A novel respiratory motion compensation strategy combining gated beam delivery and mean target position concept --a compromise between small safety margins and long duty cycles.

PURPOSE To evaluate a novel respiratory motion compensation strategy combining gated beam delivery with the mean target position (MTP) concept for pulmonary stereotactic body radiotherapy (SBRT). MATERIALS AND METHODS Four motion compensation strategies were compared for 10 targets with motion amplitudes between 6mm and 31mm: the internal target volume concept (plan(ITV)); the MTP concept where safety margins were adapted based on 4D dose accumulation (plan(MTP)); gated beam delivery without margins for motion compensation (plan(gated)); a novel approach combining gating and the MTP concept (plan(gated&MTP)). RESULTS For 5/10 targets with an average motion amplitude of 9mm, the differences in the mean lung dose (MLD) between plan(gated) and plan(MTP) were <10%. For the other 5/10 targets with an average motion amplitude of 19mm, gating with duty cycles between 87.5% and 75% reduced the residual target motion to 12mm on average and 2mm safety margins were sufficient for dosimetric compensation of this residual motion in plan(gated&MTP). Despite significantly shorter duty cycles, plan(gated) reduced the MLD by <10% compared to plan(gated&MTP). The MLD was increased by 18% in plan(MTP) compared to that of plan(gated&MTP). CONCLUSIONS For pulmonary targets with motion amplitudes >10-15mm, the combination of gating and the MTP concept allowed small safety margins with simultaneous long duty cycles.