Patient‐specific calibration of cone‐beam computed tomography data sets for radiotherapy dose calculations and treatment plan assessment

• Published in: Journal of applied clinical medical physics Vol. 19; no. 2; pp. 249 - 257
• Main Authors: MacFarlane, Michael, Wong, Daniel, Hoover, Douglas A., Wong, Eugene, Johnson, Carol, Battista, Jerry J., Chen, Jeff Z.
• Format: Journal Article
• Language: English
• Published: United States John Wiley & Sons, Inc 01-03-2018; John Wiley and Sons Inc
• Subjects: Calibration; CBCT; DIR; dose tracking; Methods; Planning; Radiation Oncology Physics; Radiation therapy; treatment plan assessment; dose tracking; treatment plan assessment; DIR; CBCT
• ISSN: 1526-9914; 1526-9914
• DOI: 10.1002/acm2.12293

Purpose
In this work, we propose a new method of calibrating cone beam computed tomography (CBCT) data sets for radiotherapy dose calculation and plan assessment. The motivation for this patient‐specific calibration (PSC) method is to develop an efficient, robust, and accurate CBCT calibration process that is less susceptible to deformable image registration (DIR) errors.
Methods
Instead of mapping the CT numbers voxel‐by‐voxel with traditional DIR calibration methods, the PSC methods generates correlation plots between deformably registered planning CT and CBCT voxel values, for each image slice. A linear calibration curve specific to each slice is then obtained by least‐squares fitting, and applied to the CBCT slice's voxel values. This allows each CBCT slice to be corrected using DIR without altering the patient geometry through regional DIR errors.
A retrospective study was performed on 15 head‐and‐neck cancer patients, each having routine CBCTs and a middle‐of‐treatment re‐planning CT (reCT). The original treatment plan was re‐calculated on the patient's reCT image set (serving as the gold standard) as well as the image sets produced by voxel‐to‐voxel DIR, density‐overriding, and the new PSC calibration methods. Dose accuracy of each calibration method was compared to the reference reCT data set using common dose‐volume metrics and 3D gamma analysis. A phantom study was also performed to assess the accuracy of the DIR and PSC CBCT calibration methods compared with planning CT.
Results
Compared with the gold standard using reCT, the average dose metric differences were ≤ 1.1% for all three methods (PSC: −0.3%; DIR: −0.7%; density‐override: −1.1%). The average gamma pass rates with thresholds 3%, 3 mm were also similar among the three techniques (PSC: 95.0%; DIR: 96.1%; density‐override: 94.4%).
Conclusions
An automated patient‐specific calibration method was developed which yielded strong dosimetric agreement with the results obtained using a re‐planning CT for head‐and‐neck patients.