Time, gravity, and the relativity of tumor position in free-breathing lung stereotactic body radiation therapy

Stereotactic body radiation therapy (SBRT) has transformed the treatment of early-stage non-small cell lung cancer (NSCLC) and continues to become more precise as imaging and dose-delivery techniques evolve, supporting high rates of local control [1,2]. Advanced motion-management strategies, such as respiratory gating or real-time tracking decrease the dose to the surrounding normal tissue but may increase treatment time and, depending on the technique, require fiducial marker placement. Consequently, many patients are still treated in free breathing in combination with an internal target volume concept because of the inherent simplicity of this approach [3,4].