Author: Hulya Ozdemir Buss, Julius G. Ojwang, Neelu Soni 👨🔬
Affiliation: Mercy Hospital Springfield 🌍
Purpose: The CyberKnife (CK) Synchrony system provides sub-millimeter motion-tracking accuracy during stereotactic body radiotherapy (SBRT). For lung tumors near the diaphragm, we evaluated the feasibility of using the liver dome, a diaphragmatic region that moves synchronously with respiration, as a surrogate for real-time tumor motion tracking. The goal is to maintain adaptive motion management while reducing radiation exposure to healthy tissues.
Methods: Patients eligible for lung SBRT with tumors near the diaphragm were scanned using the CyberKnife (CK) lung protocol on a Philips Brilliance BigBore CT simulator with 1.5 mm slice thickness. Four-dimensional CT (4DCT) scans confirmed that liver dome motion correlated with tumor motion during respiration. The liver dome, comprising approximately 13 slices from its tip, was contoured as the Tumor Tracking Volume (TTV) on breath-hold exhale CT datasets and exported to the Precision Treatment Planning System. For each case, a treatment simulation was performed prior to the treatment decision. Tumor tracking visualization simulations on the CK system validated the motion correlation model between the liver dome and respiratory motion in the superior-inferior (SI) and anterior-posterior (AP) directions, as well as with external breathing profiles. Key performance metrics included x-axis target pairing (dxAB), uncertainty (%), and correlation error. After successful simulations, treatments were delivered based on the validated motion correlation model.
Results: The metric dxAB was consistently below 1 mm, uncertainty was less than 20%, and correlation error remained under 2 mm. End-to-end test for Synchrony lung with respiratory tracking accuracy was robust, further supporting confidence in this method.
Conclusion: The liver dome is a reliable surrogate for respiratory motion tracking in CK Synchrony SBRT for lung tumors near the diaphragm. By eliminating the need for internal target volumes (ITVs) or fiducial markers, this technique maintains treatment precision, minimizes radiation exposure to healthy tissues, and improves patient outcomes.