Author: Darren Kaplan, Chenguang Liu, Mark Pankuch 👨🔬
Affiliation: Northwestern Medicine Proton Center 🌍
Purpose: This study evaluates the hypothesis that surface-guided radiation therapy for the breast or chest wall using proton PBS provides comparable target coverage and OARs sparing to x-ray-based IGRT.
Methods: A total of 10 patients who received breast or CW proton PBS were included in this study. Each patient underwent routine volumetric CT rescans, which were registered to the planning CT using both bony landmarks and nearby skin. The registration mimics the two different setup techniques: orthogonal X-ray films and surface imaging. The approved plan was recalculated on the CT rescans based on both registration methods. Target coverage, organ-at-risk (OAR) doses, and the offsets between the two registration methods were compared. Patients with metal expanders were excluded from the study.
Results: The CTV eval V95% Rx was 99.1% on the planning CT, compared to 98.5% on the rescan registered to bony anatomy and 98.6% on the rescan registered to skin. The CTV SCL V95% Rx was 94.8% on the planning CT, compared to 94.3% on the rescan registered to bony anatomy and 95.6% on the rescan registered to skin. The CTV IMN V95% Rx was 99.9% on the planning CT, compared to 99.3% on the rescan registered to bony anatomy and 99.8% on the rescan registered to skin. The ipsilateral lung V20 Gy Rx was 12.7% on the planning CT, compared to 13.2% on the rescan registered to bony anatomy and 13.4% on the rescan registered to skin.
Conclusion: Despite an average 3.1 mm shift between orthogonal X-ray film registration and surface-based registration, the dosimetry results indicate that surface-based registration offers comparable target coverage and OAR sparing to the X-ray orthogonal film-based setup in proton PBS plans. It is noted that this analysis does not account for setup uncertainty.