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Background: Advancements in deep‐learning based synthetic computed tomography (sCT) image conversion methods have enabled the development of magnetic resonance imaging (MRI)‐only based radiotherapy treatment planning (RTP) of the brain. Purpose: This study evaluates the clinical feasibility of a commercial, deep‐learning based MRI‐only RTP method with respect to dose calculation and patient positioning verification performance in RTP of the brain. Methods: Clinical validation of dose calculation accuracy was performed by a retrospective evaluation for 25 glioma and 25 brain metastasis patients. Dosimetric and image quality of the studied MRI‐only RTP method was evaluated by a direct comparison of the sCT‐based and computed tomography (CT)‐based external beam radiation therapy (EBRT) images and treatment plans. Patient positioning verification accuracy of sCT images was evaluated retrospectively for 10 glioma and 10 brain metastasis patients based on clinical cone‐beam computed tomography (CBCT) imaging. Results: An average mean dose difference of Dmean = 0.1% for planning target volume (PTV) and 0.6% for normal tissue (NT) structures were obtained for glioma patients. Respective results for brain metastasis patients were Dmean = 0.5% for PTVs and Dmean=1.0% for NTs. Global three‐dimensional (3D) gamma pass rates using 2%/2 mm dose difference and distance‐to‐agreement (DTA) criterion were 98.0% for the glioma subgroup, and 95.2% for the brain metastasis subgroup using 1%/1 mm criterion. Mean distance differences of <1.0 mm were observed in all Cartesian directions between CT‐based and sCT‐based CBCT patient positioning in both subgroups. Conclusions: In terms of dose calculation and patient positioning accuracy, the studied MRI‐only method demonstrated its clinical feasibility for RTP of the brain. The results encourage the use of the studied method as part of a routine clinical workflow. [ABSTRACT FROM AUTHOR]

Background:The clinical feasibility of using pseudo-computed tomography (pCT) images derived from magnetic resonance (MR) images for external bean radiation therapy (EBRT) planning for prostate cancer patients has been well demonstrated. This paper investigates the feasibility of applying an MR-derived, pCT planning approach to additional types of cancer in the pelvis. Material and methods:Fifteen patients (five prostate cancer patients, five rectal cancer patients, and five gynecological cancer patients) receiving EBRT at Turku University Hospital (Turku, Finland) were included in the study. Images from an MRCAT (Magnetic Resonance for Calculating ATtenuation, Philips, Vantaa, Finland) pCT method were generated as a part of a clinical MR-simulation procedure. Dose calculation accuracy was assessed by comparing the pCT-based calculation with a CT-based calculation. In addition, the degree of geometric accuracy was studied. Results:The median relative difference of PTV mean dose between CT and pCT images was within 0.8% for all tumor types. When assessing the tumor site-specific accuracy, the median [range] relative dose differences to the PTV mean were 0.7 [−0.11;1.05]% for the prostate cases, 0.3 [−0.25;0.57]% for the rectal cases, and 0.09 [−0.69;0.25]% for the gynecological cancer cases. System-induced geometric distortion was measured to be less than 1 mm for all PTV volumes and the effect on the PTV median dose was less than 0.1%. Conclusions:According to the comparison, using pCT for clinical EBRT planning and dose calculation in the three investigated types of pelvic cancers is feasible. Further studies are required to demonstrate the applicability to a larger cohort of patients. [ABSTRACT FROM PUBLISHER]

Purpose. This study introduces methods to conduct image-guided radiotherapy (IGRT) of the pelvis with either cone-beam computed tomography (CBCT) or planar localization images by relying solely on magnetic resonance imaging (MRI)-based reference images. Material and methods. Feasibility of MRI-based reference images for IGRT was evaluated against kV CBCT (50 scans, 5 prostate cancer patients) and kV & MV planar (5 & 5 image pairs and patients) localization images by comparing the achieved patient position corrections to those obtained by standard CT-based reference images. T1/T2*-weighted in-phase MRI, Hounsfield unit conversion-based heterogeneous pseudo-CT, and bulk pseudo-CT images were applied for reference against localization CBCTs, and patient position corrections were obtained by automatic image registration. IGRT with planar localization images was performed manually by 10 observers using reference digitally reconstructed radiographs (DRRs) reconstructed from the pseudo-CTs and standard CTs. Quality of pseudo-DRRs against CT-DRRs was evaluated with image similarity metrics. Results. The SDs of differences between CBCT-to-MRI and CBCT-to-CT automatic gray-value registrations were ≤ 1.0 mm & ≤ 0.8° and ≤ 2.5 mm & ≤ 3.6° with 10 cm diameter cubic VOI and prostate-shaped VOI, respectively. The corresponding values for reference heterogeneous pseudo-CT were ≤ 1.0 mm & ≤ 0.7° and ≤ 2.2 mm & ≤ 3.3°, respectively. Heterogeneous pseudo-CT was the only type of MRI-based reference image working reliably with automatic bone registration (SDs were ≤ 0.9 mm & ≤ 0.7°). The differences include possible residual errors from planning CT to MRI registration. The image similarity metrics were significantly (p ≤ 0.01) better in agreement between heterogeneous pseudo-DRRs and CT-DRRs than between bulk pseudo-DRRs and CT-DRRs. The SDs of differences in manual registrations (3D) with planar kV and MV localization images were ≤ 1.0 mm and ≤ 1.7 mm, respectively, between heterogeneous pseudo-DRRs and CT-DRRs, and ≤ 1.4 mm and ≤ 2.1 mm between bulk pseudo-DRRs and CT-DRRs. Conclusion. This study demonstrated that it is feasible to conduct IGRT of the pelvis with MRI-based reference images. [ABSTRACT FROM AUTHOR]

Purpose: The lack of electron density information in magnetic resonance images (MRI) poses a major challenge for MRI-based radiotherapy treatment planning (RTP). In this study the authors convert MRI intensity values into Hounsfield units (HUs) in the male pelvis and thus enable accurate MRI-based RTP for prostate cancer patients with varying tissue anatomy and body fat contents. Methods: T1/T2*-weighted MRI intensity values and standard computed tomography (CT) image HUs in the male pelvis were analyzed using image data of 10 prostate cancer patients. The collected data were utilized to generate a dual model HU conversion technique from MRI intensity values of the single image set separately within and outside of contoured pelvic bones. Within the bone segment local MRI intensity values were converted to HUs by applying a second-order polynomial model. This model was tuned for each patient by two patient-specific adjustments: MR signal normalization to correct shifts in absolute intensity level and application of a cutoff value to accurately represent low density bony tissue HUs. For soft tissues, such as fat and muscle, located outside of the bone contours, a threshold-based segmentation method without requirements for any patient-specific adjustments was introduced to convert MRI intensity values into HUs. The dual model HU conversion technique was implemented by constructing pseudo-CT images for 10 other prostate cancer patients. The feasibility of these images for RTP was evaluated by comparing HUs in the generated pseudo-CT images with those in standard CT images, and by determining deviations in MRI-based dose distributions compared to those in CT images with 7-field intensity modulated radiation therapy (IMRT) with the anisotropic analytical algorithm and 360° volumetric-modulated arc therapy (VMAT) with the Voxel Monte Carlo algorithm. Results: The average HU differences between the constructed pseudo-CT images and standard CT images of each test patient ranged from −2 to 5 HUs and from 22 to 78 HUs in soft and bony tissues, respectively. The average local absolute value differences were 11 HUs in soft tissues and 99 HUs in bones. The planning target volume doses (volumes 95%, 50%, 5%) in the pseudo-CT images were within 0.8% compared to those in CT images in all of the 20 treatment plans. The average deviation was 0.3%. With all the test patients over 94% (IMRT) and 92% (VMAT) of dose points within body (lower than 10% of maximum dose suppressed) passed the 1 mm and 1% 2D gamma index criterion. The statistical tests ( t- and F-tests) showed significantly improved ( p ≤ 0.05) HU and dose calculation accuracies with the soft tissue conversion method instead of homogeneous representation of these tissues in MRI-based RTP images. Conclusions: This study indicates that it is possible to construct high quality pseudo-CT images by converting the intensity values of a single MRI series into HUs in the male pelvis, and to use these images for accurate MRI-based prostate RTP dose calculations. [ABSTRACT FROM AUTHOR]

Purpose: Magnetic resonance (MR) images are used increasingly in external radiotherapy target delineation because of their superior soft tissue contrast compared to computed tomography (CT) images. Nevertheless, radiotherapy treatment planning has traditionally been based on the use of CT images, due to the restrictive features of MR images such as lack of electron density information. This research aimed to measure absorbed radiation doses in material behind different bone parts, and to evaluate dose calculation errors in two pseudo-CT images; first, by assuming a single electron density value for the bones, and second, by converting the electron density values inside bones from T1/T2*-weighted MR image intensity values. Methods: A dedicated phantom was constructed using fresh deer bones and gelatine. The effect of different bone parts to the absorbed dose behind them was investigated with a single open field at 6 and 15 MV, and measuring clinically detectable dose deviations by an ionization chamber matrix. Dose calculation deviations in a conversion-based pseudo-CT image and in a bulk density pseudo-CT image, where the relative electron density to water for the bones was set as 1.3, were quantified by comparing the calculation results with those obtained in a standard CT image by superposition and Monte Carlo algorithms. Results: The calculations revealed that the applied bulk density pseudo-CT image causes deviations up to 2.7% (6 MV) and 2.0% (15 MV) to the dose behind the examined bones. The corresponding values in the conversion-based pseudo-CT image were 1.3% (6 MV) and 1.0% (15 MV). The examinations illustrated that the representation of the heterogeneous femoral bone (cortex denser compared to core) by using a bulk density for the whole bone causes dose deviations up to 2% both behind the bone edge and the middle part of the bone (diameter <2.5 cm), but in the opposite directions. The measured doses and the calculated ones in the standard CT image were within 0.4% (through gelatine only) and 0.9% (behind bones). Conclusions: This study indicates that the decrease in absorbed dose is not dependent on the bone diameter with all types of bones. Thus, performing dose calculation in a pseudo-CT image by assuming a single electron density value for the bones can lead to a substantial misrepresentation of the dose distribution profile. This work showed that dose calculation accuracy can be improved by using a pseudo-CT image in which the electron density values have been converted from the MR image intensity values inside bones. [ABSTRACT FROM AUTHOR]