Comparison of breath hold and abdominal compression for liver SABR on C-arm Linac and MR Linac: A single institution retrospective planning study

BACKGROUND: Respiratory motion is a challenge for accurate stereotactic ablative radiotherapy (SABR) to the liver. This study compares the dosimetric impact of abdominal compression (AC), breath hold (BH) and free-breathing (FB) strategies for liver SABR on conventional C-arm linear accelerators (Linacs) and magnetic resonance Linacs (MR Linacs). METHODS: Five patients undergoing liver SABR were imaged using T2-weighted turbo-spin echo and cine-MRI with FB and AC, and mDixon with BH. The gross tumour volume (GTV) was expanded based on quantified motion to generate the internal target volume (ITV) for FB and AC, and 5 mm PTV margin added. Plans were generated for FB, AC, and BH, prescribing 4000 cGy in 5 fractions to the planning target volume (PTV). Target coverage and organ at risk (OARs) doses were assessed. RESULTS: BH plans provided superior target coverage and minimized dose to most OARs across both platforms. Median GTVexhale size was 17.77 cc, with median PTV sizes of 51.96 cc (BH), 89.99 cc (AC), and 79.59 cc (FB). On C-arm Linac, BH achieved highest mean Dmin (2969 cGy) and D95% (3888 cGy). On MR Linac, BH also showed highest mean Dmin (3056 cGy) and D95% (3916 cGy). AC and FB plans resulted in higher OAR doses. CONCLUSION: All evaluated motion management strategies produced acceptable plans, but BH offered the best overall target coverage and reduced OAR doses on both platforms. Although differences between strategies are overall small. Motion management strategies should therefore be selected on a patient-by-patient basis based on patient acceptability and dosimetric benefit.