Computed tomography (CT) scan projection radiographs, or scannograms, are used increasingly to make distance measurements for pelvimetry purposes. A request was received from a local X-ray Department to investigate the efficacy of this method prior to clinical use. One of the factors investigated was the accuracy of cursor measurements in the direction of movement of the couch and transversely across the fan beam. Cursor accuracy was assessed using the method described by Wade [1].
Background and PurposeMagnetic Resonance (MR)-only radiotherapy enables the use of MR without the uncertainty of MR–Computed Tomography (CT) registration. This requires a synthetic CT (sCT) for dose calculations, which can be facilitated by a novel Zero Echo Time (ZTE) sequence where bones are visible and images are acquired in 65 seconds. This study evaluated the dose calculation accuracy for pelvic sites of a ZTE-based Deep Learning sCT algorithm developed by GE Healthcare.Materials and MethodsZTE and CT images were acquired in 56 pelvic radiotherapy patients in the radiotherapy position. A 2D U-net convolutional neural network was trained using pairs of deformably registered CT and ZTE images from 36 patients. In the remaining 20 patients the dosimetric accuracy of the sCT was assessed using cylindrical dummy Planning Target Volumes (PTVs) positioned at four different central axial locations, as well as the clinical treatment plans (for prostate (n = 10), rectum (n = 4) and anus (n = 6) cancers). The sCT was rigidly and deformably registered, the plan recalculated and the doses compared using mean differences and gamma analysis.ResultsMean dose differences to the PTV D98% were ≤ 0.5% for all dummy PTVs and clinical plans (rigid registration). Mean gamma pass rates at 1%/1 mm were 98.0 ± 0.4% (rigid) and 100.0 ± 0.0% (deformable), 96.5 ± 0.8% and 99.8 ± 0.1%, and 95.4 ± 0.6% and 99.4 ± 0.4% for the clinical prostate, rectum and anus plans respectively.ConclusionsA ZTE-based sCT algorithm with high dose accuracy throughout the pelvis has been developed. This suggests the algorithm is sufficiently accurate for MR-only radiotherapy for all pelvic sites.
Background and PurposeMagnetic resonance (MR)-only treatment pathways require either the MR-simulation or synthetic-computed tomography (sCT) as an alternative reference image for cone beam computed tomography (CBCT) patient position verification. This study assessed whether using T2 MR or sCT as CBCT reference images introduces systematic registration errors as compared to CT for anal and rectal cancers.Materials and MethodsA total of 32 patients (18 rectum,14 anus) received pre-treatment CT- and T2 MR- simulation. Routine treatment CBCTs were acquired. sCTs were generated using a validated research model. The local clinical registration protocol, using a grey-scale registration algorithm, was performed for 216 CBCTs using CT, MR and sCT as the reference image. Linear mixed effects modelling identified systematic differences between modalities.ResultsSystematic translation and rotation differences to CT for MR were −0.3 to + 0.3 mm and −0.1 to 0.4° for anal cancers and −0.4 to 0.0 mm and 0.0 to 0.1° for rectal cancers, and for sCT were −0.4 to + 0.8 mm, −0.1 to 0.2° for anal cancers and −0.6 to + 0.2 mm, −0.1 to + 0.1° for rectal cancers.ConclusionsT2 MR or sCT can successfully be used as reference images for anal and rectal cancer CBCT position verification with systematic differences to CT <±1 mm and <±0.5°. Clinical enabling of alternative modalities as reference images by vendors is required to reduce challenges associated with their use.
Purpose This study investigates the dosimetric accuracy as well as the robustness of a bulk density assignment approach to magnetic resonance imaging (MRI)‐only based treatment planning of the prostate, with bulk density regions automatically identified using atlas‐based segmentation (ABS). Methods Twenty prostate radiotherapy patients received planning computed tomography (CT) and MRI scans and were treated with volumetric modulated arc therapy (VMAT). Two bulk densities were set, one for bone and one for soft tissue. The bone contours were created by using ABS followed by manual modification if considered necessary. A range of soft tissue and bone density pairs, between 0.95 and 1.03 g/cm 3 with increments of 0.01 for soft tissue, and between 1.15 and 1.65 g/cm 3 with increments of 0.05 for bone, were evaluated. Using the density pair giving the lowest dose difference compared to the CT‐based dose, dose differences were calculated using both the manually modified bone contours and the bone contours from ABS. Contour overlap measurements between the ABS contours and the manually modified contours were calculated. Results The dose comparison shows a very good agreement with the CT when using 0.98 g/cm 3 for soft tissue and 1.20 g/cm 3 for bone, with a dose difference less than 1 % in average dose in all regions of interest. The mean Dice similarity coefficient for bone was 0.94 and the Mean Distance to Agreement was <1 mm in most cases. Conclusions Using bulk density assignment on MR images with suitable densities for bone and soft tissue results in clinically acceptable dose differences compared to dose calculated on the CT, for both atlas‐based and manual bone contours. This demonstrates that an integrated MRI‐only pathway utilizing a bulk density assignment for two tissue types is a feasible and robust approach for patients with prostate cancer treated with VMAT.
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