Comparing dosimetric performance of robust and planning target volume-based optimization in photon lung stereotactic radiotherapy

Abstract

Background: Stereotactic body radiation therapy (SBRT) is an effective treatment for lung tumors but it is challenged by respiratory motion and setup uncertainties. These are traditionally managed by optimizing on a planning target volume (PTV). Robust optimization offers an alternative by accounting for uncertainties through error scenarios. Although well established in proton therapy, its application in photon-based lung SBRT has mostly been limited to setup uncertainties and internal target volume (ITV)-based planning. This study is the first to evaluate robust optimization applied directly to the gross tumor volume (GTV) in combination with tracking modalities of the CyberKnife installation. Purpose: This study evaluates the benefit of using robust optimization combined with CyberKnife tracking modalities on ipsilateral lung dose in lung SBRT. Methods: CyberKnife (Accuray Inc., USA) lung plans were optimized using both PTV-based and robust approaches. Two tracking modalities were evaluated: Synchrony (real-time respiratory motion compensation) and x-sight spine (static tracking). The robustness of the plans was evaluated by simulating setup uncertainties and tumor motion using 4D-CT. GTV coverage and lung dose metrics—V20Gy, V5Gy, mean lung dose (MLD)—were compared across uncertainty scenarios. Results: Both planning methods achieved more than 99.5% of scenarios meeting the GTV coverage constraint for both tracking modalities. Robust optimization reduced lung dose. The V20Gy was reduced from 4.5% to 3.2% for Synchrony and from 9.3% to 6.9% for x-sight spine. The V5Gy decreased from 22.3% to 18.0% for Synchrony and from 35.0% to 30.2% for x-sight spine. The MLD was reduced with robust optimization by 0.6 Gy for Synchrony and 1.3 Gy for x-sight spine. Conclusions: This study demonstrates the benefit of combining robust optimization with tracking techniques in photon lung SBRT for reducing lung dose and introduces a novel treatment planning strategy with the potential to improve both dosimetric outcomes and clinical consistency.