kV Cone Beam CT Research Articles

An in-house software program for quantitative image quality evaluation of linac cone-beam CT system

The study presents application of an in-house software program to calculate image quality parameters of kV cone-beam CT (CBCT) system in radiotherapy. The parameters include uniformity, noise, contrast-to-noise ratio, CT number linearity, spatial resolution and geometric distortion as recommended in AAPM TG142 and TG179 quality assurance (QA) programme for image quality performance of kV CBCT. The computer algorithm packaged in the software program is developed using MATLAB (MathWorks, Natick, MA). The algorithm was optimised for two CT image quality phantoms, Catphan 600 (The Phantom Laboratory, Salem, NY) and CIRS 062QA (CIRS, Norfolk, VA). The phantoms were scanned with XVI CBCT system (Elekta AB, Stockholm, Sweden) using an optimised CT imaging protocol. The algorithm measures the image quality metrices from the corresponding image quality test patterns in the reconstructed CT images. The algorithm provides image quality parameters from both phantoms quantitatively. The software program allows the results to be exported and recorded in MS Excel for monitoring the performance trend of the image quality parameters. The software reduces measurement uncertainties from qualitative human observation of the test patterns.

Breast board combined with a thermoplastic head mask immobilization can improve the reproducibility of the treatment setup for breast cancer patients receiving whole breast and supraclavicular nodal region irradiation

Objective To quantify the setup errors measured with kV cone-beam CT (CBCT) using breast board with or without a thermoplastic head mask in breast cancer patients who received whole breast and supraclavicular nodal region irradiation. Clinical target volume (CTV) to planning target volume (PTV) in 3 directions were also calculated. Methods The study included thirty patient receiving both whole breast and supraclavicular nodal region irradiation on Elekta Versa HD linear accelerators (Elekta Oncology Systems, Crawley, UK) between June 2016 and January 2018.The setup error data were retrospectively analyzed. All the patients were immobilized in the supine position on a breast board system (Carbon fibre breast board, Civco, Iowa, USA) with both arms raised. Twenty of the patients added an extra thermoplastic head mask to immobilize the neck. A CBCT scan was taken before treatment at the first 3 to 5 fractions and then once every week. Registration with the planning CT was performed and adjusted to match the target volume of the supraclavicular nodal region and the cervical vertebra by experienced medical staff, the position of the arms and the breast were also considered. For all patients the couch shifts in left-right (x), superior-inferior (y), anterior-posterior (z) were recorded. Statistical analysis included two-tails significance tests (t-Student and Manne-Whitney test for means, χ2 test for variances). Population of the two groups was compared in terms of distribution of the mean shift (systematic error) and their standard deviations (random error). Van Herk′s setup margin defined as MPTV=2.5Σ+ 0.7δ was calculated in patients with or without mask immobilization. Results Altogether, 56 images and 109 images were acquired in breast board only group and head mask group, respectively. Shifts of the breast board only group and the head mask group in x, y, z were 0.212±0.174 cm vs.0.272±0.242 cm (P=0.070); 0.364±0.246 cm vs.0.242±0.171 cm (P=0.001); 0.423±0.302 cm vs.0.364±0.269 cm (P=0.204). Proportion of the shift less than 5 mm in the breast board only group and the head mask group were 91.07% vs. 85.32%(P=0.294); 67.86% vs.89.91%(P=0.001); 67.86% vs.74.31%(P=0.381). The CTV to PTV margin in x, y, z were 0.645 cm, 0.981 cm, 1.317 cm in breast board only group and 0.873 cm, 0.709 cm, 0.961 cm in head mask group, respectively. Setup error in the x direction was significantly correlated with BMI (P=0.001). Conclusions For the alignment and immobilization of patients who received whole breast and supraclavicular nodal region irradiation, using a breast board in combination of a thermoplastic head mask may significantly help to reduce the shift variance in superior-inferior direction compared to using breast board only. The anterior-posterior error is relatively large, other immobilization device or patient alignment methods are needed to be further explored to improve the accuracy. Key words: Breast neoplasms/radiotherapy; Setup error; Cone-beam CT

Comparative Study between Electronic Portal Imaging Device (EPID) and Cone Beam Computed Tomography (CBCT) for Radiation Treatment Verifications

Introduction: Electronic Portal Imaging Device (EPID) and Cone Beam Computed Tomography (CBCT) are the preferred tools of Image Guided Radiotherapy (IGRT) and Dose Guided Radiotherapy (DGRT) which have been used for Radiotherapy treatment verifications. As a result, the number of publications dealing with these two tools for radiation treatment verification has increased considerably over the past years. This paper aims to compare and contrast the overall differences and similarities of EPID and CBCT tools based on their set-up verification accuracy, dosimetric verification accuracy, calibration and quality control methods, image quality, dose measurement, calculation, and evaluation methods for radiation treatment verifications. Materials and Methods: The relevant articles published in English were searched based on the keywords of EPID and CBCT for radiation treatment verifications using PubMed, EMBASE, Scopus, and Web of Science databases and finally exported to EndNote for screening, downloading, and reference management processes. Results: A total of 2512 articles were searched and subsequently screened by removing duplications and reading title and abstract to identify original articles that directly related with radiation treatment verifications using EPID and CBCT for different cases. This yielded 226 relevant articles. After downloaded and read full articles, only compared results between EPID and CBCT treatment verifications were selected. This yielded, a final total of 23 articles to be included for this work. Conclusion: EPID and MV-CBCT have almost equal potential to indicate the set-up error but since they used the LINAC’s energy, no cross-calibration procedure is needed because the imaging geometry is the same as the treatment geometry though both of them have poor image quality. The image quality of KV-CBCT is superior to both EPID and MV-CBCT and is better to indicate the set-up error. CBCT in conjunction with energy fluence maps from EPID could be used to verify the dose delivered. In this instance, a cross-calibration between the KV CBCT and EPID would be required which would not be necessary with MV CBCT. Though MV CBCT handles inhomogeneity better, soft tissue visualization is superior with KV CBCT. EPID is used to calibrate the LINAC machine. Generally, both EPID and CBCT have their own different advantages and need further study to solve their limitations.

Dosimetric Variations Assessed with CBCT for Head and Neck Cancer Radiation Therapy

Head & Neck (HN) cancers treated with conventional fractionations likely experience changes in the volume and shape of both targets and normal tissues over the long course of treatment. This study investigated the dosimetric variations of both targets and normal tissues related to the organ deformation throughout treatment. Eleven H&N cancer patients treated to the oropharynx cancer were retrospectively included in the study. Treatment was delivered with IMRT to 70 Gy in 35 fractions for PTV/CTV. Standard image guided radiation therapy (IGRT) procedure was applied for daily setup corrections, which utilized cone beam computed tomography (CBCT) with the auto rigid registration to account for translational as well as rotational setup errors. For each patient, the daily CBCT image set acquired during setup from every other fraction of treatment were imported to a third party software, VelocityTM, for the deformable registration with the planning CT image set. The registration process transformed the plan dose to each CBCT image set and deformed it as the delivered fraction dose for each structure. The accumulated dose for both targets and normal tissues was calculated with the actual deformed dose to CBCT. It is observed that dose distribution for both targets and normal tissues has changed over the course of treatment. The variation of the accumulated dose from the plan dose is organ and patient dependent. The largest variation was up to 150% in minimum dose. Maximum dose and mean dose had less variations in general. The percent variation of minimum, maximum and mean dose is summarized in a table for CTV, GTV and normal tissues. The change in the shape and volume of each structure was also observed by the translational deviation of the DICOM center of a structure more than a centimeter in x, y or z direction for some patients. The weekly breakdown of the variations in accumulated dose and structural volumetric center presents a consistent deformation of some structures over the course of treatment. Conventional fractionation H&N cancer radiotherapy in is a standard protocol. IGRT has been employed to correct the patient positioning by kV orthogonal or CBCT imaging. Considerable variations have been observed in dose distribution over the course of treatment for both targets and normal tissues due to the organ deformation. The results of this study can be used to determine whether or when adaptive therapy is needed throughout the treatment or to calculate any remedy necessary at the end of the course.Abstract TU_38_3267: Table 1StructureMax Variation (%) relative to the planDoseMeanMinMaxCTV 70Gy-1.1-42.0-0.2CTV 59.5 Gy-2.3-68.8-0.2GTV-0.8-39.4-0.2Brainstem30.5150.511.4Spinal Cord5.320.98.2Mandible-8.2-55.9-2.4Oral Cavity-11.4-48.5-2.6Right Parotid11.151.8-4.3Left Parotid10.479.6-2.5Right Brachial plexus10.8-62.332.2Left Brachial plexus-6.6102.65.5Right Submandibular-9.88.4-8.5Left Submandibular-3.5-17.7-1.2 Open table in a new tab

P269] Absorbed dose calculation based on CBCT data for head and neck cancer patients

Purpose In a radiotherapy magnetic resonance imaging (MRI) only workflow, a synthetic CT (sCT) data set is generated from the MRI data as a substitute to the computed tomography (CT). Since no CT scan will be acquired, no such data is available for quality control (QC) of the sCT. It has been suggested to use kV-cone beam CT (CBCT) data for QC in the head region [1] . The aim of this study is to evaluate the possibility to use CBCT for QC of the sCT for head and neck (H&N) cancer by comparing H&N treatment plans calculated using the CT and CBCT data, respectively. Methods Five patients with one CT and two CBCT data sets each, were evaluated. All CBCT data sets were deformably registered to the CT data, remediating errors originating from repositioning and/or anatomical changes between the imaging sessions. The CBCT and CT data were adjusted to the same field of view (FOV) in the longitudinal direction. A clinical treatment plan was created based on the CT data for each patient. Treatment volumes, organs-at-risk (OAR) and treatment plan were copied from the CT to the deformable registered CBCT. The treatment plan was then recalculated, with the standard CT HU-RED conversion curve. Dosimetric differences between the dose distributions for the treatment plans were calculated and evaluated by comparing dose-volume constrains. Results The mean local difference for the mean absorbed dose to the clinical target volume (CTV) was 0.41% with a maximum and minimum value of 1.37% and 0.15% respectively. The CTV D 98 % criteria had a difference of 0.19% and CTV D 2 % criteria a difference of 0.38%. The planning OAR volume (PRV) of the spinal cord had a difference of 0.39% and the parotid right and left a difference of 0.36% and 0.08%, respectively. Conclusions The dosimetric difference between the dose distributions for treatment plans calculated based on CT and CBCT data were minor. This indicates that CBCT data could be a good candidate for QC of sCT data in an MRI only workflow for H&N cancer patients.

P198] Cone beam CT as a clinical QA tool for prostate MRI-only workflows

Purpose Magnetic resonance imaging (MRI) only radiotherapy has in recent years been introduced as a clinical workflow for prostate cancer patients. MRI only radiotherapy is performed without computed tomography (CT) and instead, a synthetic CT (sCT) is used for treatment planning. With the introduction of new procedures for treatment planning, development of quality assurance (QA) tools must be considered. Today, no such tool has yet been presented. The aim of this study was to develop a clinically feasible QA procedure for sCT using the kV-cone beam CT (CBCT), in an MRI-only workflow for prostate cancer patients. Methods Three criteria were addressed; 1. Stability in Hounsfield Units (HUs), 2. Deviations in HUs between the CT and CBCT and 3. Validation of the QA procedure. For the two first criteria, phantom measurements were performed. For the third criteria, sCT, CT and CBCT for ten patients were used. Treatment plans were created based on the sCT (MriPlannerTM). CT and CBCT images were registered to the sCT. The treatment plan was recalculated on the CT and CBCT. Dose-volume histogram (DVH) metrics were used to evaluate dosimetric differences between the sCT plan and the recalculated CT and CBCT plans. Well defined errors were artificially introduced in the sCT for one patient to evaluate efficacy of the QA procedure. Results The kV-CBCT system was stable in HU over time (SD Conclusions The results in this study indicates that CBCT can be used as a clinically feasible QA tool for MRI-only radiotherapy of prostate cancer patients. The proposed method could easily be integrated in any modern image guided radiotherapy workflow without extensive work.

ALARA and IGRT in paediatric radiotherapy patients – A review

Purpose As low as reasonably achievable (ALARA) based protocols have been widely implemented to significant effect in diagnostic computed tomography (CT) scans for paediatric patients, but investigation of dose reduction techniques in paediatric cone beam CT (CBCT) IGRT are lacking. The purpose of this review is to report the level of dose from paediatric CBCT, the implications of this, and to investigate the potential for dose reduction. Methods A search and review of published studies investigating kV CBCT dose (in-field and out-of-field); long term effect of kV CBCT dose and dose reduction interventions was carried out. Dose studies included paediatric radiotherapy patients aged zero to nineteen years old, adult radiotherapy patients, and both paediatric and adult anthropomorphic phantoms. Results Nineteen articles were selected for review. Over a course of treatment using daily CBCT, the cumulative imaging dose can be comparable to the dose of a single treatment fraction and has the potential to be greater than peripheral doses from IMRT techniques. A number of dose reduction interventions have been proposed, which include: reducing tube energy and current; shielding radiosensitive structures; reducing the scan length/volume; reducing scan frequency; reducing rotational angle and using bow tie filters. Conclusion Paediatric patients are at an increased risk of developing second malignancies with daily CBCT. Particular consideration must be given to the use of CBCT in very young; female patients and close to radiosensitive structures. The imaging dose to these patients should be minimised, provided the image quality is sufficient for verification purposes.

OA148] A multi-center dosimetry study to evaluate imaging dose from Elekta XVI and varian OBI KV-CONE beam CT systems to cardiovascular implantable electronic devices (CIEDS)

Purpose By increasing the use of daily CBCT, additional delivered imaging dose to the patient became a matter of concern. This can also raise concerns about patients with CIEDs since accumulated dose to these devices highly needs to be limited. The aim of this study is to investigate the imaging dose to the CIEDs from Elekta XVI and Varian OBI kV-CBCT systems performed by a multi-institutional study. Methods Four geographically separated centers with different CBCT imaging systems and three different dosimetric measurement methods including Gafchromic films, metal-oxide-semiconductor field-effect transistor (MOSFET) and thermoluminescence dosimeters (TLDs) participated in this multi-center study. At each center an anthropomorphic phantom (Alderson RANDO phantom) was used and the CIED was placed in the alderson’s left clavicular region using the same setup. The dosimeter was fixed on top of the device over which a 1-cm thick bolus was used to simulate patient’s skin. Different standard CBCT-imaging protocols which are commonly used in clinical practice of each center were applied namely, Head&Neck (HN), Chest, Thorax, Symmetry, Prostate and Pelvis. Results The maximum values of imaging doses were found once CIEDs were inside the CBCT field. They were between the following values: HNS20 (XVI):2.57 mGy/100 mAs, Thorax (OBI):2.92 mGy/100 mAs, ChestM15-left (XVI): 1.52 mGy/100 mAs, Symmetry-left (XVI): 5.67 mGy/100 mAs. However, they received much lower dose once the CIEDs were not inside the field. Conclusion This study shows that although imaging doses received by CIEDs outside the CBCT field is negligible, a special attention should be paid to this value when CIEDs are inside the CBCT field because daily use of CBCT can make a considerable contribution to the total dose received by CIED. Based on the obtained results, it is possible that in some protocol like symmetry total imaging dose to CIED in a 30-fraction reach as high as 50 cGy. Therefore in such cases, this additional imaging dose should not be ignored when classifying CIED patients into risk groups.

Early Changes in Serial CBCT-Measured Parotid Gland Biomarkers Predict Chronic Xerostomia After Head and Neck Radiation Therapy

To determine whether serial cone beam computed tomography (CBCT) images taken during head and neck radiation therapy (HNR) can improve chronic xerostomia prediction. In a retrospective analysis, parotid glands (PGs) were delineated on daily kV CBCT images using deformable image registration for 119 HNR patients (60 or 70Gy in 2Gy fractions over 6 or 7weeks). Deformable image registration accuracy for a subset of deformed contours was quantified using the Dice similarity coefficient and mean distance to agreement in comparison with manually drawn contours. Average weekly changes in CBCT-measured mean Hounsfield unit intensity and volume were calculated for each PG relative to week 1. Dose-volume histogram statistics were extracted from each plan, and interactions among dose, volume, and intensity were investigated. Univariable analysis and penalized logistic regression were used to analyze association with observer-rated xerostomia at 1year after HNR. Models including CBCT delta imaging features were compared with clinical and dose-volume histogram-only models using area under the receiver operating characteristic curve (AUC) for grade ≥1 and grade ≥2 xerostomia prediction. All patients experienced end-treatment PG volume reduction with mean (range) ipsilateral and contralateral PG shrinkage of 19.6% (0.9%-58.4%) and 17.7% (4.4%-56.3%), respectively. Midtreatment volume change was highly correlated with mean PG dose (r=-0.318, P<1e-6). Incidence of grade ≥1 and grade ≥2 xerostomia was 65% and 16%, respectively. For grade ≥1 xerostomia prediction, the delta-imaging model had an AUC of 0.719 (95% confidence interval [CI], 0.603-0.830), compared with 0.709 (95% CI, 0.603-0.815) for the dose/clinical model. For grade ≥2 xerostomia prediction, the dose/clinical model had an AUC of 0.692 (95% CI, 0.615-0.770), and the addition of contralateral PG changes modestly improved predictive performance, with an AUC of 0.776 (0.643-0.912). The rate of CBCT-measured PG image feature changes improves prediction over dose alone for chronic xerostomia prediction. Analysis of CBCT images acquired for treatment positioning may provide an inexpensive monitoring system to support toxicity-reducing adaptive radiation therapy.

Set-up errors in head and neck cancer treated with IMRT technique assessed by cone-beam computed tomography: a feasible protocol.

PurposeTo investigate set-up errors, suggest the adequate planning target volume (PTV) margin and image-guided radiotherapy frequency in head and neck (H&N) cancer treated with intensity-modulated radiotherapy (IMRT) assessed by kV cone-beam computed tomography (CBCT).MethodsWe analyzed 360 CBCTs in 60 patients with H&N cancer treated with IMRT. The target delineation was contoured according to ICRU62. PTVs were generated by adding a 3–5 mm margin in all directions to the respective clinical target volumes. The kV CBCT images were obtained at first three days of irradiation and weekly thereafter. The overall mean displacement, range, systematic (∑) and random (σ) errors were calculated. Adequate PTV margins were calculated according to the van Herk formula (2.5∑ + 0.7r).ResultsThe mean of set-up errors was less than 2 mm in any direction. The overall frequency of set-up displacements greater than 3 mm was 3.9% in medial-lateral (ML) direction, 8% in superior-inferior (SI) direction, and 15.5% in anterior-posterior (AP) direction. The range of translations shifts was 0–9 mm in ML direction, 0–5 mm in SI direction and 0–10 mm in AP direction, respectively. After systematic set-up errors correction, the adequate margin to overcome the problem of set-up errors was found to be less than 3 mm.ConclusionImage-guided kV CBCT was effective for the evaluation of set-up accuracy in H&N cancer. The kV CBCT at first three fractions and followed-by weekly appears adequate for reducing significantly set-up errors in H&N cancer treated with IMRT technique. Finally, 3–5 mm PTV margins appear adequate and safe to overcome the problem of set-up errors.

Acuros CTS: A fast, linear Boltzmann transport equation solver for computed tomography scatter - Part II: System modeling, scatter correction, and optimization.

To correct for scatter in kV cone-beam CT (CBCT) projection data on a clinical system using a new tool, Acuros® CTS, that estimates scatter images rapidly and accurately by deterministically solving the linear Boltzmann transport equation. Phantom and patient CBCT scans were acquired on TrueBeam® radiotherapy machines. A first-pass reconstruction was used to create water and bone density maps of the imaged object, which was updated to include a more accurate representation of the patient couch. The imaging system model accounted for the TrueBeam x-ray source (polychromatic spectrum, beam filtration, bowtie filter, and collimation hardware) and x-ray detection system (antiscatter grid, flat-panel imager). Acuros CTS then used the system and object models to estimate the scatter component of each projection image, which was subtracted from the measured projections. The corrected projections were then reconstructed to produce the final result. We examined the tradeoff between run time and accuracy using a Pareto optimization of key parameters, including the voxel size of the down-sampled object model, the number of pixels in the down-sampled detector, and the number of scatter images (angular down-sampling). All computations and reconstructions were performed on a research workstation containing two graphics processing units (GPUs). In addition, we established a method for selecting a subset of projections for which scatter images were calculated. The projections were selected to minimize interpolation errors in the remaining projections. Image quality improvement was assessed by measuring the accuracy of the reconstructed phantom and patient images. The Pareto optimization yielded a set of parameters with an average run time of 26 seconds for scatter correction while maintaining high accuracy of scatter estimation. This was achieved in part by means of optimizing the projection angles that were processed, thus favoring the use of more angles in the lateral (i.e., horizontal) direction and fewer angles in the AP direction. In a 40 cm solid water phantom reconstruction, nonuniformities were decreased from 217 HU without scatter correction to 51 HU with conventional (kernel-based) scatter correction to 17 HU with Acuros CTS-based scatter correction. In clinical pelvis scans, nonuniformities in the bladder were reduced from 85 HU with conventional scatter correction to 14 HU with Acuros CTS. Acuros CTS is a promising new tool for fast and accurate scatter correction for CBCT imaging. By carefully modeling the imaging chain and optimizing several parameters, we achieved high correction accuracies with computation times compatible with the clinical workflow. The improvement in image quality enables better soft-tissue visualization and potentially enables applications such as adaptive radiotherapy.

Leveraging multi-layer imager detector design to improve low-dose performance for megavoltage cone-beam computed tomography

While megavoltage cone-beam computed tomography (CBCT) using an electronic portal imaging device (EPID) provides many advantages over kilovoltage (kV) CBCT, clinical adoption is limited by its high doses. Multi-layer imager (MLI) EPIDs increase DQE(0) while maintaining high resolution. However, even well-designed, high-performance MLIs suffer from increased electronic noise from each readout, degrading low-dose image quality. To improve low-dose performance, shift-and-bin addition (ShiBA) imaging is proposed, leveraging the unique architecture of the MLI. ShiBA combines hardware readout-binning and super-resolution concepts, reducing electronic noise while maintaining native image sampling. The imaging performance of full-resolution (FR); standard, aligned binned (BIN); and ShiBA images in terms of noise power spectrum (NPS), electronic NPS, modulation transfer function (MTF), and the ideal observer signal-to-noise ratio (SNR)—the detectability index (d′)—are compared. The FR 4-layer readout of the prototype MLI exhibits an electronic NPS magnitude 6-times higher than a state-of-the-art single layer (SLI) EPID. Although the MLI is built on the same readout platform as the SLI, with each layer exhibiting equivalent electronic noise, the multi-stage readout of the MLI results in electronic noise 50% higher than simple summation. Electronic noise is mitigated in both BIN and ShiBA imaging, reducing its total by ~12 times. ShiBA further reduces the NPS, effectively upsampling the image, resulting in a multiplication by a sinc2 function. Normalized NPS show that neither ShiBA nor BIN otherwise affects image noise. The LSF shows that ShiBA removes the pixilation artifact of BIN images and mitigates the effect of detector shift, but does not quantifiably improve the MTF. ShiBA provides a pre-sampled representation of the images, mitigating phase dependence. Hardware binning strategies lower the quantum noise floor, with 2 × 2 implementation reducing the dose at which DQE(0) degrades by 10% from 0.01 MU to 0.004 MU, representing 20% improvement in d′.

Analytical Calculation of the Compton Single Scatter Component of Pencil Beam Scatter Kernel for Scatter Correction in kV Cone Beam CT (kV-CBCT)

The accuracy of conventional superposition or convolution methods for scatter correction in kV-CBCT is usually compromised by the spatial variation of pencil-beam scatter kernel (PBSK) due to finite size, irregular external contour and heterogeneity of the imaged object. This study aims to propose an analytical method to quantify the Compton single scatter (CSS) component of the PBSK, which dominates the spatial distribution of total scatter assuming that multiple scatter can be estimated as a constant background and Rayleigh scatter is the secondary source of scatter. The CSS component of PBSK is the line integration of scatter production by incident primary photons along the beam line followed by the post-scattering attenuation as the scattered photons traverse the object. We propose to separate the object-specific attenuation term from the line integration and equivalently replace it with an average value such that the line integration of scatter production is object independent but only beam specific. We derived a quartic function formula as an approximate solution to the spatial distribution of the unattenuated CSS component of PBSK. The “effective scattering center” is introduced to calculate the average attenuation. The proposed analytical framework to calculate the CSS was evaluated using parameter settings of the On-Board Imager kV-CBCT system and was found to be in high agreement with the reference results. The proposed method shows highly increased computational efficiency compared to conventional analytical calculation methods based on point scattering model. It is also potentially useful for correcting the spatial variant PBSK in adaptive superposition method.

Abdominal imaging dose in radiology and radiotherapy – Phantom point dose measurements, effective dose and secondary cancer risk

PurposeTo compare abdominal imaging dose from 3D imaging in radiology (standard/low-dose/dual-energy CT) and radiotherapy (planning CT, kV cone-beam CT (CBCT)). MethodsDose was measured by thermoluminescent dosimeters (TLD’s) placed at 86 positions in an anthropomorphic phantom. Point, organ and effective dose were assessed, and secondary cancer risk from imaging was estimated. ResultsOverall dose and mean organ dose comparisons yield significantly lower dose for the optimized radiology protocols (dual-source and care kV), with an average dose of 0.34±0.01 mGy and 0.54±0.01 mGy (average ± standard deviation), respectively. Standard abdominal CT and planning CT involve considerably higher dose (13.58 ± 0.18 mGy and 18.78±0.27 mGy, respectively). The CBCT dose show a dose fall-off near the field edges. On average, dose is reduced as compared with the planning or standard CT (3.79 ± 0.21 mGy for 220° rotation and 7.76 ± 0.37 mGy for 360°), unless the high-quality setting is chosen (20.30 ± 0.96 mGy). The mean organ doses show a similar behavior, which translates to the estimated secondary cancer risk. The modelled risk is in the range between 0.4 cases per million patient years (PY) for the radiological scans dual-energy and care kV, and 300 cases per million PY for the high-quality CBCT setting. ConclusionsModern radiotherapy imaging techniques (while much lower in dose than radiotherapy), involve considerably more dose to the patient than modern radiology techniques. Given the frequency of radiotherapy imaging, a further reduction in radiotherapy imaging dose appears to be both desirable and technically feasible.

Evaluation of Differences Between Estimated Delivered Dose and Planned Dose in Nasopharynx Patients Using Deformable Image Registration and Dose Accumulation

To investigate differences in planned dose and estimated delivered dose using deformable image registration (DIR) and dose accumulation for clinical target volumes (CTV) and organs at risk (OAR). Fifty-two nasopharynx patients treated with chemoradiotherapy [70 Gy (high-dose) and 56Gy (low-dose) in 35 fractions] from 2013 and 2015 were included. All clinical plans used 6 MV intensity modulated radiation therapy (IMRT) and daily 3D kV cone beam CT (CBCT) image guidance. Retrospective dose accumulation was performed in RayStation v4.5.2 as follows: DICOM RT data (CT image, plan, dose, and contours) were imported from the clinical planning system. Daily CBCTs were also imported and deformably registered to the planning CT. Daily delivered dose was calculated on the CBCTs and deformed back to and accumulated onto the planning CT to estimate the delivered dose. The planned and delivered doses were compared using representative dose metrics for primary CTVs and OARs including brainstem, mandible, spinal cord, and right and left parotid glands. The impact of daily set-up variation and anatomical changes on target coverage and OAR doses was evaluated by comparing planned dose vs. delivered dose. Relative differences in CTV D98 calculated as percentage of planned dose was used to assess target coverage. The mean relative difference in D98 for 70 Gy CTVs was 0.8% (median 1.1%, range -1.8 to2.6%). The mean relative difference in D98 for 56 Gy CTVs was negligible (median 0.1%, range -2.8 to2.2%). The median (range) maximum dose to 0.1 cc of the brainstem was similar between planned and delivered: 53.0 Gy (40.7-61.5 Gy) and 52.2 Gy (40.0-61.7 Gy). Although some cases exceeded the nominal tolerance dose of 54 Gy in the plan, other cases planned under this tolerance did not exceed 54 Gy in estimated delivered dose. The median maximum dose to 0.1cc of the spinal cord and mandible was 37.8 Gy and 72.0 Gy and delivered dose (range) was 38.8 Gy (30.4-45.8 Gy) and 72.2 Gy (62.9-75.4 Gy), respectively. The D50 planned median dose, delivered median dose (range) for the right and left parotid was 34.4 Gy, 39.3 Gy (23.5-72.3 Gy), and 37.3 Gy, 40.6 Gy (18.6-68.1 Gy). The median planned, delivered dose (range) for the right and left parotid mean dose was 38.4 Gy, 40.3 Gy (30.1-69.8 Gy) and 40.5 Gy, 41.7 Gy (25.3-61.9 Gy). No large differences were seen between planned and delivered OAR doses for the evaluated metrics. Outliers were identified (±2 SD from the average) for the parotids, indicating that variation can be expected for this OAR and warrants further investigation. There was minimal dosimetric change on the serial OARs; the right and left parotids received greater dose than planned. Further studies are required to investigate the impact of this dosimetric change and to identify patients who can benefit from adaptive radiotherapy.

Improvement of Dose Calculation with Scattered Photon-Corrected kV Cone-Beam CT Image Using the Kernel Deconvolution Method

Intensity modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT) has to be accomplished with a dose distribution in terms of changes in the tumor size and position for minimizing adverse effects on neighboring organs. If cone-beam computed tomography (CBCT) would facilitate dose calculation attached to the IMRT or SBRT apparatus, we could use the CBCT-based adaptive radiation therapy (ART). However, a CBCT image cannot achieve quantitative electron density values owing to scattered photons; thus, we cannot use it directly for dose calculation. In this study, we calculated the distribution of scattered photons in CBCT images using the Monte Carlo (MC) method and corrected the scattered photon effect with scatter kernels in the process of image reconstruction with the maximum likelihood expectation maximization (MLEM) method. Moreover, we investigated the accuracy of dose calculation by this segmented region method in comparison with typical methods by replacing CBCT values with ideal density values. We calculated the distribution of scattered photons with slab phantoms using the MC method and acquired scattered kernels of three materials (bone, water, and lungs) having different thicknesses until 20 cm every 0.1 cm. These kernels were used in the MLEM method for correcting scattered photons in lung and pelvis CBCT images. Next, we segmented CBCT and multislice CT (MSCT) images into three regions (lungs, bones, and soft tissues) and compared segmentation accuracy between MSCT and scatter-corrected CBCT images. Moreover, we evaluated the effectiveness of dose calculation by CBCT images. In the CBCT to density conversion method, we converted the CBCT value using individual three tables in each organ, and in the density overriding method, we observed calculated doses on accurate organ-segmented images using γ and dose–volume histogram (DVH) analyses. Segmentation results showed that the threshold value of bone and soft tissue separation was similar to the ideal threshold value used in MSCT images; the segmentation error ratio improved (w/o: 63.0%, w/: 4.7%). Similarly, segmentation error ratios for lungs and soft tissues improved by 8.6% compared with those observed on the original CBCT image. In the CBCT to density conversion method, differences in target doses of MSCT and CBCT with scatter corrections were small in the DVH analysis. In the density overriding method, γ analysis results showed that the calculated dose distributions with scatter-corrected images were similar to those seen in MSCT; the pass rate was significantly higher with an accurate segmentation effect (w/o: 70.1%, w/: 95.9%). We evaluated the effectiveness of scattered photon-corrected CBCT image using the kernel deconvolution method for organ segmentation and the effect of this contribution on dose calculation. Improvement of the segmentation accuracy with scattered photon correction can be observed using CBCT dose calculation for ART.

Dosimetric Evaluation of a Gamma Ray System for Advanced Radiation Therapy Treatments

Gamma ray systems have exhibited superior dosimetric advantages for stereotactic radiosurgery/radiotherapy (SRS/SRT) of intracranial tumors due to the use of non-coplanar multiple sources and the rapid dose fall-off outside the treatment target as a result of lower secondary electron energies. In this work we performed a dosimetric planning comparison study on a novel Gamma ray system to evaluate its potential clinical benefits for SRS of intracranial tumors and stereotactic body radiotherapy (SBRT) of other body sites. The new Gamma ray system (CybeRT, OUR United RT Group, China) consists of a ring gantry with two treatment heads: a focusing head with 16 cobalt-60 sources and an adjustable (1-2cm) cobalt-60 source equipped with a multileaf collimator (MLC). The MLC has 60 paired leaves, and the maximum field size is 40cmx40cm (40 pairs of 0.5cm central leaves, 20 pairs of 1cm outer leaves). The treatment heads can provide a total of 35° non-coplanar beam incidence. The treatment couch provides 6-degrees-of-freedom motion compensation and the kV cone-beam CT system has a spatial resolution of 0.4mm. Monte Carlo simulations were performed to obtain dose distributions and compare with measurements. A retrospective study of 27 previously treated patients was performed using the Prowess RT Pro (a prototype version), Accuray MultiPlan and Varian Eclipse TPS to compare cobalt plans with 6 MV x-ray plans on the CybeRT, CyberKnife and linac systems. Monte Carlo results confirmed the new Gamma ray system design parameters including output factors and 3D dose distributions. Its beam penumbra/dose gradient was similar to or better than that of 6MV photon beams with adjustable source sizes that also resulted in variable dose rates required for treatment optimization and VMAT delivery. CybeRT had a 0.3mm isocenter accuracy. The low-dose acquisition mode of the CBCT system provided fluoroscopy and 3D imaging at a dose level 3 times less than conventional CBCT systems. Since cobalt beams produced lower-energy secondary electrons the new Gamma ray system exhibited better dose properties in low-density lung tissues. Because of their rapid depth dose falloff, cobalt beams were favorable for peripheral lung tumors with partial-arc deliveries to spare the opposite lung and critical structures. The skin dose with CybeRT for head and neck treatments was similar to that with CyberKnife and conventional linacs and only 2-3% higher for abdominal and pelvic treatments. Superior treatment plans were obtained using the focusing Gamma treatment head for small (up to 50cc) tumor volumes and using the Gamma MLC treatment head for large (> 30cc) tumor volumes for intracranial, head and neck, breast, lung, liver and prostate patients investigated in this study. The unique dosimetric properties of cobalt beams and the accurate stereotaxy/dose delivery techniques make the new Gamma ray system an ideal RT system for advanced SRS/SBRT as well as conventional radiation therapy.

Initial experience with intra-fraction motion monitoring using Calypso guided volumetric modulated arc therapy for definitive prostate cancer treatment.

IntroductionAccurate delivery of radiation while reducing dose to organs at risk is essential in prostate treatment. The Calypso motion management system detects and corrects both inter‐ and intra‐fraction motion which offers potential benefits over standard alignment to fiducial markers. The aims of this study were to implement Calypso with Dynamic Edge™ gating and to assess both the motion seen, and interventions required.MethodsAn implementation group was formed which assessed changes needed to standard workflows. Three patients had Calypso beacons inserted into their prostate. All patients were treated using volumetric modulated arc therapy to a dose of 80 Gy in 40 fractions. Standard inter‐fraction motion correction using either kilovoltage (kV) orthogonal paired imaging or cone beam computed tomography (CBCT) image‐guided radiotherapy techniques, were used along with the Calypso system to compare accuracy. A gating threshold of >0.5 cm was used during treatment. Workflow variations along with inter‐ and intra‐fraction motion and interventions required were assessed.ResultsA total of 116 fractions were treated using Calypso with Dynamic Edge™ gating. There was a strong concordance between aligning beacons using kV orthogonal imaging or CBCT and Calypso (mean variation ≤0.06 cm). The mean intra‐fraction motion detected was ≤0.2 cm in all directions with the largest motion recorded being 2.2 cm in the left direction while the treatment beam was off. Prostate rotation was largest in the pitch direction and 28 fractions exceeded the 10° tolerance. A total of 78 couch shift corrections of ≥0.3 cm were required, usually following standard imaging, and before treatment starting. Three gating events due to intra‐fraction motion occurred during treatment.ConclusionsIntra‐fraction motion monitoring with Calypso was successfully implemented. Greatest movement was seen between time of standard imaging and treatment starting with more than half the treatments requiring a ≥0.3 cm adjustment. This would not have been detected without intra‐fraction monitoring.

Assessment of female breast dose for thoracic cone-beam CT using MOSFET dosimeters.

Objective: To assess the breast dose during a routine thoracic cone-beam CT (CBCT) check with the efforts to explore the possible dose reduction strategy.Materials and Methods: Metal oxide semiconductor field-effect transistor (MOSFET) dosimeters were used to measure breast surface doses during a thorax kV CBCT scan in an anthropomorphic phantom. Breast doses for different scanning protocols and breast sizes were compared. Dose reduction was attempted by using partial arc CBCT scan with bowtie filter. The impact of this dose reduction strategy on image registration accuracy was investigated.Results: The average breast surface doses were 20.02 mGy and 11.65 mGy for thoracic CBCT without filtration and with filtration, respectively. This indicates a dose reduction of 41.8% by use of bowtie filter. It was found 220° partial arc scanning significantly reduced the dose to contralateral breast (44.4% lower than ipsilateral breast), while the image registration accuracy was not compromised.Conclusions: Breast dose reduction can be achieved by using ipsilateral 220° partial arc scan with bowtie filter. This strategy also provides sufficient image quality for thorax image registration in daily patient positioning verification.

Accuracy of dose calculations on kV cone beam CT images of lung cancer patients.

To develop a clinically feasible method for dose calculations on cone beam CT (CBCT) images of two different vendors, and to determine the accuracy of these dose calculations for lung cancer patients. Lung cancer patients with CBCT imaging (n = 10 for Elekta, n = 6 for Varian) and a repeated planning CT scan on the same day were selected. For CBCT dose calculations, an adapted Hounsfield units-to-mass density table (HU table) was used which was obtained by comparing CT values of corresponding points on the CBCT and the repeated planning CT scan. Dose calculations with three different HU tables were compared: a patient-specific, a general thorax-CBCT, and the standard CT HU table. Planning CT data were used to compensate for the limited field of view (FOV) (Elekta) or scan length (Varian) of the CBCT. For evaluation, clinically relevant dose metrics were compared between the repeated CT and CBCT to assess the accuracy of dose calculations on CBCT for both vendors. For both vendors, isodose lines and dose volume histograms were very similar between dose calculation on CBCT and CT. For Varian, average differences between CT and CBCT dose calculations were 2%-3% for most dose metrics when the standard CT HU table was used. A better agreement was observed when a thorax-CBCT HU table was used, with differences of 1%-2%. No added value was found by using a patient-specific HU table, showing similar results as the general thorax-CBCT HU table. For Elekta, the dose metrics showed large deviations when the CT HU table was used, but using a patient-specific HU table resulted in similar accuracy as for Varian CBCT dose calculations, with average differences between repeated CT and CBCT dose metrics below 3%, and for most dose metrics even below 2%. Differences between Elekta and Varian CBCT, including hardware, reconstruction software, HU calibration, FOV, and scan length, resulted in different challenges for CBCT dose calculations for the different vendors. For Elekta CBCT scans, the procedure with a patient-specific HU table resulted in similar accuracy as for Varian CBCT dose calculations with a general HU correction for all thorax patients. The vendor-specific corrective methods used in this study resulted in dose calculations feasible for treatment re-evaluation for both Elekta and Varian CBCT scans.