Accuracy of the doses computed by the Eclipse treatment planning system near and inside metal elements.

Metal artefacts degrade clinical image quality which decreases the confidence of using computed tomography (CT) for the delineation of key structures for treatment planning and leads to dose errors in affected areas. In this work, we investigated accuracy of doses computed by the Eclipse treatment planning system near and inside metallic elements for two different computation algorithms. An impact of CT metal artefact reduction methods on the resulting calculated doses has also been assessed. A water phantom including Gafchromic film and metal inserts was irradiated (max dose 5 Gy) using a 6 MV photon beam. Three materials were tested: titanium, alloy 600, and tungsten. The phantom CT images were obtained with the pseudo-monoenergetic reconstruction (PMR) and the iterative metal artefact reduction (iMAR). Image sets were used for dose calculation using an Eclipse treatment planning station (TPS). Monte Carlo (MC) simulations were used to predict the true dose distribution in the phantom allowing for comparison with doses measured by film and calculated by TPS. Measured and simulated percentage depth doses (PDDs) were not statistically different (p > 0.618). Regional differences were observed at edges of metallic objects (max 8% difference). However, PDDs simulated with and without film were statistically different (p < 0.002). PDDs calculated by the Acuros XB algorithm based on the dose-to-medium approach best matched the MC reference regardless of the CT reconstruction methods and inserts used (p > 0.078). PDDs obtained using other algorithms significantly differ from the MC values (p < 0.011). The Acuros XB algorithm with a dose-to-medium approach provides reliable dose calculation in all metal regions when using the Varian system. The inability of the AAA algorithm to model backscatter dose significantly limits its clinical application in the presence of metal. No significant impact on the dose calculation was found for a range of metal artefact reduction strategies.

Combination of dual-energy computed tomography and iterative metal artefact reduction to increase general quality of imaging for radiotherapy patients with high dense materials. Phantom study.

PURPOSE: To evaluate the use of pseudo-monoenergetic reconstructions (PMR) from dual-energy computed tomography, combined with the iterative metal artefact reduction (iMAR) method. METHODS: Pseudo-monoenergetic CT images were obtained using the dual-energy mode on the Siemens Somatom Definition AS scanner. A range of PMR combinations (70-130 keV) were used with and without iMAR. A Virtual Water™ phantom was used for quantitative assessment of error in the presence of high density materials: titanium, alloys 330 and 600. The absolute values of CT number differences (AD) and normalised standard deviations (NSD) were calculated for different phantom positions. Image quality was assessed using an anthropomorphic pelvic phantom with an embedded hip prosthesis. Image quality was scored blindly by five observers. RESULTS: AD and NSD values revealed differences in CT number errors between tested sets. AD and NSD were reduced in the vicinity of metal for images with iMAR (p < 0.001 for AD/NSD). For ROIs away from metal, with and without iMAR, 70 keV PMR and pCT AD values were lower than for the other reconstructions (p = 0.039). Similarly, iMAR NSD values measured away from metal were lower for 130 keV and 70 keV PMR (p = 0.002). Image quality scores were higher for 70 keV and 130 keV PMR with iMAR (p = 0.034). CONCLUSION: The use of 70 keV PMR with iMAR allows for significant metal artefact reduction and low CT number errors observed in the vicinity of dense materials. It is therefore an attractive alternative to high keV imaging when imaging patients with metallic implants, especially in the context of radiotherapy planning.

Comparison of three different phantoms used for Winston-Lutz test with Artiscan software.

BACKGROUND: One of the most important test in every quality assurances process of medical linear accelerators is the Winston-Lutz test, allowing an evaluation of the treatment isocentre in the light of uncertainty of the position of the collimator, the gantry and the couch. AIM: The purpose of this work was analysis of the results of the Winston-Lutz test performed with three different phantoms for two different accelerators. MATERIALS AND METHODS: Measurements were performed on two Varian machines: TrueBeam equipped with aS1200 EPID and TrueBeam equipped with aS1000 EPID. During the study three different phantoms dedicated for verification of the radiation isocentre were used: PTW Isoball, AQUILAB Isocentre Phantom and Varian Isocentre Cube. Analysis of the DICOM images was performed in Artiscan software. RESULTS: For TrueBeam with as1200 EPID, gantry MV isocentre was about 0.18 mm larger for Varian Isocentre Cube than for two other phantoms used in this study. The largest variability of this parameter was observed for the couch. The results differed to 1.16 mm. For TrueBeam with as1000 EPID, results for collimator isocentre with PTW Isoball phantom were about 0.10 mm larger than for two other phantoms. For the gantry, results obtained with Varian Isocentre Cube were 0.21 mm larger. CONCLUSION: The obtained results for all three phantoms are within the accepted tolerance range. The largest differences were observed for treatment couch, which may be related to the phantom mobility during couch movement.

Quality evaluation of monoenergetic images generated by dual-energy computed tomography for radiotherapy: A phantom study.

PURPOSE: Quantification analysis for monoenergetic computed tomography (CT) images obtained from dual-energy CT scanning was performed in the light of their potential use for structures delineation during radiotherapy. METHODS: Parameters that describe the quality of the images are: linearity, low and high contrast resolution, uniformity, noise and signal to noise ratio (SNR). To evaluate these parameters, a Catphan phantom was scanned using a dual-energy mode at Somatom Definition AS. Based on the polyenergetic CT images, sixteen monoenergetic series (ranged from 40 keV to 190 keV) were created by CT scanner software and automatically analyzed using Artiscan software. RESULTS: Analysis of linearity shows that a potential use of any monoenergetic images in radiotherapy planning requires that individual calibration curves are implemented for each of them. While the results of the high contrast resolution analysis were comparable for each energy (5 lp/cm), the results of the analyses for uniformity, low contrast resolution, noise and SNR allowed us to select the best imaging energies. The highest relative uniformity was detected for images reconstructed for energies of 60 keV and 70 keV (98.54% and 98.61%). Similar results were observed for low contrast resolution, where the largest number of disks was detected for these energies, and the noise values (0.42% for 60 keV, 0.44% for 70 keV). The best SNR was observed for images reconstructed for energy of 60 keV. CONCLUSIONS: Taking into account these results, the energy of 70 keV was selected as potentially the best for reconstruction of monoenergetic images used for structures delineation during radiotherapy.

Conversion of the helical tomotherapy plans to the step-and-shoot IMRT plans for patients with hip prosthesis during radiotherapy for prostate cancer.

PURPOSE: To evaluate the SharePlan software in conversion of helical tomotherapy (HT) to a step and shoot IMRT (sIMRT) for patients with high-risk prostate cancer and hip prosthesis. METHODS: Analysis was performed for 16 consecutive patients treated on HT. The HT plans were converted to sIMRT plans. 3DCRT, sliding window IMRT (dIMRT) and VMAT plans for a c-arm linear accelerator (CLA) were created manually. The doses in planning target volume (PTV), bladder, rectum, bowels, femoral heads and hip prosthesis were compared using: (i) a qualitative analysis of doses in averaged dose-volume histograms, (ii) a quantitative, ranking procedure performed for each patient separately, and (iii) statistical testing based on the Friedman ANOVA and Nemenyi method. RESULTS: For the bladder, rectum, and femoral head, the best dose distributions were observed for HT and sIMRT and then for dIMRT, VMAT, and finally for 3DCRT (p-values were, respectively, 0.002, 0.004 and p = 0.024). For the bowels, 3DCRT was significantly different from the rest of the techniques (p = 0.009). For the hip prosthesis, the differences were only between 3DCRT and HT/sIMRT (p = 0.038). CONCLUSION: The SharePlan is an efficient tool for the conversion of HT plans for patients with prostate cancer and hip prosthesis. Dose distributions in sIMRT and in HT plans are similar and are generally better than in CLA plans.