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. 2021 Aug;22(8):156-167.
doi: 10.1002/acm2.13341. Epub 2021 Jul 26.

Geometric and dosimetric accuracy of deformable image registration between average-intensity images for 4DCT-based adaptive radiotherapy for non-small cell lung cancer

Yulun He  1 , Guillaume Cazoulat  1 , Carol Wu  2 , Christine Peterson  3 , Molly McCulloch  1 , Brian Anderson  1 , Julianne Pollard-Larkin  4 , Peter Balter  4 , Zhongxing Liao  5 , Radhe Mohan  4 , Kristy Brock  1
Affiliations

Geometric and dosimetric accuracy of deformable image registration between average-intensity images for 4DCT-based adaptive radiotherapy for non-small cell lung cancer

Yulun He et al. J Appl Clin Med Phys. 2021 Aug.

Abstract

Purpose: Re-planning for four-dimensional computed tomography (4DCT)-based lung adaptive radiotherapy commonly requires deformable dose mapping between the planning average-intensity image (AVG) and the newly acquired AVG. However, such AVG-AVG deformable image registration (DIR) lacks accuracy assessment. The current work quantified and compared geometric accuracies of AVG-AVG DIR and corresponding phase-phase DIRs, and subsequently investigated the clinical impact of such AVG-AVG DIR on deformable dose mapping.

Methods and materials: Hybrid intensity-based AVG-AVG and phase-phase DIRs were performed between the planning and mid-treatment 4DCTs of 28 non-small cell lung cancer patients. An automated landmark identification algorithm detected vessel bifurcation pairs in both lungs. Target registration error (TRE) of these landmark pairs was calculated for both DIR types. The correlation between TRE and respiratory-induced landmark motion in the planning 4DCT was analyzed. Global and local dose metrics were used to assess the clinical implications of AVG-AVG deformable dose mapping with both DIR types.

Results: TRE of AVG-AVG and phase-phase DIRs averaged 3.2 ± 1.0 and 2.6 ± 0.8 mm respectively (p < 0.001). Using AVG-AVG DIR, TREs for landmarks with <10 mm motion averaged 2.9 ± 2.0 mm, compared to 3.1 ± 1.9 mm for the remaining landmarks (p < 0.01). Comparatively, no significant difference was demonstrated for phase-phase DIRs. Dosimetrically, no significant difference in global dose metrics was observed between doses mapped with AVG-AVG DIR and the phase-phase DIR, but a positive linear relationship existed (p = 0.04) between the TRE of AVG-AVG DIR and local dose difference.

Conclusions: When the region of interest experiences <10 mm respiratory-induced motion, AVG-AVG DIR may provide sufficient geometric accuracy; conversely, extra attention is warranted, and phase-phase DIR is recommended. Dosimetrically, the differences in geometric accuracy between AVG-AVG and phase-phase DIRs did not impact global lung-based metrics. However, as more localized dose metrics are needed for toxicity assessment, phase-phase DIR may be required as its lower mean TRE improved voxel-based dosimetry.

Keywords: 4DCT; adaptive radiotherapy; deformable image registration accuracy; non-small cell lung cancer.

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Conflict of interest statement

Yulun He has nothing to disclose; Dr. Cazoulat has nothing to disclose; Dr. Wu has nothing to disclose; Dr. Peterson has nothing to disclose; Dr. McCulloch has nothing to disclose; Brian Anderson has nothing to disclose; Dr. Pollard‐Larkin has nothing to disclose; Dr. Balter reports grants from RaySearch Laboratories, grants from Varian Associates, outside the submitted work; Dr. Liao has nothing to disclose; Dr. Mohan has nothing to disclose; Dr. Brock reports grants from RaySearch Laboratories, Helen Black Image Guided Cancer Therapy Research Fund and Tumor Measurement Initiative of University of Texas MD Anderson Cancer Center, during the conduct of the study. In addition, Dr. Brock has a licensing agreement with RaySearch Laboratories with royalties paid.

Figures

FIGURE 1
FIGURE 1
Manual landmark identification process to validate the automatic landmark workflow. The planning four‐dimensional computed tomography (4DCT) and the mid‐treatment 4DCT are shown. Coronal views of the end‐inhalation phase (T0), mid‐ventilation phase (T3), and end‐exhalation phase (T5), as well as the average‐intensity image (AVG), are shown in the left column. Simplified cartoons of the two 4DCTs are shown on the right. Deformable image registrations (DIRs) were established between corresponding phases and AVGs of the two 4DCTs. A landmark at the lower vessel bifurcation in the right lung was identified with different colors in different phases. The landmark's locations in these phases were transferred onto the AVG of each corresponding 4DCT
FIGURE 2
FIGURE 2
Target registration error (TRE) differences between deformable image registration (DIR) across average‐intensity images and DIR across phases for each patient. Each data point represents a labeled patient. The result of linear regression is TRE (auto)=0.5 × TRE (manual) +1.4. Coefficient of determination is 0.81
FIGURE 3
FIGURE 3
Boxplots of mean target registration error (TRE) of the 28 patients. Each color pair of boxplots represents the mean TRE of phase pairs for the corresponding phase‐phase DIR and for AVG‐AVG DIR. The standard deviation of TRE ranged from 1.0 to 3.4 mm (not shown). *AVG, average‐intensity image; DIR, deformable image registration; T0, end‐inhalation phase; T3, mid‐ventilation phase; T5, end‐exhalation phase
FIGURE 4
FIGURE 4
This figure shows, for patient 14, deformed doses (left panels) and their difference (right panel) on an axial slice of the average‐intensity image (AVG) of the mid‐treatment week. The upper‐left and lower‐left panels show the deformed planned dose with AVG‐AVG DIR and with T5‐T5 DIR respectively. *DIR, deformable image registration; T5, end‐exhalation phase
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