ESTRO 2024 - Abstract Book

S4264

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

1467

Digital Poster

Markerless tracking of tumor location for SBRT of sarcoma and melanoma metastases to the lung

Xin Wang 1 , Sara Lynn Thrower 1 , Dershan Luo 1 , Ahsan S Farooqi 2 , Devarati Mitra 2 , Bishop J Andrew 2 , Ashleigh Guadagnolo 2 1 MD Anderson Cancer Center, Radiation Physics, Houston, USA. 2 MD Anderson Cancer Center, Radiation Oncology, Houston, USA

Purpose/Objective:

CBCT is widely used for initial alignment in lung Stereotactic Body Radiation Therapy (SBRT). It provides 3D anatomy with high contrast visibility of tumor. However, CBCT is usually performed with the couch in a center position to avoid clearance issues and therefore is not suitable for real-time verification. Other imaging modalities which can provide real-time tracking of the target in the final treatment position during beam delivery are desired. The purpose of this study is to investigate the visibility of sarcoma and melanoma lung metastases using orthogonal kV/MV imaging along with an ExacTrac X-ray system.

Material/Methods:

At our institution, orthogonal image pairs have been used to verify the target position after the couch has been shifted to the final treatment location according to CBCT alignment. The final isocenter location can be assessed in the 2D projections using nearby bony structures and their relative position to soft tissue target revealed in CBCT, or the best with target itself when visible. In this study, we analyzed the dependency of tumor visibility on 2D image modality, tumor volume, location, and density. The cohort includes 77 cases of lung metastases from sarcoma and melanoma primaries treated with SBRT since 2021. Among them MV(AP)-MV(LAT), MV(AP)-kV(LAT) and kV(AP)-MV(LAT) image pairs were acquired for 41, 24, and 12 cases respectively. The results were used to optimize the orthogonal pair usage in our workflow. In addition, we assessed the feasibility of using Brainlab ExacTrac X-ray system to perform real-time verification of tumor alignment when treating with Deep Inspiration Breath Hold (DIBH). Our scope includes identifying the predictors of tumor visibility in ExacTrac kV image and the ability to verify tumor within PTV margin in current version of ExacTrac® and ExacTrac Dynamic®.

Results:

Tumors were visible in 49% (32 out of 65) of the cases in the AP projection of MV images. Visibility reduced to 17% (9 out of 53) in the LAT MV ports. kV images showed similar trend. Tumor’s visualization in the AP and LAT direction of kV images were 33% (4 out of 12) and 13% (3 out of 24) respectively. For both MV and kV, tumor was visible in LAT only when it was also visible in AP. We identified certain characteristics on tumors that were visible on kV imaging, including average HU > -70; volume > 10 cc, and separation from mediastinum, diaphragm, and chest wall. Using these criteria, we identified 5 DIBH cases, where tumor was likely to be visible on the ExacTrac X-ray system. Tumors were clearly visible in at least 1 panel in 4 of the 5 cases (80%), and in 2 of the 5 cases(40%) tumor was visible in both panels. This allowed for intrafractional assessment of treatment position relative to PTV alignment.