An assessment of the number of CT slices necessary to plan breast radiotherapy

Aims: The aim of this study was to evaluate the number of CT slices required to produce satisfactory dose distribution for tangential field irradiation of the chest wall and breast and to assess correlation of this with the volume of breast tissue treated. Forty-six patients underwent a CT scan of the thorax. An optimized plan was produced by assessing dose distribution on the central axis (CAX) slice only. This plan was then recalculated using the entire CT data set without any changes to the beam parameters. A separate optimized plan was generated using the CAX slice and two slices indicative of the upper and lower level of the field. This three-slice plan was then calculated using the entire CT data set. Finally an optimized 3D plan was generated using the entire CT data set. The different planning methods were compared using dose-volume histograms (DVH). Dose inhomogeneity was defined as any treatment volume outside the ICRU 50 dose distribution recommendations. Results: Fifty-two percent of single-slice plans and 21% of three-slice plans (when assessed volumetrically) had greater volumes of breast tissue outside the ICRU 50 report guidelines suggesting that better homogeneity could be achieved by assessing a greater number of slices. Seventy-nine percent of three-slice plans showed no homogeneity improvement if the plan was calculated with the entire 3D data set. Conclusions: We conclude that a single-slice plan is unsatisfactory in providing sufficient information about the dose variation across the treatment volume and that ideally a 3D plan with DVHs should be produced. If the required data is unavailable then a minimum of three slices should be used as an approximation. We also propose a software tool for treatment planning systems, which calculates the percentage of the total PTV having dose outside the ICRU 50 radiation dose distribution homogeneity guideline range. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.