Markerless liver online adaptive stereotactic radiotherapy: feasibility analysis

Abstract Objective . Radio-opaque markers are recommended for image-guided radiotherapy in liver stereotactic ablative radiotherapy (SABR), but their implantation is invasive. We evaluate in this in-silico study the feasibility of cone-beam computed tomography-guided stereotactic online-adaptive radiotherapy (CBCT-STAR) to propagate the target volumes without implanting radio-opaque markers and assess its consequence on the margin that should be used in that context. Approach . An emulator of a CBCT-STAR-dedicated treatment planning system was used to generate plans for 32 liver SABR patients. Three target volume propagation strategies were compared, analysing the volume difference between the GTV Propagated and the GTV Conventional , the vector lengths between their centres of mass ( l CoM ), and the 95th percentile of the Hausdorff distance between these two volumes (HD95). These propagation strategies were: (1) structure-guided deformable registration with deformable GTV propagation; (2) rigid registration with rigid GTV propagation; and (3) image-guided deformable registration with rigid GTV propagation. Adaptive margin calculation integrated propagation errors, while interfraction position errors were removed. Scheduled plans (Plan Non-adaptive ) and daily-adapted plans (Plan Adaptive ) were compared for each treatment fraction. Main results. The image-guided deformable registration with rigid GTV propagation was the best propagation strategy regarding to l CoM (mean: 4.3 +/− 2.1 mm), HD95 (mean 4.8 +/− 3.2 mm) and volume preservation between GTV Propagated and GTV Conventional . This resulted in a planning target volume (PTV) margin increase (+69.1% in volume on average). Online adaptation (Plan Adaptive ) reduced the violation rate of the most important dose constraints (‘priority 1 constraints’, 4.2 versus 0.9%, respectively; p < 0.001) and even improved target volume coverage compared to non-adaptive plans (Plan Non-adaptive ). Significance . Markerless CBCT-STAR for liver tumours is feasible using Image-guided deformable registration with rigid GTV propagation. Despite the cost in terms of PTV volumes, daily adaptation reduces constraints violation and restores target volumes coverage.