SU‐E‐T‐380: Evaluation of Interfraction Motion of the Strut‐Adjusted Volume Implant (SAVI) Using 3D Reconstruction from CT Scout Images

Purpose: The Strut‐Adjusted Volume Implant (SAVI) is a partial breast irradiation applicator. We developed a method to reconstruct the 3D device location using scout images to provide applicator position and proper expansion verification. We also used this technique to evaluate interfraction motion. Methods: The SAVI device was implanted in a lumpectomy cavity. The patient was aligned by CT lasers and skin tattoos to ensure reproducible setup. A post‐operative CT scan was performed for treatment planning. The patient was treated in 10 fractions over the course of 5 days. Daily CT scans and anterior and lateral scout scans were acquired prior to each fraction. Radio‐opaque markers located on three of the struts were localized using a peak detection filter. The location of each marker on the 2D scout image was backprojected towards the CT x‐ray source. Each 3D marker position was reconstructed at the backprojection intersection. The 3D marker position was compared to the location in the 3D CTimage. The interfractional displacement of the device was assessed from the reconstructed marker locations. Results: The average distance (standard deviation) between the marker positions reconstructed using the scout images and the CTimages was 0.76 (0.28) mm. Using the scout image data, the average interfractional device movement (standard deviation) in the SI, AP, and LR directions, and 3D was 0.51 (0.46) mm, 0.95 (0.81) mm, 0.73 (0.61) mm, and 1.56 (0.68) mm. Conclusion: SAVI interfraction motion can be accurately measured using scout images. The patient setup for partial breast brachytherapy can be improved by correcting the applicator displacement. This proposed technique eliminates the need for CT verifications, and therefore additional dose to the patient, while still accurately identifying applicator displacement.