2735 - Adaptive Radiotherapy in Head and Neck Cancers - When do We Replan?

Purpose/Objective(s): It is well known that patients undergoing head and neck radiotherapy have anatomical changes in organs at risk (OAR’s) and gross tumor volume during the course of treatment which can lead to change in dose distribution to OAR’s and gross tumor volume. Several studies have shown the change in OAR volume and increase in dose received by OAR’s than planning scan. But the optimal timing to replan the treatment is still unknown which can lead to clinically significant benefit. Hence, in our study we have attempted to find the optimal timing of replanning the treatment in head and neck radiotherapy patients which can give clinically significant dose reduction to OAR’s, thereby reducing the side effects.

Materials/Methods: 20 head and neck cancer patients were retrospectively enrolled in our study. We assessed change in volume of target and OARs in each week for 6 weeks by averaging the volume change on daily CBCT of each week. Actual Dmean for ipsilateral and contralateral Parotids of each week was calculated by averaging the Dmean obtained by replanning on daily CBCT of that week. Difference in Dmean on planning CT scan (Dmean_p ) and average Dmean of each week (Dmean_n, n is average Dmean of that particular week) was calculated without adaptive replan (Dmean_{Diff_noART}) and with adaptive replan (Dmean_{Diff_ART}) and compared. We also compared accumulated Dmean with (AccDmean__ART) and without adaptive replan (AccDmean__noART) and the difference between accumulated Dmean and planning Dmean with and without adaptive replan.

Results: The average change in volume of target for all patients were 2 %, 4 %, 6%, 12%, 15% and 25% respectively from 1^st to 6^th week. There was statistically significant reduction in target volume after 4^th week (p value - 0.0001). Average change in Parotid volumes were 4%, 5%, 12%, 15%, 15% and 21% respectively from 1^st to 6^th week. There was statistically significant reduction in parotid volumes after 3^rd week (p value - 0.04). The difference in Dmean of Parotids without adaptive replan was 0.83, 1.45, 1.86, 2.13, 2.86, 2.87 respectively from 1^st to 6^th week. The difference in Dmean of Parotids after adaptive replan was 0.43, 0.86, 0.88, 1.02, 1.32, 1.34 respectively from 1^st to 6^th week. There was statistically significant reduction in Dmean received by Parotids after adaptive replan in 4^th week (p value - 0.01). The accumulated Dmean for parotid volumes before adaptive replan at the end of 6^th week was 29.41 Gy and the accumulated Dmean for parotid volumes after adaptive replan at the end of 6^th week was 26.08 Gy. It was statistically significant (p – 0.003).

Conclusion: We conclude that optimal timing of replanning should be after 4^th week of radiation to reduce the dose received by Parotids and account for significant reduction in target volume. We warrant further study with larger sample size and clinical comparison for toxicities to validate our results.