(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose:
To evaluate breast PBS proton plans in which pencil beams go through metal tissue expanders and develop an optimal treatment planning procedure for this scenario.
Methods:
We analyzed in total a set of 56 CBCT images of 2 patients with metal tissue expander implants. Tissue expander motion for each fraction was derived by comparing the expander positioning differences between each CBCT and planning CT. For patients with tissue expanders in place, a 2- to 4-field plan with multi-field optimization (rMFO) with robustness optimization was utilized. A field specific target for each field was created to avoid beams going through the expander for that field. For comparison, proton plans with beams going through the metal tissue expander were also created using a similar beam arrangement and optimization parameters.
Results:
Out of 56 CBCT images, the tissue expander in 47 CBCT images (84%) stayed within 2mm of the position in the planning CT. The overall average tissue expander moved 1.4mm; however, the maximum tissue expander movement observed was 4.5mm. We also observed mild tissue expander rotation during treatment. Comparing plans with and without beams going through metal tissue expanders, we found that target coverage and maximum dose to the left lung were similar, while the V20 to the left lung increased by 16%, maximum dose to the heart increased by 1.4%, and the mean dose to heart increased from 3.2Gy to 3.9Gy.
Conclusion:
As tissue expanders display internal motion, robustness optimization must be used for treatment planning, as without it target coverage can be reduced significantly. Based on these findings, we recommend using a 2- to 4-field plan with robustness optimization for PBS proton.
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