Abstract
Aim: In prone breast treatments, a carbon fiber support device resides under the contralateral breast. Tangent beams are designed to encompass the treated breast and these often pass through the board at a shallow angle, resulting in significant attenuation. Our planners account for this attenuation by adding field-in-field dose to the deep part of the breast, through the board. Concern was raised about how accurate the treatment delivery is when the inherent uncertainties of patients’ positions are accounted for. Furthermore, transmission measurements are usually carried out perpendicular to the board, a non-clinical situation. The goal of this study is to evaluate the dosimetric effect of the board and the robustness of the plan to positional uncertainty. Materials and Methods: Twenty-two breast patients treated on a commercial prone breast board between 2017 and 2020 were selected for this retrospective study. To evaluate the board’s attenuation, we compared the plans with the board removed from the dose calculation. To quantify the robustness of this technique, we moved the beam isocenter with respect to the patient and board. Results: Our results showed that when the breast board is removed from a plan which was designed to account for the board attenuation, the average point dose increases by 7.48%, with a maximum of 22%. Comparing results with a mixed Analysis of Variance (ANOVA) and a least-square means analysis, our robustness evaluation indicates that anterior shifts at every magnitude (1 mm through 5 mm) make a significant difference in all dose statistics (D95, max, 95% prescription coverage and homogeneity index) investigated. In/out and right/left shifts resulted in an insignificant change in dose statistics. Conclusion: Prone breast boards can add significant dosimetric uncertainty into the treatment delivery process. Accounting for plan robustness in the design of the plan is highly recommended. A prone breast board design with support moved away from the beam path is warranted.
Highlights
The introduction of prone breast treatment in the US stems from an investigation into an alternative technique for irradiating the breast following breast-conserving surgery
Our results showed that when the breast board is removed from a plan which was designed to account for the board attenuation, the average point dose increases by 7.48%, with a maximum of 22%
We investigated prone breast board attenuation and prone breast treatment plan robustness for a commercially available Bionix breast support system
Summary
The introduction of prone breast treatment in the US stems from an investigation into an alternative technique for irradiating the breast following breast-conserving surgery. Tangent beams are used to deliver a uniform dose to the ipsilateral breast [1]-[9] as it is a simple planning technique to design and treat, intensity modulation has been successfully used [10] [11]. These tangent beams often pass through the prone breast board, resulting in beam attenuation dependent on the angle of the beam with respect to the board (Figure 1). It is accounting for the attenuation that poses the difficulty with this technique
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