Search

Aims Interstitial brachytherapy (ISBT) plays an important role in the management of locally advanced gynaecological malignancies. However, the relationship between urinary toxicity and dose to the urethra is not well understood. We sought to evaluate the correlation between urethral dose and the incidence of genitourinary complications among patients undergoing vaginal high dose rate ISBT. Materials and methods Eighty-three patients treated with ISBT between August 2014 and April 2018 were retrospectively reviewed. CTCAE version 5.0 was used to grade toxicity. Individual treatment plans were evaluated to collect dose parameters. Urethral contours were added to the structure sets using a uniform 1 cm diameter brush and minimum doses to the hottest 0.1, 0.2 and 0.5 cm3 (D0.1cm3, D0.2cm3 and D0.5cm3) of the urethra were obtained. Total (ISBT ± external beam radiotherapy) equivalent doses in 2 Gy fractions (EQD2) received by the targets and organs at risk were calculated. Numerical counts (%) and medians (interquartile range) were used to characterise the data. Fisher's exact and the Mann–Whitney–Wilcox tests were used as appropriate. Receiver operator curve analysis was used to define the urethral threshold dose that correlated to genitourinary toxicity. Results The median age and follow-up times were 67 years (59–75) and 25 months (16–37), respectively. Patients had predominantly primary endometrial (49%) and vaginal (37%) cancer, with four (5%) patients with metastatic rectal cancer to the vagina. Twenty-four of 79 (30%) patients experienced acute genitourinary toxicity and 34 of 71 (48%) experienced late genitourinary toxicity. In both analyses, the median urethral dose was significantly higher among those with toxicity. Receiver operator curve analysis indicated that D0.1cm3, D0.2cm3 and D0.5cm3 of the urethra were associated with the development of toxicity at doses >78, >71 and >62 Gy, respectively. Conclusion Urethral dose seems to predict genitourinary toxicity in ISBT of vaginal tumours. Further study with an expanded cohort and longer follow-up is warranted.

PURPOSE While brachytherapy is an effective treatment for localized prostate cancer, there has been a noticeable decline in its use. Training opportunity for prostate brachytherapy has been in steady decline, with some residents receiving little to no hands-on training. This work was developed to design a training environment that uses a phantom-based simulator to teach the process of TRUS-based prostate brachytherapy METHODS AND MATERIALS A prostate phantom was fabricated from a representative prostate patient TRUS scan. Three materials were used: gelatin powder, graphite powder, and water. The prostate was developed using 9% gelatin and 0.3% graphite per 100 ml water. Five radiation oncologists were asked to qualitatively score the phantom according to image quality, haptic feedback, needle insertion quality, and its compatibility with operative tools. The contrast-to-noise ratio (CNR) was estimated using different concentrations of graphite. The elasticity of the phantom was evaluated based on ultrasound elastography measurements RESULTS The prostate phantom had an average CNR of 3.94 ± 1.09 compared to real prostate images with a CNR of 2 ± 1.8. The average Young's modulus was computed to be 58.03 ± 6.24 kPa compared to real prostate tissue (58.8 ± 8.2 kPa). Oncologists ranked the phantom as “very good” for overall quality of the phantom. They reported that needle insertion quality was “very good” during a simulated brachytherapy procedure. CONCLUSION We have developed a 3D printing prostate phantom to be used for training purposes during prostate brachytherapy. The phantom has been evaluated for image quality and elasticity. The reconstructed phantom could be used as an anthropomorphic surrogate to train residents on prostate brachytherapy procedures.

Magnetic resonance imaging (MRI) offers excellent soft-tissue contrast enabling the contouring of targets and organs at risk during gynecological interstitial brachytherapy procedure. Despite its advantage, one of the main obstacles preventing a transition to an MRI-only workflow is that implanted plastic catheters are not reliably visualized on MR images. This study aims to evaluate the feasibility of a deep-learning-based algorithm for semiautomatic reconstruction of interstitial catheters during an MR-only workflow. MR images of 20 gynecological patients were used in this study. Note that 360 catheters were reconstructed using T1- and T2-weighted images by five experienced brachytherapy planners. The mean of the five reconstructed paths were used for training (257 catheters), validation (15 catheters), and testing/evaluation (88 catheters). To automatically identify and localize the catheters, a two-dimensional (2D) U-net algorithm was used to find their approximate location in each image slice. Once localized, thresholding was applied to those regions to find the extrema, as catheters appear as bright and dark regions in T1- and T2-weighted images, respectively. The localized dwell positions of the proposed algorithm were compared to the ground truth reconstruction. Reconstruction time was also evaluated. A total of 34 009 catheter dwell positions were evaluated between the algorithm and all planners to estimate the reconstruction variability. The average variation was 0.97 ± 0.66 mm. The average reconstruction time for this approach was 11 ± 1 min, compared with 46 ± 10 min for the expert planners. This study suggests that the proposed deep learning, MR-based framework has potential to replace the conventional manual catheter reconstruction. The adoption of this approach in the brachytherapy workflow is expected to improve treatment efficiency while reducing planning time, resources, and human errors.

Background and purpose To identify if, in intermediate risk prostate cancer (IR-PCa), the absolute percentage of biopsied tissue positive for pattern 4 disease (APP4) may be a predictor of outcome. Materials and methods 411 patients with IR-PCa were retrospectively reviewed. APP4 was calculated based on biopsy reports. Multivariable competing risk analysis was then performed on optimized APP4 cutpoints to predict for biochemical failure (BF), androgen deprivation use for BF (ADT-BF) and development of metastases (MD). Results Median follow-up for the cohort was 5.2 (Inter Quartile Range: 2.9–6.6) years. Median baseline PSA was 7.3 (5.3–9.8) ng/mL. 234 (56.9%) patients had T1 and 177 (43.1%) had T2 disease. Median APP4 was 2.00 (0.75–7.50)%. 38 (9.3%) patients experienced BF. The optimal cutpoint of APP4 for BF was >3.3% with an area under the curve (AUC) of 0.66. 17 (4.1%) received ADT-BF. The ADT-BF cutpoint was >6.6% with an AUC of 0.72. Eight (2.0%) developed MD. The MD cutpoint was >17.5% with an AUC of 0.86. Using APP4 >3.3 vs ≤ 3.3, log-transformed baseline PSA ln(PSA) (HR 2.5, 1.1–6.1; p = 0.037) and APP4 (HR 2.3, 1.1–4.7; p = 0.031) predicted for BF. Using APP4 >6.6 vs ≤ 6.6, ln(PSA) (HR 4.2, 1.4–12.4; p = 0.010) and APP4 (HR 3.7, 1.4–10.0; p = 0.009) were predictive of ADT-BF. APP4 >17.5 vs ≤ 17.5 alone was predictive of MD (HR 25.7, 4.9–135.3; p Conclusion APP4 cutpoints of >3.3%, >6.6% and >17.5% were strongly associated with increased risk of BF, ADT-BF and developing MD respectively. These findings may inform future practice when treating IR-PCa but require external validation.

PURPOSE/OBJECTIVE(S) Little is known about optimal dose constraints when high dose rate (HDR) brachytherapy is used as monotherapy for prostate cancer. Most dose constraints for HDR monotherapy were extrapolated from predictors of toxicity when HDR brachytherapy was combined with external beam radiotherapy as a boost. We sought to determine clinical and dosimetric predictors of toxicity and health related quality of life (HRQOL) in men treated with HDR brachytherapy as monotherapy. MATERIALS/METHODS Eligible patients were treated with single fraction HDR brachytherapy as monotherapy on two prospective clinical trials at a single institution. Patients in the first trial (HDR-mono) received a single fraction of 19 Gy without a dominant intraprostatic lesion (DIL) boost, and patients from the second trial (MARS) received 19 Gy in a single fraction with an MRI-guided DIL boost to ≥ 23 Gy. ADT was not used. Univariable and multivariable logistic regression was used to evaluate clinical (age, IPSS score, prostate volume, alpha blocker use at baseline, and receipt of a DIL boost) and dosimetric (prostate V100, V150, V200, D90, Urethral Dmax and D10, and Rectal Dmax and V80) predictors of CTCAE v4 acute/late toxicity and HRQOL changes measured with expanded prostate index composite (EPIC). Three classifications of late minimally clinically important changes (MCICs) were evaluated: small (> 0.5 standard deviation [SD]), moderate (> 1.0 SD) and severe (> 2.0 SD) declines compared to baseline. RESULTS 147 patients were included (87 from HDR-mono, 60 from MARS). Median follow-up was 62.8 months. Only increasing prostate size predicted acute GU toxicity ≥ grade 2 (OR 1.05, 95% CI 1.01-1.09, P = 0.021). On multivariable regression, predictors of late GU toxicity ≥ grade 2 were not receiving a DIL boost (OR 3.78, 95% CI 1.88-7.83, P < 0.001) and higher baseline IPSS score (OR 1.89, 95% CI 1.89-3.18, P = 0.015). Rectal and urethral dose constraints were not associated with late GU/GI toxicity ≥ grade 2. Contrary to our hypothesis, small (OR 0.91, 95% CI 0.81-0.98, P = 0.032) and moderate (OR 0.91, 95% CI 0.80-0.98, P = 0.037) urinary MCICs were less frequent in those with higher urethral Dmax. No impact of urethra D10 on urinary MCICs was seen. Rectal Dmax and V80 did not predict bowel MCIC changes at any threshold. CONCLUSION We were unable to identify dose constraints that were predictive of late toxicity or relevant HRQOL changes in single fraction HDR brachytherapy. This suggests dose parameters used in these trials were safe, and that minor variability in plan dosimetry likely has little clinical significance. While increasing urethral Dmax paradoxically predicted for less frequent urinary MCICs, we do not believe this finding is clinically relevant. Further research exploring optimal dose constraints to be used in HDR brachytherapy as monotherapy for prostate cancer is warranted.

The purpose of this study was to examine RBE variation as a function of distance from the radioactive source, and the potential impact of this variation on a realistic prostate brachytherapy treatment plan.Three brachytherapy sources (The RBEThe large increase RBE

Purpose Identifying dominant intraprostatic lesions (DILs) on transrectal ultrasound (TRUS) images during prostate high-dose-rate brachytherapy treatment planning remains a significant challenge. Multiparametric MRI (mpMRI) is the tool of choice for DIL identification; however, the geometry of the prostate on mpMRI and on the TRUS may differ significantly, requiring image registration. This study assesses the dosimetric impact attributed to differences in DIL contours generated using commonly available MRI to TRUS automated registration: rigid, semi-rigid, and deformable image registration, respectively. Methods and Materials Ten patients, each with mpMRI and TRUS data sets, were included in this study. Five radiation oncologists with expertise in TRUS-based high-dose-rate brachytherapy were asked cognitively to transfer the DIL from the mpMRI images of each patient to the TRUS image. The contours were analyzed for concordance using simultaneous truth and performance level estimation (STAPLE) algorithm. The impact of DIL contour differences due to registration variability was evaluated by comparing the STAPLE-DIL dosimetry from the reference (STAPLE) plan with that from the evaluation plans (manual and automated registration) for each patient. The dosimetric impact of the automatic registration approach was also validated using a margin expansion that normalizes the volume of the autoregistered DILs to the volumes of the STAPLE-DILs. Dose metrics including D90, Dmean, V150, and V200 to the prostate and DIL were reported. For urethra and rectum, D10 and V80 were reported. Results Significant differences in DIL coverage between reference and evaluation plans were found regardless of the algorithm methodology. No statistical difference was reported in STAPLE-DIL dosimetry when manual registration was used. A margin of 1.5 ± 0.8 mm, 1.1 ± 0.8 mm, and 2.5 ± 1.6 mm was required to be added for rigid, semi-rigid, and deformable registration, respectively, to mitigate the difference in STAPLE-DIL coverage between the evaluation and reference plans. Conclusion The dosimetric impact of integrating an MRI-delineated DIL into a TRUS-based brachytherapy workflow has been validated in this study. The results show that rigid, semi-rigid, and deformable registration algorithms lead to a significant undercoverage of the DIL D90 and Dmean. A margin of at least 1.5 ± 0.8 mm, 1.1 ± 0.8 mm, and 2.5 ± 1.6 mm is required to be added to the rigid, semi-rigid, and deformable DIL registration to be suitable for DIL-boosting during prostate brachytherapy.

Magnetic resonance imaging (MRI) offers excellent soft-tissue contrast enabling the contouring of targets and organs at risk (OARs) during gynecological interstitial brachytherapy procedure. Despite its benefit, one of the main challenges toward MRI-only workflows is that the implanted catheters are not reliably visualized on MR images. This study aims to evaluate the feasibility of MR-only workflow using an in-house MR line marker during interstitial gynecological high-dose-rate (HDR) brachytherapy.Ten patients diagnosed with locally advanced cervical cancer treated with HDR brachytherapy were included in this study. The hybrid CT/MR-treated plan was used as the study reference plan. Five users manually reconstructed the catheter's path on MR images (3D T1- and T2-weighted). Subsequently, the dwell positions from the users' plans were superimposed on the reference plans to evaluate the dosimetric impact of the using MR-only for catheter reconstruction in comparison with hybrid CT/MR approach. Variability of dwell positions between users and reconstruction time was also evaluated.More than 96.90% of catheter reconstruction variations were2 mm. No statistical differences were reported between MR-only and hybrid CT/MR in gross tumor volume DThe feasibility of MR-only workflow using MR line marker during interstitial gynecological HDR brachytherapy has been validated in this study. The results show that the MR-only workflow is equivalent to the conventional hybrid CT/MR approach in terms of gross tumor volume and high-risk clinical target volume coverage and respecting of OARs dose limits.

The purpose of this study was to quantitatively assess the CT metal-induced artifacts from a novel direction-modulated brachytherapy (DMBT) tandem applicator prototype, recently designed for cervical cancer treatments.A water-based pelvic phantom was constructed for CT scanning. The DMBT applicator was imaged using our institutional protocol, one with higher kVp and mAs settings, and repetition of these protocols using 3-mm slices. A conventional stainless steel applicator was also scanned. In addition to the standard reconstructed images, applicator images were reconstructed using a commercial metal artifact-reduction (MAR) algorithm and an in-house-developed research algorithm. Subsequently, image quality and artifact severity were evaluated.Artifact severity, measured in terms of SDs in CT numbers, decreased asymptotically to background water levels with the distance away from the applicator. Artifact-reduction algorithms lead to significant and visible improvements in image quality, with50% and20% decrease in artifact severity achieved at a 10-mm distance for the DMBT and stainless steel applicators, respectively. Differences in artifact severity were minimal between the four imaging protocols. DMBT dimensions were the same on images with and without the commercial MAR algorithm, within1 mm of the theoretical value. Both the commercial and in-house algorithms restored the CT numbers outside the applicator, albeit a better performance was achieved by the in-house algorithm.The artifacts produced by both applicators were minimized with the use of MAR algorithms. Adoption of the DMBT and stainless steel applicators for CT-guided brachytherapy is anticipated as MAR algorithms are widely available on CT scanners.

Purpose The purposes of this study were to investigate the pattern of relapse in patients with low- or intermediate-risk prostate cancer treated with 19-Gy high-dose-rate brachytherapy (HDR-BT) and to calculate the dose received by the area of recurrence. Methods and Materials Patients included in this analysis were treated under a Phase II randomized trial that evaluated the role of 19-Gy HDR-BT monotherapy in low- and intermediate-risk prostate cancers. Multiparametric prostate MRI and prostate biopsy were performed in patients with suspicious local recurrence. The site of local relapse was compared with the initial site of disease. The dose received by the site of recurrence was investigated through registration of the posttreatment multiparametric prostate MRI with the HDR-BT treatment plan. Results Eight of 87 treated patients were found to have local recurrence after 19-Gy HDR-BT. Seven of the eight recurrences were at the site of initial bulk disease. Seven patients were found to have a more aggressive histology in the posttreatment biopsy. The mean volume of prostate that had received 100% of prescription dose was 97%. Mean dose to area of recurrence was 29.1 Gy, whereas dose to 98% and dose to 90% of the recurrence were 21.6 Gy and 23.2 Gy, respectively. Conclusions The relapse pattern after a single 19-Gy HDR-BT is predominantly associated with the site of initial disease. This lends some rationale to future strategies of further focused dose escalation to initial site of disease, notwithstanding the fact that the calculated biologically equivalent dose using linear–quadratic assumptions is already very high.

Purpose. To investigate the feasibility of using a single MRI acquisition for fiducial marker identification and synthetic CT (sCT) generation towards MRI-only treatment planning for prostate external beam radiation therapy (EBRT). Methods. Seven prostate cancer patients undergoing EBRT, each with three implanted gold fiducial markers, participated in this study. In addition to the planning CT scan, all patients were scanned on a 3 T MR scanner with a 3D double-echo gradient echo (GRE) sequence. Quantitative susceptibility mapping (QSM) was performed for marker localization. QSM-derived marker positions were compared to those from CT. The bulk density assignment technique for sCT generation was adopted. The magnitude GRE images were segmented into muscle, bone, fat, and air using a combination of unsupervised intensity-based classification of soft tissue and convolutional neural networks (CNN) for bone segmentation. Results. All implanted markers were visualized and accurately identified (average error: 0.7 ± 0.5 mm). QSM generated distinctive contrast for hemorrhage, calcifications, and gold fiducial markers. The estimated susceptibility/HU values on QSM/CT for gold and calcifications were 31.5 ± 2.9 ppm/1220 ± 100 HU and 14.6 ± 0.9 ppm/440 ± 100 HU, respectively. The intensity-based soft tissue classification resulted in an average Dice score of 0.97 ± 0.02; bone segmentation using CNN resulted in an average Dice score of 0.93 ± 0.03. Conclusion. This work indicates the feasibility of simultaneous fiducial marker identification and sCT generation using a single MRI acquisition. Future works includes evaluation of the proposed method in a large cohort of patients with optimized acquisition parameters as well as dosimetric evaluations.

A new GPU‐based Monte Carlo dose calculation algorithm (GPUMCD), developed by the vendor Elekta for the Monaco treatment planning system (TPS), is capable of modeling dose for both a standard linear accelerator and an Elekta MRI linear accelerator. We have experimentally evaluated this algorithm for a standard Elekta Agility linear accelerator. A beam model was developed in the Monaco TPS (research version 5.09.06) using the commissioned beam data for a 6 MV Agility linac. A heterogeneous phantom representing several scenarios — tumor‐in‐lung, lung, and bone‐in‐tissue — was designed and built. Dose calculations in Monaco were done using both the current clinical Monte Carlo algorithm, XVMC, and the new GPUMCD algorithm. Dose calculations in a Pinnacle TPS were also produced using the collapsed cone convolution (CCC) algorithm with heterogeneity correction. Calculations were compared with the measured doses using an ionization chamber (A1SL) and Gafchromic EBT3 films for 2×2 cm2,5×5 cm2, and 10×2 cm2 field sizes. The percentage depth doses (PDDs) calculated by XVMC and GPUMCD in a homogeneous solid water phantom were within 2%/2 mm of film measurements and within 1% of ion chamber measurements. For the tumor‐in‐lung phantom, the calculated doses were within 2.5%/2.5 mm of film measurements for GPUMCD. For the lung phantom, doses calculated by all of the algorithms were within 3%/3 mm of film measurements, except for the 2×2 cm2 field size where the CCC algorithm underestimated the depth dose by ∼5% in a larger extent of the lung region. For the bone phantom, all of the algorithms were equivalent and calculated dose to within 2%/2 mm of film measurements, except at the interfaces. Both GPUMCD and XVMC showed interface effects, which were more pronounced for GPUMCD and were comparable to film measurements, whereas the CCC algorithm showed these effects poorly. PACS number(s): 87.53.Bn, 87.55.dh, 87.55.km

Purpose: This paper provides a comparison between a fast, commercial, in-patient Monte Carlodose calculation algorithm (GPUMCD) and geant4. It also evaluates the dosimetric impact of the application of an external 1.5 T magnetic field. Methods: A stand-alone version of the Elekta™ GPUMCD algorithm, to be used within the Monaco treatment planning system to model dose for the Elekta™ magnetic resonance imaging(MRI)Linac, was compared against geant4 (v10.1). This was done in the presence or absence of a 1.5 T static magnetic field directed orthogonally to the radiation beam axis. Phantoms with material compositions of water, ICRU lung, ICRU compact-bone, and titanium were used for this purpose. Beams with 2 MeV monoenergetic photons as well as a 7 MV histogrammed spectrum representing the MRILinac spectrum were emitted from a point source using a nominal source-to-surface distance of 142.5 cm. Field sizes ranged from 1.5 × 1.5 to 10 × 10 cm2. Dose scoring was performed using a 3D grid comprising 1 mm3 voxels. The production thresholds were equivalent for both codes. Results were analyzed based upon a voxel by voxel dose difference between the two codes and also using a volumetric gamma analysis. Results: Comparisons were drawn from central axis depth doses, cross beam profiles, and isodose contours. Both in the presence and absence of a 1.5 T static magnetic field the relative differences in doses scored along the beam central axis were less than 1% for the homogeneous water phantom and all results matched within a maximum of ±2% for heterogeneous phantoms. Volumetric gamma analysis indicated that more than 99% of the examined volume passed gamma criteria of 2%—2 mm (dose difference and distance to agreement, respectively). These criteria were chosen because the minimum primary statistical uncertainty in dose scoring voxels was 0.5%. The presence of the magnetic field affects the dose at the interface depending upon the density of the material on either sides of the interface. This effect varies with the field size. For example, at the water-lung interface a 33.94% increase in dose was observed (relative to the Dmax), by both GPUMCD and geant4 for the field size of 2 × 2 cm2 (compared to no B-field case), which increased to 47.83% for the field size of 5 × 5 cm2 in the presence of the magnetic field. Similarly, at the lung-water interface, the dose decreased by 19.21% (relative to Dmax) for a field size of 2 × 2 cm2 and by 30.01% for 5 × 5 cm2field size. For more complex combinations of materials the dose deposition also becomes more complex. Conclusions: The GPUMCD algorithm showed good agreement against geant4 both in the presence and absence of a 1.5 T external magnetic field. The application of 1.5 T magnetic field significantly alters the dose at the interfaces by either increasing or decreasing the dose depending upon the density of the material on either side of the interfaces.

Purpose The purpose of this study was to examine RBE variation as a function of distance from the radioactive source, and the potential impact of this variation on a realistic prostate brachytherapy treatment plan. Methods Three brachytherapy sources (125I, 192Ir, and 169Yb) were modelled in Geant4 Monte Carlo code, and the resulting electron energy spectrum in water in 3D space around these sources was scored (voxel size of 2 mm3). With this energy spectrum, microdosimetric techniques were used to calculate the maximum RBE, RBEM, as a function of distance from the source. RBEM of 125I relative to 192Ir was calculated in order to validate simulations against literature; all other RBEM calculations were done by normalizing electron fluence at various distances to the source position. In order to examine the impact of RBEM variation in treatment planning, a realistic 192Ir prostate plan was re-evaluated in terms of RBE instead of absorbed dose. Results The RBEM of 125I, 192Ir, and 169Yb at 8 cm away from the source was 0.994 (+/-0.002), 1.030 (+/-0.003), and 1.066 (+/-0.008), respectively. RBEM in the HDR prostate treatment plan exhibited several hot (+3.6% in RBEM) spots. Conclusions The large increase RBEM observed in 169Yb has not yet been described in the literature. Despite the presence of radiobiological hotspots in the HDR treatment, these variations are likely nominal and clinically insignificant.

Magnetic resonance-guided radiation therapy (MR-GRT) offers great potential to improve radiation treatment outcomes by providing more accurate and patient-tailored therapy. Despite superior soft tissue contrast in MRI, one of the challenges towards MRI-only workflows is that the process often requires some sort of 'MR-invisible' metal-based devices. In this study, the feasibility of quantitative susceptibility mapping (QSM) for visualization of some MR-invisible radiation therapy devices was studied. Our recently proposed QSM-based algorithm for brachytherapy seed visualization was modified and the feasibility of the optimized algorithm for visualization of different devices including: brachytherapy seeds, plastic interstitial needles, CT-markers and obturators, and different types of fiducial markers in agar, prostate and meat phantoms were studied. All phantoms were scanned using 3T MR scanner with a 3D multi-echo gradient recalled echo (ME-GRE) pulse sequence. The QSM results in all phantoms were compared to CT images for spatial accuracy of the QSM. The applied post-processing algorithm was found to be insensitive to the seeds' type; also, presence of nearby calcifications had no effect on seed visualization. QSM successfully generated positive contrast for both types of investigated fiducial markers with high spatial accuracy compared to CT. Interstitial needles containing both aluminum-based CT-maker and titanium-based obturators were accurately depicted on the QSM. The proposed QSM-based technique relies on the standard MR pulse sequences and visualize the conventional MR-invisible metallic devices with CT-like positive contrast solely through post-processing. Upon in vivo validation of the technique, QSM may have the potential to replace CT for an MR-only guided radiation therapy.

Purpose: To evaluate the performance of a model based image reconstruction method in reducing metal artifacts in the megavoltage computed tomography (MVCT) images of a phantom representing bilateral hip prostheses and to compare with the filtered-backprojection (FBP) technique. Methods: An iterative maximum likelihood polychromatic algorithm for CT (IMPACT) is used with an additional model for the pair/triplet production process and the energy dependent response of the detectors. The beam spectra for an in-house bench-top and TomoTherapy Trade-Mark-Sign MVCTs are modeled for use in IMPACT. The empirical energy dependent response of detectors is calculated using a constrained optimization technique that predicts the measured attenuation of the beam by various thicknesses (0-24 cm) of solid water slabs. A cylindrical (19.1 cm diameter) plexiglass phantom containing various cylindrical inserts of relative electron densities 0.295-1.695 positioned between two steel rods (2.7 cm diameter) is scanned in the bench-top MVCT that utilizes the bremsstrahlung radiation from a 6 MeV electron beam passed through 4 cm solid water on the Varian Clinac 2300C and in the imaging beam of the TomoTherapy Trade-Mark-Sign MVCT. The FBP technique in bench-top MVCT reconstructs images from raw signal normalized to air scan and corrected for beam hardening using a uniformmore » plexiglass cylinder (20 cm diameter). The IMPACT starts with a FBP reconstructed seed image and reconstructs the final image in 150 iterations. Results: In both MVCTs, FBP produces visible dark shading in the image connecting the steel rods. In the IMPACT reconstructed images this shading is nearly removed and the uniform background is restored. The average attenuation coefficients of the inserts and the background are very close to the corresponding values in the absence of the steel inserts. In the FBP images of the bench-top MVCT, the shading causes 4%-9.5% underestimation of electron density at the central inserts with an average of (6.3 {+-} 1.8)% for the range of electron densities studied. In the uniform plexiglass background, the shadow creates 0.8%-4.7% underestimation of electron density with an average of (2.9 {+-} 1.2)%. In the corresponding IMPACT images, the underestimation in the shaded plexiglass background is 0.3%-1.8% with an average of (0.9 {+-} 0.5)% and 1.4%-6.8% with an average of (2.8 {+-} 2.7)% in the central insert region. In the FBP images of the TomoTherapy Trade-Mark-Sign MVCT, this shading creates 2.6%-6.7% underestimation of electron density with an average of (3.7 {+-} 1.4)% at the central inserts and 5.9%-7.2% underestimation in the background with an average of (6.4 {+-} 0.5)%. In the IMPACT images, the uniform background between the steel rods is restored with 0.3%-1.0% underestimation of electron density with an average of (0.7 {+-} 0.3)%. The corresponding underestimation at the central inserts of the IMPACT images is -0.4%-0.1% with an average of (-0.1 {+-} 0.2)%. Conclusions: The shading metal artifact has been nearly removed in MVCT images using the IMPACT algorithm with the accurate geometry of the system, proper modeling of energy dependent response of detectors, and all relevant photon interaction processes. This results less than 1% difference in electron density in the background plexiglass and less than 3% averaged over the range of electron densities investigated.« less

Large magnetoresistance effects were observed in Ni50Mn50−xSnx Heusler alloys for concentrations 13⩽x⩽16. The effects occur at the respective martensitic transition temperatures (TM), which can be tuned in temperature through variations in Sn concentration. The largest observed values of the magnetoresistance (Δρ/ρo) were −42% for x=14 at T≈250 K, and −23% above room temperature (T≈320 K) for the sample with x=13, where the magnetic field change was ΔH=5 T. Magnetization and X-ray diffraction measurements indicate that the origin of the observed magnetoresistance is a magnetically induced magnetostructural transition from a complex 10 M modulated orthorhombic phase containing some antiferromagnetic coupling to a ferromagnetic cubic L21 structure.

Purpose: An MRI-linear accelerator is currently being developed by the vendor Elekta™. The treatment planning system that will be used to model dose for this unit uses a Monte Carlo dose calculation algorithm, GPUMCD, that allows for the application of a magnetic field. We tested this radiation transport code against an independent Monte-Carlo toolkit Geant4 (v.4.10.01) both with and without the magnetic field applied. Methods: The setup comprised a 6 MeV mono-energetic photon beam emerging from a point source impinging on a homogeneous water phantom at 100 cm SSD. The comparisons were drawn from the percentage depth doses (PDD) for three different field sizes (1.5 x 1.5 cm2, 5 x 5 cm2, 10 x 10 cm2) and dose profiles at various depths. A 1.5 T magnetic field was applied perpendicular to the direction of the beam. The transport thresholds were kept the same for both codes. Results: All of the normalized PDDs and profiles agreed within ± 1 %. In the presence of the magnetic field, PDDs rise more quickly reducing the depth of maximum dose. Near the beam exit point in the phantom a hot spot is created due to the electron return effect. This effect is more pronounced for the larger field sizes. Profiles selected parallel to the external field show no effect, however, the ones selected perpendicular to the direction of the applied magnetic field are shifted towards the direction of the Lorentz force applied by the magnetic field on the secondary electrons. It is observed that these profiles are not symmetric which indicates a lateral build up of the dose. Conclusion: There is a good general agreement between the PDDs/profiles calculated by both algorithms thus far. We are proceeding towards clinically relevant comparisons in a heterogeneous phantom for polyenergetic beams. Funding for this work has been provided by Elekta.

Purpose To evaluate the metal artifacts in diagnostic kilovoltage computed tomography (kVCT) images of patients that are corrected by use of a normalized metal artifact reduction (NMAR) method with megavoltage CT (MVCT) prior images: MVCT-NMAR. Methods and Materials MVCT-NMAR was applied to images from 5 patients: 3 with dual hip prostheses, 1 with a single hip prosthesis, and 1 with dental fillings. The corrected images were evaluated for visualization of tissue structures and their interfaces and for radiation therapy dose calculations. They were compared against the corresponding images corrected by the commercial orthopedic metal artifact reduction algorithm in a Phillips CT scanner. Results The use of MVCT images for correcting kVCT images in the MVCT-NMAR technique greatly reduces metal artifacts, avoids secondary artifacts, and makes patient images more useful for correct dose calculation in radiation therapy. These improvements are significant, provided the MVCT and kVCT images are correctly registered. The remaining and the secondary artifacts (soft tissue blurring, eroded bones, false bones or air pockets, CT number cupping within the metal) present in orthopedic metal artifact reduction corrected images are removed in the MVCT-NMAR corrected images. A large dose reduction was possible outside the planning target volume (eg, 59.2 Gy to 52.5 Gy in pubic bone) when these MVCT-NMAR corrected images were used in TomoTherapy treatment plans without directional blocks for a prostate cancer patient. Conclusions The use of MVCT-NMAR corrected images in radiation therapy treatment planning could improve the treatment plan quality for patients with metallic implants.

A systematic study of electric-field-tuned ferromagnetic resonance (FMR) of a ferroelectric/ferromagnetic/semiconductor multiferroic heterostructure, consisting of a Co${}_{2}$MnSb epitaxial film grown on a GaAs substrate bonded to a lead zinc niobate--lead titanate crystal, is reported. The films, grown by pulsed laser deposition, were studied for their crystallographic structure, magnetocrystalline anisotropy, and magnetostrictive and ferromagnetic resonance properties. Ferromagnetic resonance measurements were carried out at $X$-band frequency under the application of electric fields with external magnetic fields applied along the [110], [100], [$1\overline{1}0$], and [001] directions of the Heusler film. Magnetic anisotropy fields were derived from the angular dependence of FMR measurements, yielding an in-plane fourth-order anisotropy constant ${K}_{1}=\ensuremath{-}150\ifmmode\times\else\texttimes\fi{}{10}^{3}$ erg/cm${}^{3}$ and a perpendicular second-order anisotropy constant ${K}_{\ensuremath{\perp}}=12\ifmmode\times\else\texttimes\fi{}{10}^{3}$ erg/cm${}^{3}$. A theoretical model, which includes the effects of electric-field tuning, is presented to calculate the tunability of the ferromagnetic resonance frequency of the multiferroic heterostructure. The multiferroic heterostructure exhibits a frequency tuning of 450 MHz under the application of an electric field of 10 kV cm${}^{\ensuremath{-}1}$, corresponding to a magnetoelectric coupling coefficient of 8.8 Oe cm kV${}^{\ensuremath{-}1}$. This work explores the potential of electronically controlled multiferroic devices for use in microwave integrated circuits, while concomitantly establishing the basic theoretical foundation allowing for the calculation of microwave tunability for this and other heterostructures.

Purpose: To evaluate the metal artifacts in diagnostic kVCT images of patients that are corrected using a normalized metal artifact reduction method with MVCT prior images, MVCT-NMAR. Methods: An MVCTNMAR algorithm was developed and applied to five patients: three with bilateral hip prostheses, one with unilateral hip prosthesis and one with dental fillings. The corrected images were evaluated for visualization of tissue structures and their interfaces, and for radiotherapy dose calculations. They were also compared against the corresponding images corrected by a commercial metal artifact reduction technique, O-MAR, on a Phillips™ CT scanner. Results: The use of MVCT images for correcting kVCT images in the MVCT-NMAR technique greatly reduces metal artifacts, avoids secondary artifacts, and makes patient images more useful for correct dose calculation in radiotherapy. These improvements are significant over the commercial correction method, provided the MVCT and kVCT images are correctly registered. The remaining and the secondary artifacts (soft tissue blurring, eroded bones, false bones or air pockets, CT number cupping within the metal) present in O-MAR corrected images are removed in the MVCT-NMAR corrected images. Large dose reduction is possible outside the planning target volume (e.g., 59.2 Gy in comparison to 52.5 Gy in pubic bone) whenmore » these MVCT-NMAR corrected images are used in TomoTherapy™ treatment plans, as the corrected images no longer require directional blocks for prostate plans in order to avoid the image artifact regions. Conclusion: The use of MVCT-NMAR corrected images in radiotherapy treatment planning could improve the treatment plan quality for cancer patients with metallic implants. Moti Raj Paudel is supported by the Vanier Canada Graduate Scholarship, the Endowed Graduate Scholarship in Oncology and the Dissertation Fellowship at the University of Alberta. The authors acknowledge the CIHR operating grant number MOP 53254.« less

Purpose: A new commercial GPU-based Monte Carlo dose calculation algorithm (GPUMCD) developed by the vendor Elekta™ to be used in the Monaco Treatment Planning System (TPS) is capable of modeling dose for both a standard linear accelerator and for an Elekta MRI-Linear accelerator (modeling magnetic field effects). We are evaluating this algorithm in two parts: commissioning the algorithm for an Elekta Agility linear accelerator (the focus of this work) and evaluating the algorithm’s ability to model magnetic field effects for an MRI-linear accelerator. Methods: A beam model was developed in the Monaco TPS (v.5.09.06) using the commissioned beam data for a 6MV Agility linac. A heterogeneous phantom representing tumor-in-lung, lung, bone-in-tissue, and prosthetic was designed/built. Dose calculations in Monaco were done using the current clinical algorithm (XVMC) and the new GPUMCD algorithm (1 mm3 voxel size, 0.5% statistical uncertainty) and in the Pinnacle TPS using the collapsed cone convolution (CCC) algorithm. These were compared with the measured doses using an ionization chamber (A1SL) and Gafchromic EBT3 films for 2×2 cm2, 5×5 cm2, and 10×10 cm2 field sizes. Results: The calculated central axis percentage depth doses (PDDs) in homogeneous solid water were within 2% compared to measurements for XVMC and GPUMCD. For tumor-in-lung and lung phantoms, doses calculated by all of the algorithms were within the experimental uncertainty of the measurements (±2% in the homogeneous phantom and ±3% for the tumor-in-lung or lung phantoms), except for 2×2 cm2 field size where only the CCC algorithm differs from film by 5% in the lung region. The analysis for bone-in-tissue and the prosthetic phantoms are ongoing. Conclusion: The new GPUMCD algorithm calculated dose comparable to both the XVMC algorithm and to measurements in both a homogeneous solid water medium and the heterogeneous phantom representing lung or tumor-in-lung for 2×2 cm2-10×10 cm2 field sizes. Funding support was obtained from Elekta.

Purpose: To evaluate the performance of a model based image reconstruction in reducing metal artifacts in MVCT systems, and to compare with filtered‐back projection (FBP) technique. Methods: Iterative maximum likelihood polychromatic algorithm for CT (IMPACT) is used with pair/triplet production process and the energy dependent response of detectors. The beam spectra for in‐house bench‐top and TomotherapyTM MVCT are modelled for use in IMPACT. The energy dependent gain of detectors is calculated using a constrained optimization technique and measured attenuation produced by 0 – 24 cm thick solid water slabs. A cylindrical (19 cm diameter) plexiglass phantom containing various central cylindrical inserts (relative electron density of 0.28–1.69) between two steel rods (2 cm diameter) is scanned in the bench‐top [the bremsstrahlung radiation from 6 MeV electron beam passed through 4 cm solid water on the Varian Clinac 2300C] and TomotherapyTM MVCTs. The FBP reconstructsimages from raw signal normalised to air scan and corrected for beam hardening using a uniform plexi‐glass cylinder (20 cm diameter). IMPACT starts with FBP reconstructed seed image and reconstructs final image at 1.25 MeV in 150 iterations. Results: FBP produces a visible dark shading in the image between two steel rods that becomes darker with higher density central insert causing 5–8 % underestimation of electron density compared to the case without the steel rods. In the IMPACT image the dark shading connecting the steel rods is nearly removed and the uniform background restored. The average attenuation coefficients of the inserts and the background are very close to the corresponding theoretical values at 1.25 MeV. Conclusions: The dark shading metal artifact due to beam hardening can be removed in MVCT using the iterative reconstruction algorithm such as IMPACT. However, the accurate modelling of detectors' energy dependent response and physical processes are crucial for successful implementation. Funding support for the research is obtained from “Vanier Canada Graduate Scholarship” and “Canadian Institute of Health Research”.

Purpose: High density/high atomic number metallic objects create shading and streaking metal artifacts in the CT image that can cause inaccurate delineation of anatomical structures or inaccurate radiation dose calculation. A modified iterative maximum-likelihood polychromatic algorithm for CT (mIMPACT) that models the energy response of detectors, photon interaction processes and beam polychromaticity has successfully reduced metal artifacts in MVCT. Our extension of mIMPACT in kVCT did not significantly reduce metal artifacts for high density metal like steel. We hypothesize that photon starvation may result in the measured data in a commercial kVCT imaging beam. Methods: We measured attenuation of a range of steel plate thicknesses, sandwiched between two 12cm thick solid water blocks, using a Phillips Big Bore CTTM scanner in scout acquisition mode with 120kVp and 200mAs. The transmitted signal (y) was normalized to the air scan signal (y0) to get attenuation [i.e., ln(y/y0)] data for a detector. Results: Below steel plate thickness of 13.4mm, the variations in measured attenuation as a function of view number are characterized by a quantum noise and show increased attenuation with metal thickness. On or above this thickness the attenuation shows discrete levels in addition to the quantum noise. Some views have saturated attenuation value. The histograms of the measured attenuation for up to 36.7mm of steel show this trend. The detector signal is so small that the quantization levels in the analog to digital (A-to-D) converter are visible, a clear indication of photon starvation. Conclusion: Photons reaching the kVCT detector after passing through a thick metal plate are either so low in number that the signal measured has large quantum noise, or are completely absorbed inside the plate creating photon starvation. This is un-interpretable by the mIMPACT algorithm and cannot reduce metal artifacts in kVCT for certain realistic thicknesses of steel hip implants. Moti Raj Paudel is supported by the Vanier Canada Graduate Scholarship, the Endowed Graduate Scholarship in Oncology and the Dissertation Fellowship at the University of Alberta. The authors acknowledge the CIHR operating grant number MOP 53254.

Purpose: To evaluate the metal artifacts in kilovoltage computed tomography (kVCT) images that are corrected using a normalized metal artifact reduction (NMAR) method with megavoltage CT (MVCT) prior images. Methods: Tissue characterization phantoms containing bilateral steel inserts are used in all experiments. Two MVCT images, one without any metal artifact corrections and the other corrected using a modified iterative maximum likelihood polychromatic algorithm for CT (IMPACT) are translated to pseudo-kVCT images. These are then used as prior images without tissue classification in an NMAR technique for correcting the experimental kVCT image. The IMPACT method in MVCT included an additional model for the pair/triplet production process and the energy dependent response of the MVCT detectors. An experimental kVCT image, without the metal inserts and reconstructed using the filtered back projection (FBP) method, is artificially patched with the known steel inserts to get a reference image. The regular NMAR image containing the steel inserts that uses tissue classified kVCT prior and the NMAR images reconstructed using MVCT priors are compared with the reference image for metal artifact reduction. The Eclipse treatment planning system is used to calculate radiotherapy dose distributions on the corrected images and on the reference image using the Anisotropic Analytical Algorithm with 6 MV parallel opposed 5 × 10 cm2 fields passing through the bilateral steel inserts, and the results are compared. Gafchromic film is used to measure the actual dose delivered in a plane perpendicular to the beams at the isocenter. Results: The streaking and shading in the NMAR image using tissue classifications are significantly reduced. However, the structures, including metal, are deformed. Some uniform regions appear to have eroded from one side. There is a large variation of attenuation values inside the metal inserts. Similar results are seen in commercially corrected image. Use of MVCT prior images without tissue classification in NMAR significantly reduces these problems. The radiation dose calculated on the reference image is close to the dose measured using the film. Compared to the reference image, the calculated dose difference in the conventional NMAR image, the corrected images using uncorrected MVCT image, and IMPACT corrected MVCT image as priors is ∼15.5%, ∼5%, and ∼2.7%, respectively, at the isocenter. Conclusions: The deformation and erosion of the structures present in regular NMAR corrected images can be largely reduced by using MVCT priors without tissue segmentation. The attenuation value of metal being incorrect, large dose differences relative to the true value can result when using the conventional NMAR image. This difference can be significantly reduced if MVCT images are used as priors. Reduced tissue deformation, better tissue visualization, and correct information about the electron density of the tissues and metals in the artifact corrected images could help delineate the structures better, as well as calculate radiation dose more correctly, thus enhancing the quality of the radiotherapy treatment planning.

Co2MnSb thin films were grown on glass and GaAs (001) substrates using pulsed laser deposition. The films were grown in magnetic fields (HG = 500 Oe and 0 Oe) that were applied in the plane of the substrate during the deposition process. Angle-dependent magneto-optic Kerr effect measurements for films grown on glass revealed a uniaxial magnetic anisotropy in the direction of the applied growth field. Films grown on GaAs (001) exhibited more complicated magnetic anisotropy behavior, due to additional contributions from the substrate. Point contact Andreev reflection spectroscopy measurements indicated that the spin polarizations of the films were about P ∼ 50%, with negligible difference between films grown in zero and non-zero applied fields.

Purpose: To evaluate the performance of a model based image reconstruction in reducing metal artifacts in MVCT system in the image of a phantom representing bilateral hip prostheses, and to compare with traditionally reconstructed image.Method and Materials: A cylindrical plexiglass (19 cm diameter) phantom containing two steel inserts (2 cm diameter) was scanned using bremsstrahlung radiation from 6MeV electron beam passing through 4 cm thick solid water in Varian Clinac 2300C with bench‐top MVCT system. Iterative maximum‐likelihood polychromatic algorithm for CT (IMPACT) modified to include pair and triplet production for the MV spectrum was used with air, plexiglass, bone, and iron as base substances. Detector calibrated signal without beam‐hardening correction was used to get filtered back‐projected (FBP) image and used as an initial image in IMPACT. The final image at 1.25 MeV was obtained after 130 iterations. The second image was reconstructed using FBP with traditional signal and beam‐hardening corrections. The quantitative analysis included calculation of average attenuation coefficient in various regions of interest and their comparisons with theoretical value. Results: Visual inspection of the traditional image shows that the region between two steel inserts is dark with additional white streaks. Attenuation coefficients in ROIs fluctuate largely and deviate from theoretical values. In the iteratively reconstructed image, metal artifacts are remarkably reduced leaving behind only faint shadings in between the inserts. The average attenuation coefficients in ROIs were close to theoretical values. Image profile through the steel inserts shows restored uniform background between them. Conclusion: This experiment emphasizes the importance of model based image reconstruction and MVCT system for the metal artifact reduction. The beam‐hardening correction applied in conventional image reconstruction creates severe darkening and white streaks in the image. The complete removal of the metal artifacts requires further modeling and strategies which will be employed in future studies.

We present the structural and element specific magnetic properties of Co2MnSn and Co2MnSb thin films grown on GaAs (100) substrates using pulsed laser deposition. X-ray magnetic circular dichroism (XMCD) spectra were measured for 400A thick films at the L2,3 edges of Co and Mn. Element specific moments for Co and Mn in Co2MnSn were calculated from the x-ray absorption and XMCD spectra using the XMCD sum rules. The ratios of orbital to spin magnetic moments for Co and Mn were calculated for Co2MnSn and Co2MnSb.

Heusler alloy Co2MnSn1−xSbx (x=0.0, 0.5, and 1.0) thin films were grown on GaAs (001) substrates using pulsed laser deposition techniques. Growth parameters have been determined that result in highly magnetically anisotropic, crystalline, and oriented (001) films. The angular dependences, relative to the GaAs (001) crystallographic directions, of the coercive field Hc(θ) and the remanence Mr(θ) were determined from angle dependent magneto-optic Kerr effect (MOKE) measurements. It was found that Hc(θ) revealed higher order symmetry contributions to the magnetic anisotropy than did Mr(θ). The Fourier analysis of rotational MOKE data was used to determine the symmetry contributions to the total anisotropy.