The effects of computed tomography image characteristics and knot spacing on the spatial accuracy of B-spline deformable image registration in the head and neck geometry

Objectives: To explore the effects of computed tomography (CT) image characteristics and B-spline knot spacing (BKS) on the spatial accuracy of a B-spline deformable image registration (DIR) in the head-and-neck geometry. Methods: The effect of image feature content, image contrast, noise, and BKS on the spatial accuracy of a B-spline DIR was studied. Phantom images were created with varying feature content and varying contrast-to-noise ratio (CNR), and deformed using a known smooth B-spline deformation. Subsequently, the deformed images were repeatedly registered with the original images using different BKSs. The quality of the DIR was expressed as the mean residual displacement (MRD) between the known imposed deformation and the result of the B-spline DIR. Finally, for three patients, head-and-neck planning CT scans were deformed with a realistic deformation field derived from a rescan CT of the same patient, resulting in a simulated deformed image and an a-priori known deformation field. Hence, a B-spline DIR was performed between the simulated image and the planning CT at different BKSs. Similar to the phantom cases, the DIR accuracy was evaluated by means of MRD. Results: In total, 162 phantom registrations were performed with varying CNR and BKSs. MRD-values = +/- 250 HU and noise <+/- 200 HU. Decreasing the image feature content resulted in increased MRD-values at all BKSs. Using BKS = 15 mm for the three clinical cases resulted in an average MRD <1.0 mm. Conclusions: For synthetically generated phantoms and three real CT cases the highest DIR accuracy was obtained for a BKS between 10-20 mm. The accuracy decreased with decreasing image feature content, decreasing image contrast, and higher noise levels. Our results indicate that DIR accuracy in clinical CT images (typical noise levels <+/- 100 HU) will not be effected by the amount of image noise

Differences in delineation guidelines for head and neck cancer result in inconsistent reported dose and corresponding NTCP

AbstractPurposeTo test the hypothesis that delineation of swallowing organs at risk (SWOARs) based on different guidelines results in differences in dose–volume parameters and subsequent normal tissue complication probability (NTCP) values for dysphagia-related endpoints.Materials and methodsNine different SWOARs were delineated according to five different delineation guidelines in 29 patients. Reference delineation was performed according to the guidelines and NTCP-models of Christianen et al. Concordance Index (CI), dosimetric consequences, as well as differences in the subsequent NTCPs were calculated.ResultsThe median CI of the different delineation guidelines with the reference guidelines was 0.54 for the pharyngeal constrictor muscles, 0.56 for the laryngeal structures and 0.07 for the cricopharyngeal muscle and esophageal inlet muscle. The average difference in mean dose to the SWOARs between the guidelines with the largest difference (maxΔD) was 3.5±3.2Gy. A mean ΔNTCP of 2.3±2.7% was found. For two patients, ΔNTCP exceeded 10%.ConclusionsThe majority of the patients showed little differences in NTCPs between the different delineation guidelines. However, large NTCP differences >10% were found in 7% of the patients. For correct use of NTCP models in individual patients, uniform delineation guidelines are of great importance

3D Variation in delineation of head and neck organs at risk

<p>Abstract</p> <p>Background</p> <p>Consistent delineation of patient anatomy becomes increasingly important with the growing use of highly conformal and adaptive radiotherapy techniques. This study investigates the magnitude and 3D localization of interobserver variability of organs at risk (OARs) in the head and neck area with application of delineation guidelines, to establish measures to reduce current redundant variability in delineation practice.</p> <p>Methods</p> <p>Interobserver variability among five experienced radiation oncologists was studied in a set of 12 head and neck patient CT scans for the spinal cord, parotid and submandibular glands, thyroid cartilage, and glottic larynx. For all OARs, three endpoints were calculated: the Intraclass Correlation Coefficient (ICC), the Concordance Index (CI) and a 3D measure of variation (3D SD).</p> <p>Results</p> <p>All endpoints showed largest interobserver variability for the glottic larynx (ICC = 0.27, mean CI = 0.37 and 3D SD = 3.9 mm). Better agreement in delineations was observed for the other OARs (range, ICC = 0.32-0.83, mean CI = 0.64-0.71 and 3D SD = 0.9-2.6 mm). Cranial, caudal, and medial regions of the OARs showed largest variations. All endpoints provided support for improvement of delineation practice.</p> <p>Conclusions</p> <p>Variation in delineation is traced to several regional causes. Measures to reduce this variation can be: (1) guideline development, (2) joint delineation review sessions and (3) application of multimodality imaging. Improvement of delineation practice is needed to standardize patient treatments.</p

Applicability of the Matrixx detector combined with the compass system to detect leaf positioning errors of an elekta beam modulator

This study explores the capability of the COMPASS system (IBA Dosimetry, Germany) to detect leaf positioning errors of a high resolution MLC with 4mm leaf width using a lower resolution (7.62mm detector spacing) 2D matrix of ionisation chamber

An efficient and independent 3D dose calculationtool for quality assurance of complex radiotherapy treatment plans

To validate the model-based 3D dose calculation performed by the COMPASS system, and to evaluate the use of the combination of model-based and measurement-based 3D dose determination for clinical QA purposes

Reconstruction of high-resolution 3D dose from matrix measurements: error detection capability of the COMPASS correction kernel method

The COMPASS system (IBADosimetry) is a quality assurance (QA) tool which reconstructs 3D doses inside a phantom or a patient CT. The dose is predicted according to the RT plan with a correction derived from 2D measurements of a matrix detector. This correction method is necessary since a direct reconstruction of the fluence with a high resolution is not possible because of the limited resolution of the matrix used, but it comes with a blurring of the dosewhich creates inaccuracies in the dose reconstruction. This paper describes the method and verifies its capability to detect errors in the positioning of a MLC with 10 mm leaf width in a phantom geometry. Dose reconstruction was performed forMLC position errors of various sizes at various locations for both rectangular and intensity-modulated radiotherapy (IMRT) fields and compared to a reference dose. It was found that the accuracy with which an error inMLC position is detected depends on the location of the error relative to the detectors in the matrix. The reconstructed dose in an individual rectangular field for leaf positioning errors up to 5 mm was correct within 5% in 50% of the locations. At the remaining locations, the reconstruction of leaf position errors larger than 3 mm can show inaccuracies, even though these errors were detectable in the dose reconstruction. Errors larger than 9 mm created inaccuracies up to 17% in a small area close to the penumbra. The QA capability of the system was tested through gamma evaluation. Our results indicate that themean gamma provided by the system is slightly increased and that the number of points above gamma 1 ensures error detection for QA purposes. Overall, the correction kernel method used by the COMPASS system is adequate to perform QA of IMRT treatment plans with a regular MLC, despite local inaccuracies in the dose reconstruction