2677 - Novel Use of SCOUT^® Reflector for Target Localization in Preoperative Breast Radiotherapy

N. Ghassemi¹, D. Abul-Enin¹, K. Gerber¹, M. Lazar², S. Burke¹, P. R. Anne³, Y. Vinogradskiy⁴, J. P. Shames¹, S. Wan⁵, and N. L. Simone⁴; ¹Thomas Jefferson University, Philadelphia, PA, ²Department of Surgery, Sidney Kimmel Medical College & Cancer Center at Thomas Jefferson University, Philadelphia, PA, ³Department of Radiation Oncology, Sidney Kimmel Medical College & Cancer Center at Thomas Jefferson University, Philadelphia, PA, ⁴Department of Radiation Oncology, Sidney Kimmel Medical College at Thomas Jefferson University, Philadelphia, PA, ⁵Department of Radiation Oncology, Sidney Kimmel Medical College at Thomas Jefferson University, Philadephia, PA

Purpose/Objective(s): Numerous trials have demonstrated the benefit of partial breast preoperative radiation to produce tumor shrinkage to decrease the extent of surgery and increase the likelihood of negative margins with surgery, and to minimize long-term cosmetic effects of radiation. One of the main challenges of preoperative partial breast radiation is localizing the tumor. Prior studies have placed fiducial markers in the tumor bed, but this is an extraneous procedure just for radiation localization. For the first time, we sought to determine the novel use of the SCOUT^® reflector, a radar localization system used by ~40% breast surgeons nationwide for surgical localization, to optimize target localization in preoperative breast radiation delivery. Materials/

Methods: Prior to CT simulation, a SCOUT^® reflector (diameter=1.6 mm, length=12 mm) was placed by a radiologist under ultrasound or mammographic guidance. Contours for tumor and normal structures were created as per standard of care, with the reflector contoured with a 3 mm margin. A plan was developed to treat the partial breast PTV to 40Gy in 5 fractions every other day delivered by a VMAT dual-partial-arc plan. Cone beam Computed Tomography (CBCT) was used for initial alignment and real-time kV images (RTI) images were taken every 20 degrees of gantry rotation through the arc. The study evaluated the average distance between the centroid of the reflector+3mm contour and the center of each reflector.
Results: This study conducted a comprehensive analysis of real-time imaging data from preoperative radiation therapy, reviewing a total of 292 images. Upon combining the datasets from patients, the mean real distance between the centroid of the reflector+3mm contour and the localized reflector was 1.87 ± 1.01 mm. Applying Van Herks formula to the aggregate data yielded a margin of 5.34 mm, which indicates the necessary buffer to ensure accurate localization within the margin of safety. The one-sample t-test of the collective data resulted in a highly significant level of precision in the localization method (p < 0.05). These results reinforce the reliability of the SCOUT^® reflector for target tracking in image-guided radiation therapy (IGRT). Post-treatment, the reflector was successfully detected in all surgical procedures, validating its clinical utility.
Conclusion: For the first time, this study demonstrates the SCOUT^® reflector to be a robust tool for enhancing the accuracy of tumor localization in preoperative breast radiation therapy. Real-time imaging demonstrated this technique enabled precise targeting, monitoring, and intrafractional adjustment. Future studies should consider the use of SCOUT^® reflectors for preoperative breast tumor localization which may allow for smaller PTV margins in the future.