W Xiong*, H Lin, A Zhai, M Kang, G Yu, I Choi, New York Proton Center, New York, NY

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  W Xiong

TU-F115-IePD-F6-1 (Tuesday, 7/12/2022) 1:15 PM - 1:45 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 6

Purpose: To quantitively evaluate Pencil Beam Scanning (PBS) proton plans with pencil beams passing through metal tissue expanders and develop an optimal treatment planning procedure for this scenario.

Methods: We analyzed 278 CBCT images from 11 patients with metal tissue expander implants treated with PBS proton therapy. Tissue expander motion for each fraction was derived by comparing the expander positioning difference between CBCT and planning CT images. For patients with tissue expanders in place, a 2- to 4- field plan with multi-field optimization (MFO) robust optimization was utilized. A field-specific target for each field was created to avoid beams passing through the expander for that field. For comparison, proton plans with beams passing through the metal tissue expander were also created using similar beam arrangements and optimization parameters. Dose evaluation on expander motion was performed on a set of CT images with different expander locations.

Results: Out of 278 CBCT images, the tissue expander in 210 CBCT images (76.4%) stayed within 2mm of the position in the planning CT, and there was a 9.4% chance that expanders moved >5mm. The maximum displacement was 14mm in the lateral direction and 7mm in the vertical direction. Dose evaluation demonstrated similar heart and lung doses with expander motion <10mm. Target coverage D95% was also consistent with planned values, while CTV Dmax increased from 113.6% to 119.6% with expander motion >10mm.

Conclusion: As some proportion of treatment images demonstrate significant tissue expander interfraction migration, robustness optimization and specific planning considerations must be taken into account to ensure consistent sufficient target coverage. This study also demonstrates that using more fields, i.e., 2- to the 4-field arrangement, can help in maintaining acceptable coverage for PBS proton breast treatment with tissue expanders.

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