# Initial Experience with Correlation Object–Based DRR Targeting Using Stereoscopic X-Ray Imaging in Lung SBRT

## TL;DR

This study explores using X-rays with a surrogate-based method to improve tumor targeting during lung cancer radiotherapy, showing promising results for real-time monitoring.

## Contribution

The study introduces the use of Correlation Objects for markerless tumor targeting in lung SBRT using X-rays, demonstrating consistent geometric accuracy.

## Key findings

- Surrogates improved tumor visibility on X-ray/DRR fusions from 14.3% to 75.5%.
- Geometric accuracy showed 76% of data points deviated less than 5 mm in 3D vector measurements.
- Tumor location influenced visibility, with upper lobe lesions more visible than lower lobe lesions.

## Abstract

Lung tumors are rarely visible on X-rays due to their low soft-tissue contrast; however, X-rays are easy to acquire and substantially faster to acquire than conventional cone beam computed tomography. This study investigates whether a surrogate-based strategy using Correlation Objects can facilitate the use of X-rays for positioning and intra-fraction monitoring during markerless stereotactic body radiotherapy of lung tumors. Retrospective analysis of 63 X-ray pairs provides new insights into the application of surrogate-based digitally reconstructed radiographs and demonstrates promising potential for prospective use. The implementation of this surrogate-based approach could represent a significant advancement for patients eligible for stereotactic body radiotherapy of lung tumors.

Background/Objectives: Despite significant advances in imaging technology, real-time intra-fraction monitoring of moving targets remains a challenge in markerless radiotherapy. This retrospective study investigates the use of ExacTrac Dynamic by Brainlab as an intra-fraction monitoring tool for stereotactic body radiotherapy (SBRT) in both early-stage NSCLC and oligometastatic disease. Methods: A total of 63 X-ray pairs from 21 patients were analyzed to evaluate tumor visualization with and without a surrogate approach. Statistical analysis was conducted to determine whether failures could be attributed to tumor size or localization using the Mann–Whitney U-test and Fisher’s exact test. The accuracy of the X-ray/digitally reconstructed radiograph (DRR) surrogate-based fusion was assessed by calculating and comparing the corresponding 3D vectors according to the linear mixed effects model, with a random slope effect for size of surrogate and a random intercept per patient. Results: Surrogates enhanced tumor visualization on X-ray/DRR fusions from 14.3% to 75.5%. Tumor size and lung affected (left or right) did not predict visualization success. Tumor location, however, tended to influence visibility, with lesions in the upper lobes being more readily visualized (88%) than those in the lower lobes (48.1%), although no statistical significance was reported (p > 0.05). Regarding geometric accuracy, 76% of the analyzed data points deviated less than 5 mm in the 3D vector measurements, the mean values were around 4 mm (±3 mm), and the medians were within 3 mm across all conditions. No statistically significant differences (p > 0.05) were found based on the surrogate size or the triggering time of the X-ray during the breathing cycle. Conclusions: Surrogate-based DRRs, referred to as Correlation Objects, demonstrate consistent geometric accuracy across multiple surrogate sizes and X-ray acquisitions, supporting the clinical translation of markerless lung targeting workflows for lung SBRT.