Sci‐Thur PM ‐ Colourful Interactions: Highlights 06: Patient‐specific calibration of cone‐beam computed tomographic (CBCT) image sets for on‐line dose tracking and plan assessment

Abstract

Purpose:To develop a radiotherapy dose tracking and plan evaluation technique using cone‐beam computed tomography (CBCT) images.Methods:We developed a patient‐specific method of calibrating CBCT image sets for dose calculation. The planning CT was first registered with the CBCT using deformable image registration (DIR). A scatter plot was generated between the CT numbers of the planning CT and CBCT for each slice. The CBCT calibration curve was obtained by least‐square fitting of the data, and applied to each CBCT slice. The calibrated CBCT was then merged with original planning CT to extend the small field of view of CBCT. Finally, the treatment plan was copied to the merged CT for dose tracking and plan evaluation. The proposed patient‐specific calibration method was also compared to two methods proposed in literature. To evaluate the accuracy of each technique, 15 head‐and‐neck patients requiring plan adaptation were arbitrarily selected from our institution. The original plan was calculated on each method's data set, including a second planning CT acquired within 48 hours of the CBCT (serving as gold standard). Clinically relevant dose metrics and 3D gamma analysis of dose distributions were compared between the different techniques.Results:Compared to the gold standard of using planning CTs, the patient‐specific CBCT calibration method was shown to provide promising results with gamma pass rates above 95% and average dose metric agreement within 2.5%.Conclusions:The patient‐specific CBCT calibration method could potentially be used for on‐line dose tracking and plan evaluation, without requiring a re‐planning CT session.