Optimal timing of re-planning for head and neck adaptive radiotherapy

Background and purpose Adaptive radiotherapy (ART) relies on re-planning to correct treatment variations, but the optimal timing of re-planning to account for dose changes in head and neck organs at risk (OARs) is still under investigation. We aimed to find out the optimal timing of re-planning in head and neck ART. Materials and methods A total of 110 head and neck cancer patients were retrospectively enrolled. A semi auto-segmentation method was applied to obtain the weekly mean dose (Dmean) to OARs. The K-nearest-neighbour method was used for missing data imputation of weekly Dmean. A dose deviation map was built using the planning Dmean and weekly Dmean values and then used to simulate different ART scenarios consisting of 1 to 6 re-plannings. The difference between accumulated Dmean and planning Dmean before re-planning (ΔDmeanₐccₙoART) and after re-planning (ΔDmeanₐccART) were evaluated and compared. Results Among all the OARs, supraglottic showed the largest ΔDmeanₐccₙoART (1. 23±3. 13 Gy) and most cases of ΔDmeanₐccₙoART >3 Gy (26 patients). The 3rd week is suggested in the optimal timing of re-planning for 10 OARs. For all the organs except arytenoid, 2 re-plannings were able to guarantee the ΔDmeanₐccART below 3 Gy while the average |ΔDmeanₐccART| was below 1 Gy. ART scenarios of 2₄, 3₄, 3₅ (week of re-planning separated with "_") were able to guarantee ΔDmeanₐccART of 99% of patients below 3 Gy simultaneously for 19 OARs. Conclusions The optimal timing of re-planning was suggested for different organs at risk in head and neck adaptive radiotherapy. Generic scenarios of timing and frequency for re-planning can be applied to guarantee the increase of accumulated mean dose within 3 Gy simultaneously for multiple organs.