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

Abstract

PurposeIn 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.MethodsInstead 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.ResultsCompared 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%).ConclusionsAn 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.

A review of dose calculation approaches with cone beam CT in photon and proton therapy

“…Another possible option, here referred as "HU ROI ", consists in segmenting structures (ROI's) in the pCT and then transferring them to the CBCT images. The HU in the delineated ROI's is manually assigned to the mean HU value of the same regions in the pCT images [53,61,67]. The direct comparison between HU MLT and HU ROI presented by Fotina et al [53] shows that similar results were obtained for prostate and lung, but for head and neck the results are 2.5% more accurate with HU MLT .…”

“…To reduce the influence of CBCT artefacts and HU variability on the dose calculation, several studies have suggested overriding the density of the structures in the CBCT image with average HU values from the corresponding structures on the pCT/rCT [36,42,66,67,43,45,53,[61][62][63][64][65]. Some other studies generated a linear conversion curve to match the HU values in the CBCT images with the HU values in the pCT images [27,39,[68][69][70][71][72] (Table 3a, 3b).…”

“…B-spline, Demons, etc). For CBCT dose calculation purposes, DIR is used to deform either the original pCT or the rescan rCT to match one of the patient's CBCT images [27,29,[76][77][78][79]38,42,43,67,71,[73][74][75]. Thus, a deformation vector field (DVF) defining the voxel-to-voxel mapping between pCT and CBCT is generated.…”

A Method to Correct Cone Beam CT (CBCT) Images Using Planning CT for Derivation of Plan of the Day in STEREOTACTIC Body Radiotherapy (SBRT)

“…Second, the increased HU accuracy can enormously improve the precision of dose calculation using CBCT image sets. With all considerations, the improved image quality utilizing the proposed scatter correction method ought to be alluring in clinical applications [14,15].…”

Enhancement of Hounsfield unit distribution in cone-beam CT images for adaptive radiation therapy: Evaluation of a hybrid correction approach

“…The common goal was to propose promising strategies to overcome the limitations related to the high amount of scatter that reduces the quality of CBCT images compared to the pCT [4] and ensure suitable calibration in ED for an accurate dose calculation. These strategies include: the integration of physical instruments as anti-scatter grids to reduce the impact of scatter on the CBCT reconstruction quality [5][6][7], scatter measurement and correction based on the imaging system simulation [8][9][10][11], calibration of CBCT system using phantoms [12][13][14][15][16][17][18][19] and finally the use of pCT-based approaches. The latter approaches aim to map hounsfield units (HU) from pCT to CBCT images [20,21], match the peaks of their histograms [22,23] or assign to the CBCT images exact HU values extracted from the pCT [19,[23][24][25][26][27][28].…”