Monitoring anatomical changes of individual patients using statistical process control during head-and-neck radiotherapy

• Published in: Physics and imaging in radiation oncology Vol. 9; pp. 21 - 27
• Main Authors: Lowther, Nicholas J., Hamilton, David A., Kim, Han, Evans, Jamie M., Marsh, Steven H., Louwe, Robert J.W.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.01.2019; Elsevier
• Subjects: Anatomical changes; Deformable image registration; Head-and-neck; Original; Radiotherapy; Statistical process control; Anatomical changes; Statistical process control; Head-and-neck; Radiotherapy; Deformable image registration
• ISSN: 2405-6316; 2405-6316
• DOI: 10.1016/j.phro.2018.12.004

•EWMA trends from DIR data enabled timely detection of anatomical changes.•The majority of the systematic trends occurred before the 4th week of treatment.•Using SPC control limits, only 24% of patient positioning trends could be confirmed.•Using a 2 mm threshold, 82% of patient positioning trends could be confirmed.•Using SPC control limits, 90% of the soft tissue changes could be confirmed. Reduced toxicity while maintaining loco-regional control rates have been reported after reducing planning target volume (PTV) margins for head-and-neck radiotherapy (HNRT). In this context, quantifying anatomical changes to monitor patient treatment is preferred. This retrospective feasibility study investigated the application of deformable image registration (DIR) and Exponentially Weighted Moving Average (EWMA) Statistical Process Control (SPC) charts for this purpose. DIR between the computed tomography for treatment planning (pCT) images of twelve patients and their daily on-treatment cone beam computed tomography (CBCT) images quantified anatomical changes during treatment. EWMA charts investigated corresponding trends. Uncertainty analysis provided 90% confidence limits which were used to confirm whether a trend previously breached a threshold. Trends in patient positioning reproducibility occurred before the end of treatment week four in 54% of cases. Using SPC process limits, only 24% of these were confirmed at a 90% confidence level before the end of treatment. Using an a priori clinical limit of 2 mm, absolute changes in patient pose were detected in 39% of cases, of which 82% were confirmed. Soft tissue trends outside SPC process limits occurring before the end of treatment week four were confirmed in 90% of cases. Structure specific action thresholds enabled detection of systematic anatomical changes during the first four weeks of treatment. Investigation of the dosimetric impact of the observed deviations is needed to show the efficacy of SPC to timely indicate required treatment adaptation and provide a safety net for PTV margin reduction.