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 (D-mean) to OARs. The K-nearest-neighbour method was used for missing data imputation of weekly D-mean. A dose deviation map was built using the planning D-mean and weekly D-mean values and then used to simulate different ART scenarios consisting of 1 to 6 re-plannings. The difference between accumulated D-mean and planning D-mean before re-planning (Delta D-mean_acc_noART) and after re-planning (Delta D-mean_acc_ART) were evaluated and compared. Results: Among all the OARs, supraglottic showed the largest Delta D-mean_acc_noART (1.23 +/- 3.13 Gy) and most cases of Delta D-mean_acc_noART > 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 Delta D-mean_acc_ART below 3 Gy while the average |Delta D-mean_acc_ART| was below 1 Gy. ART scenarios of 2_4, 3_4, 3_5 (week of re-planning separated with "_") were able to guarantee Delta D-mean_acc_ART 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.