A Prospective Cohort Study of Gated Stereotactic Liver Radiation Therapy Using Continuous Internal Electromagnetic Motion Monitoring

Purpose: Intrafraction motion can compromise the treatment accuracy in liver stereotactic body radiation therapy (SBRT). Respiratory gating can improve treatment delivery; however, gating based on external motion surrogates is inaccurate. The present study reports the use of Calypso-based internal electromagnetic motion monitoring for gated liver SBRT. Methods and Materials: Fifteen patients were included in a study of 3-fraction respiratory gated liver SBRT guided by 3 implanted electromagnetic transponders. The planning target volume was created by a 5-mm axial and 7-mm (n=12) or 10-mm (n=3) craniocaudal expansion of the clinical target volume (CTV) and covered with 67% of the prescribed CTV mean dose. Treatment was gated to the end-exhale phase of the respiratory cycle with beam-on when the target deviated < 3 mm (lefteright/anteroposterior) and 4 mm (craniocaudal) from the planned position, according to the monitored (25-Hz) transponder centroid position. The couch was adjusted remotely if baseline drifts > 1 to 2 mm occurred. Log files of transponder motion were used to determine the geometric error and reconstruct the delivered CTV dose in the actual gated treatments and in simulated nongated treatments. Results: No severe side effects were observed in relation to transponder implantation. All 45 treatment fractions were successfully guided using the Calypso system. The mean number of couch corrections during each gated fraction was 2.8 (range 0-7). The mean duty cycle during gated treatment was 62.5% (range 29.1%-84.9%). Without gating, the mean 3-dimensional geometric error during a fraction would have been 5.4 mm (range 2.7-12.1). Gating reduced this error to 2.0 mm (range 1.2-3.0).The patient mean reduction in minimum dose to 95% of the CTV relative to the planned dose was 6.0 percentage points (range 0.7-22.0) without gating and 0.8 percentage point (range 0.2-2.0) with gating. Conclusions: Gating using internal motion monitoring was successfully applied for liver SBRT. It markedly improved the geometric and dosimetric accuracy compared with nongated standard treatment. (C) 2018 Elsevier Inc. All rights reserved.