Purpose: In breast radiotherapy with intensity modulation, it is a well established practice to extend the dose fluence outside the limit of the body contour to account for small changes in size and position of the target and the rest of the tissues due to respiration or to possible oedema. A simple approach is not applicable with RapidArc volumetric modulated are therapy not being based on a fixed field fluence delivery. In this study, a viable technical strategy to account for this need is presented.
Methods: RapidArc (RA) plans for six breast cancer patients (three right and three left cases), were optimized (PRO version III) on the original CT data set (0) and on an alternative CT (E) generated with an artificial expansion (and assignment of soft-tissue equivalent HU) of 10 mm of the body in the breast region and of the PTV contours toward the external direction. Final dose calculations for the two set of plans were performed on the same original CT data set O, normalizing the dose prescription (50 Gy) to the target mean. In this way, two treatment plans on the same CT set O for each patient were obtained: the no action plan (OO) and the alternative plan based on an expanded optimization (EO). Fixing MU, these two plans were then recomputed on the expanded CT data set and on an intermediate one (with expansion = 5 mm), to mimic, possible changes in size due to edema during treatment or residual displacements due to breathing not properly controlled. Aim of the study was to quantify the robustness of this planning strategy on dose distributions when either the OO or the EO strategies were adopted. For all the combinations, a DVH analysis of all involved structures is reported.
Results: I. The two optimization approaches gave comparable dose distributions on the original CT data set. II. When plans were evaluated on the expanded CTs (mimicking the presence of edema), the EO approach showed improved target coverage if compared to OO: on CT_10 mm, Dv = 98% [%]= 92.5 +/- 0.9 and 68.5 +/- 3.1, respectively, for EO and OO. Minor changes were registered in organs at risk sparing for both EO and OO. III. From dose distributions and DVHs, EO approach allowed to irradiate at near to prescription levels also the expanded fraction of the target: this would account also for residual intrafraction movements.
Conclusions: The proposed plan strategy could represent a robust approach to account for moderate changes in target or body volume during the course of breast radiotherapy and to account for residual intrafractional respiratory motion in volumetric modulated are therapy. The strategy, logistically simple to implement requiring only modifications to the standard planning workflow was routinely implemented at author's institute for treatment of breast patients with RapidArc.