Geometric and dosimetric comparison of four intrafraction motion adaptation strategies for stereotactic liver radiotherapy.

The accuracy of stereotactic body radiotherapy (SBRT) in the liver is limited by tumor motion. Selection of the most suitable motion mitigation strategy requires good understanding of the geometric and dosimetric consequences. This study compares the geometric and dosimetric accuracy of actually delivered respiratory gated SBRT treatments for 15 patients with liver tumors with three simulated alternative motion adaptation strategies. The simulated alternatives are MLC tracking, baseline shift adaptation by inter-field couch corrections and no intrafraction motion adaptation. The patients received electromagnetic transponder-guided respiratory gated IMRT or conformal treatments in three fractions with a 3-4 mm gating window around the full exhale position. The CTV-PTV margin was 5 mm axially and 7-10 mm cranio-caudally. The CTV and PTV were covered with 95% and 67% of the prescribed mean CTV dose, respectively. The time-resolved target position error during treatments with the four investigated motion adaptation strategies was used to calculate motion margins and the motion-induced reduction in CTV D relative to the planned dose (ΔD ). The mean (range) number of couch corrections per treatment session to compensate for tumor drift was 2.8 (0-7) with gating, 1.4 (0-5) with baseline shift adaptation, and zero for the other strategies. The motion margins were 3.5 mm (left-right), 9.4 mm (cranio-caudal) and 3.9 mm (anterior-posterior) without intrafraction motion adaptation, approximately half of that with baseline shift adaptation, and 1-2 mm with MLC tracking and gating. With 7 mm CC margins the mean (range) of ΔD for the CTV was 8.1 (0.6-29.4)%-points (no intrafraction motion adaptation), 4.0 (0.4-13.3)%-points (baseline shift adaptation), 1.0 (0.3-2.2)%-points (MLC tracking) and 0.8 (0.1-1.8)%-points (gating). With 10 mm CC margins ΔD was instead 4.8 (0.3-14.8)%-points (no intrafraction motion adaptation) and 2.9 (0.2-9.8)%-points (baseline shift adaptation). In conclusion, baseline shift adaptation can mitigate gross dose deficits without the requirement of real-time motion adaptation. MLC tracking and gating, however, more effectively ensure high similarity between planned and delivered doses.