Time series analysis for optimal offline replanning timing in head and neck proton therapy

Inter-fraction anatomical changes can compromise proton treatment quality. Offline replanning is a common approach to rectify the anatomical changes during fractionated treatment. It is important to determine the optimal timing for replanning, because offline replanning can only occur a limited number of times.

This study determines optimal replanning timing for proton head and neck patients, when one replanning is allowed.

This study proposes a methodology for identifying optimal replanning timing on a daily basis, which involved creating actual replans for 576 daily CBCTs from 18 head and neck patients. A moving average technique was used to mitigate the noise in the time series data for a more robust analysis.

Serial organs, specifically the brainstem and spinal cord, benefited the most from a single offline replan, showing improvements in D_max of 2.93 and 4.35 Gy(RBE), respectively. The high-risk and low-risk CTVs also showed improvement, with gains in D₉₈ of 1.00 and 1.99 Gy(RBE). In contrast, parallel organs benefited the least, exhibiting less than 1.00 Gy(RBE) improvement in D_mean. Replanning at fraction 15 improved the key DVH parameters of most structures, six out of eight, by at least 95% of their corresponding maximum gain from one replanning.

The study suggests replanning around the end of the third week is optimal for head and neck cancer patients receiving proton therapy. The identified adaptation window corresponds with results from studies on offline adaptive photon therapy.