose distribution is investigated by the example of the tangential breast irradiation.
Patients and Methods: In 18 patients transverse sections for treatment planning of tangential breast irradiation were made both using a diagnostic CT scanner and a therapy simulator with CT option. For patient positioning the patient outlines were compared. Furthermore, the treatment plans which were based independently on both transverse sections were compared.
Results: In the mean deviations of 1 resp. 0 mm (SD = 7 resp. 8 mm) between the ventral and lateral outlines of the breast were found. The mean breast diameter between the Z-points was 5 mm wider in the simulator CT based plans (SD = 14 mm). The field width was 6 mm wider (SD = 8 mm), the beam angle 1 degree steeper (SD = 4 degrees). The calculated monitor units differed 1% (SD = 6%). None of the differences between measured parameters reached statistical significance. A significant correlation was only seen for the interrelated contours (anteroposterior and lateral diameters).
Conclusion: The results of this investigation show no difference between treatment set-up and treatment planning using information of a CT scanner or a simulator with CT option for tangential breast irradiation.