Purpose: During liver SABR treatment, breath-hold at end exhale or deep inhale is often used to manage target motion. Typically, breath-holds are monitored with a surface monitoring technique such as the Varian RPM. In the LARK trial, we deployed a real-time IGRT system to monitor the internal target motion. Here, we investigate the differences between external and internal monitoring for breath-hold SABR.
Methods: Seven patients with colorectal metastases in the liver were treated at end-exhale (5 patients) or deep-inhale (2 patients) breath-hold on a Varian TrueBeam (dual-arc VMAT, 6FFF). A margin of 5mm in all directions was applied to the GTV to create the PTV. Three gold fiducial markers (3mm×1mm diameter) were implanted around the patient’s tumours for real-time IGRT tracking surrogate. During treatment, the patient internal motion was monitored with an in-house 3D real-time IGRT method utilizing intrafraction kilovoltage imaging at 10Hz. External surface position monitoring (Varian RPM) was also used to measure the patient’s breath-hold. Couch shifts were performed following kV pair imaging if the internal motion exceeded 3mm for two consecutive breath-holds. An isocenter shift method was used to reconstruct the delivered motion-included dose with real-time IGRT and with external monitoring alone.
Results: With real-time IGRT, the 95% confidence interval of the target motion was [-2.3, 2.5]mm, [-3.0 3.0]mm, [-2.7 3.0]mm in LR, SI and AP. With external monitoring, the 95% confidence interval of the target motion would be [-2.4, 2.9]mm, [-3.7, 5.4]mm, [-3.6 3.3]mm in LR, SI and AP. Dosimetrically, with real-time IGRT, the PTV D95 was within 2.7% of the planned value for all patients. With external monitoring alone, the PTV D95 would be underdosed by 4.5±3.8%.
Conclusion: Real-time IGRT is more accurate for target motion monitoring during breath-hold liver SABR treatments. External monitoring alone resulted in target underdose due to undetected internal motion.
Funding Support, Disclosures, and Conflict of Interest: D T Nguyen is supported by NHMRC and Cancer Institute NSW Early Career Fellowships. P Keall is supported by an NHMRC Investigator (L3) grant. Nguyen, O'Brien and Keall are listed inventors on KIM-related patents. Nguyen and Keall and stock-holders of SeeTreat Pty.