Pretreatment Cone-beam Computed Tomography Research Articles

Markerless tumor tracking using short kilovoltage imaging arcs for lung image-guided radiotherapy

The ability to monitor tumor motion without implanted markers is clinically advantageous for lung image-guided radiotherapy (IGRT). Existing markerless tracking methods often suffer from overlapping structures and low visibility of tumors on kV projection images. We introduce the short arc tumor tracking (SATT) method to overcome these issues. The proposed method utilizes multiple kV projection images selected from a nine-degree imaging arc to improve tumor localization, and respiratory-correlated 4D cone-beam CT (CBCT) prior knowledge to minimize the effects of overlapping anatomies. The 3D tumor position is solved as an optimization problem with prior knowledge incorporated via regularization. We retrospectively validated SATT on 11 clinical scans from four patients with central tumors. These patients represent challenging scenarios for markerless tumor tracking due to the inferior adjacent contrast. The 3D trajectories of implanted fiducial markers were used as the ground truth for tracking accuracy evaluation. In all cases, the tumors were successfully tracked at all gantry angles. Compared to standard pre-treatment CBCT guidance alone, trajectory errors were significantly smaller with tracking in all cases, and the improvements were the most prominent in the superior-inferior direction. The mean 3D tracking error ranged from 2.2–9.9 mm, which was 0.4–2.6 mm smaller compared to pre-treatment CBCT. In conclusion, we were able to directly track tumors with inferior visibility on kV projection images using SATT. Tumor localization accuracies are significantly better with tracking compared to the current standard of care of lung IGRT. Future work involves the prospective evaluation and clinical implementation of SATT.

Automated Delineation of the Normal Urinary Bladder on Planning CT and Cone Beam CT

PurposeTo quantify the volumetric effect of delineation variability when using manual versus semiautomated tools to contour the normal bladder on planning computed tomography (CT) and cone beam CT. MethodsFollowing research ethics board approval, 10 prostate cancer patients were selected. For each patient, one pretreatment cone beam CT (CBCT) was randomly selected from the first treatment week and registered to the planning CT (planCT). Model-based auto adaptation was used to delineate the outer bladder (OB) surface for the planCT. That contour was then propagated and manually adapted onto the CBCT. A second observer delineated OB for the planCT and CBCT using typical manual methods. These delineation procedures were repeated four times on each image set, with observers blinded to the previous contours. Metrics of volumetric, geometric, and overlap concordance were used to compare the manual and automated OB contours. ResultsThe mean pairwise difference between the manual and model-based planCT volumes was 4 cm3 (2%), and the model-based contours exhibited approximately half the observer variation of the manual ones (3 cm3, 2%). The mean of pairwise differences between the manual and propagated CBCT volumes was 13 cm3 (8%), but the propagated contours exhibited approximately half the observer related volume variation (11 cm3, 6%). Small CBCT bladder volumes displayed larger observer variation with manual methods (r2, −0.640). Variability between the automated contours was significantly smaller than for the corresponding manual observations (P = .004 and .002, respectively). Metrics of three-dimensional overlap concordance indicated excellent agreement within and between the delineation methods. Automated CBCT contours were significantly smoother than the manual ones (surface sphericity index, 1.29 vs. 1.35; P = .03). ConclusionsVolumetric, geometric, and overlap metrics all indicated that planCT and CBCT automated OB contours fell within the range of manually delineated contours. The CBCT propagated contours were significantly smoother and associated with smaller intraobserver variability, compared with manual contours. Importantly, the findings from this research suggest that contour propagation may be more robust than manual delineation, especially in the presence of poor image quality.

Biologically Optimized Online Patient Positioning for Prostate External Beam Radiation Therapy

The standard approach for target localization in prostate radiation therapy is to perform rigid registration between pretreatment cone beam CT (CBCT) and planning CT (PCT); however, due to internal organ deformation, online registration could be operator-dependent and suboptimal. We proposed a novel method to evaluate the delivered dose to target and organs-at-risk (OAR) over a range of potential treatment positions. Using this approach, we determined optimal positioning based on proper target dose coverage with minimal risk of complication to OAR. A set of 10 patients with low or intermediate-risk prostate cancer was evaluated retrospectively in this IRB-approved study. For each patient, a VMAT plan with one 358oarc was developed on the PCT to deliver 78 Gy in 39 fractions. The margin for planning target volume was 7 mm around prostate, except for 5 mm in the posterior direction. Five representative CBCT image sets were selected for each patient. The CBCT was registered to planning CT online by radiation therapists, who performed automatic match and manually adjusted for best agreement at the rectal and prostate interface. Consensus contours of prostate, bladder, and rectum were created on each CBCT by combining independent delineations from 5 oncologists, which represented the ground truth for these structures. For each CBCT, search for optimal treatment position was performed within a 7 mm radius from the online operator-matched position following two criteria. The first criterion required minimization of NTCPrectum for less NTCPbladder relative to the online matched position. The second criterion required maximization of the complication free probability [CFP: (1- NTCPrectum)×(1- NTCPbladder)]. Both criteria required better prostate Dmin and equivalent uniform dose (EUD, a = -4) than values from online matched position. NTCPrectum was calculated with the Lyman-Kutcher-Burman (LKB) model (n = 0.09, m = 0.13, TD50 = 76.9 Gy) (Michalski et al), and similarly the bladder (n = 0.00995, m = 0.022, TD50 = 77.6 Gy) (Cheung et al). The optimal treatment positons that fulfilled the first criterion were at 2 ± 3, 2 ± 2, 0 ± 2 mm (mean ± SD) right, anterior, and inferior to the online matched position, and demonstrated a reduction of 26.3 ± 23.9% in NTCPrectum (range, 0–84.7%), while attaining an average increase of prostate Dmin by 1.2 Gy (range, 0–11.8 Gy) and EUD by 0.3 Gy (range, 0–8.1 Gy) relative to those from the online matched positions. The second criterion provided an increase in CFP by 19.4 ± 18.1% (range, 0–59.8%) and an average increase of prostate Dmin by 2.8 Gy (range, 0–32.2 Gy), and EUD by 1.2 Gy (range, 0–38.5 Gy). The optimal positions were 1 ± 4, 1 ± 2, 2 ± 3 mm right, anterior, and inferior to the online matched position. These encouraging results suggest a role for optimization of patient positioning based on radiobiological optimization in the routine IGRT of prostate patients.

Dosimetric Impact of Internal and External Anatomical Changes in Stereotactic Radiosurgery (SRS) of L-Spine Tumors: Beam Configuration Matters

To investigate three treatment planning strategies to minimize the dosimetric variations due to internal and external anatomical changes in stereotactic radiosurgery (SRS) of L-spine tumors. 11 L-spine SRS patients with extensive gas filling in the bowel in the planning CT or pre-treatment cone beam CT were selected for this study. Three coplanar treatment plans were made to deliver 16Gy to 90% of GTV in one fraction using a treatment planning system: 1) Step and shoot IMRT with 9 beams equally distributed around the patient, 2) step and shoot IMRT with 9 posterior beams every 20 degree, and 3) VMAT with 360 degrees full arcs. Dose was calculated with heterogeneity correction and collapsed cone convolution algorithm. Pre-treatment CBCT was registered to the planning CT with the vertebral body as the alignment focus region. For each patient, the external body and bowel gas were contoured on the planning CT and CBCT. To estimate actual delivered dose while considering patient’s anatomy of the treatment day, the following density overrides were performed on the planning CT. To consider external anatomy change, the CBCT external that was outside of the planning CT external were assigned to density one and the CT external outside the CBCT external were assigned to density zero. To consider the internal anatomy change, bowel gas contour in the planning CT was assigned one and the bowel gas contour transferred from the CBCT was assigned to zero. The mean absolute differences (MAD) between planning and estimated delivery in target coverage (V16Gy), maximum dose (0.03cc of volume, D0.03cc) and 12 Gy received by cauda-equina (V12Gy) were evaluated. The MAD of V16Gy for the GTV was 1.8 ± 1.4% (range: 0.2% – 4.9%), 2.4 ± 1.3% (0.1% – 4.9%) and 2.6 ± 1.3% (0.3% – 5.0%) for posterior, uniform and VMAT plans, respectively. The difference was statistically significant for the posterior vs. VMAT plans (p = 0.049) but not significant for the posterior vs. uniform plans (p=0.174). Cauda-equina MAD of V12Gy was 0.9 ± 1.1% (range: 0.1% - 3.9%), 1.2 ± 1.0% (0.1% – 2.7%) and 0.8 ± 0.8% (0% – 1.8%, p=0.024 comparison to uniform) for posterior, uniform and VMAT plans, respectively. The lower cauda-equina MAD of V12Gy for VMAT was because VMAT plans achieved significantly lower V12Gy in the treatment plans (mean: 4.3 ± 1.4%) than posterior (6.7 ± 1.6%) and uniform (6.8 ± 1.6%) plans (p = 0.003). MAD of D0.03cc was 13.1 ± 8.6 cGy (range: 2 – 31 cGy), 19.2 ± 16 cGy (4 – 53 cGy) and 19.3 ± 16 cGy (0 – 59 cGy) for the posterior, uniform and VMAT plans, respectively. The difference was not statistically significant. The posterior plans were less influenced dosimetrically in tumor coverage by variations in abdominal anatomy but were not superior in terms of cauda-equina sparing.

A pilot study of root position in orthodontic diagnosis model set-up

To investigate the importance of root information in diagnosis set-up by constructing three-dimensional (3D) digital models with individual anatomic roots. Pretreatment cone-beam CT (CBCT) and laser scanning data were collected from two patients (extraction and non-extraction each) with skeletal Class I malocclusion. Threshold segmentation of the CBCT was performed to generate a 3D digital model which has individually isolated tooth. This model and the scan model were superimposed to generate an integrated model (Mo) composed of high-resolution surface scan crowns sutured to the CBCT roots. Pretreatment dentition plaster model were made into set-up model. The diagnosis model set-up was performed successively by three orthodontists and one senior orthodontic technician. Set-up model scan of each patient after tooth alignment was obtained. The isolated composite teeth were individually superimposed onto the set-up model surface scan to creat set-up model (Ma, Mb, Mc, Md) containing root position. These isolated composite teeth were also superimposed onto the posttreatment surface scan depicting the posttreatment model (M). In order to observe whether diagnosis model set-up would cause exposure of the root, Ma-Md were compared with Mo, which showed the true positions of alveolus. In order to validate the accuracy of the expected root position setup, Ma-Md were compared with the true root position represented by M. Color displacement maps generated to measure the discrepancies of root positions. Nonparallel and exposure of the root was found in all setup models. Color displacement maps through molar superimpositions showed maximum differences of 8.79 mm for the maxillary teeth and 9.96 mm for the mandibular teeth. Diagnosis model set-up based only on crown can not absolutely ensure the good alignment of roots without root exposure. It is necessary to construct the integrated model including root for diagnosis during tooth arrangement process.

Prediction of lung density changes after radiotherapy by cone beam computed tomography response markers and pre-treatment factors for non-small cell lung cancer patients

Background and purposeThis study investigates the ability of pre-treatment factors and response markers extracted from standard cone-beam computed tomography (CBCT) images to predict the lung density changes induced by radiotherapy for non-small cell lung cancer (NSCLC) patients. Methods and materialsDensity changes in follow-up computed tomography scans were evaluated for 135 NSCLC patients treated with radiotherapy. Early response markers were obtained by analysing changes in lung density in CBCT images acquired during the treatment course. The ability of pre-treatment factors and CBCT markers to predict lung density changes induced by radiotherapy was investigated. ResultsAge and CBCT markers extracted at 10th, 20th, and 30th treatment fraction significantly predicted lung density changes in a multivariable analysis, and a set of response models based on these parameters were established. The correlation coefficient for the models was 0.35, 0.35, and 0.39, when based on the markers obtained at the 10th, 20th, and 30th fraction, respectively. ConclusionsThe study indicates that younger patients without lung tissue reactions early into their treatment course may have minimal radiation induced lung density increase at follow-up. Further investigations are needed to examine the ability of the models to identify patients with low risk of symptomatic toxicity.

The detection of periapical pathoses using digital periapical radiography and cone beam computed tomography in endodontically retreated teeth - part 2: a 1year post-treatment follow-up.

Part 2 of this prospective clinical study aimed to compare the 1-year outcome of root canal retreatments, when individual roots and teeth were assessed by periapical radiographs and cone beam computed tomography (CBCT). Subjects participating in this study had been referred for management of an endodontic problem associated with one or more root filled teeth. Root canal retreatment was performed by Specialists or postgraduate students under the direct supervision of Specialist endodontic staff. A total of 98 teeth (84 patients) were reassessed clinically and radiographically 1year after completion of root canal retreatment. The postoperative periapical radiographs and CBCT scans were compared with their respective pre-treatment (diagnostic) periapical radiographs and CBCT scans. The increase or decrease in size of existing periapical radiolucencies and development of new radiolucencies were assessed by a consensus panel consisting of two calibrated examiners. They also determined an appropriate management plan for each case based on the radiographical findings. Comparison of the outcome diagnosis of individual roots and teeth and case management, when assessed by periapical radiographs and CBCT scans, was performed using chi-squared and McNemar's tests. An overall favourable result of 93% success for teeth (96% roots) was recorded when the assessment was undertaken by periapicals compared with 77% success for teeth (87% roots) when assessed by CBCT. A significant difference in outcome diagnosis of single paired roots (P<0.0001) and teeth (P= 0.0001) was observed when comparing periapicals to CBCT for the cohort of teeth as a whole. When comparing the future management plan on the basis of radiographic information alone, there was a significant difference between periapicals and CBCT-based management (P=0.01). Diagnosis using CBCT revealed a significantly lower number of favourable outcomes than periapicals in root canal retreatment. This significantly affected the future management of cases attending for a review.

Positioning reproducibility with and without rotational corrections for 2 head and neck immobilization systems

PurposeThe purpose of this study was to evaluate the impact of offline rotational corrections and assess intrafraction motion for head and neck (H&N) cancer patients immobilized with and without a custom neck cushion. Methods and materialsFifty H&N cancer patients were immobilized and imaged with pretreatment and posttreatment cone beam computed tomography (CBCT) for each treatment fraction. Of these patients, 25 had a custom neck cushion added to their immobilization. Each CBCT was registered to the simulation computed tomography offline. Registrations were performed with automatching tools and a matching volume of interest that consisted of a 5-mm expansion around the mandible, occipital bone, C1/C2, and C7/T1. To determine positioning accuracy, the registration was inspected to confirm these bony anatomy structures were contained within a 3- or 5-mm expansion of the simulation position. If not, the registration was repeated with rotational corrections included and re-evaluated. For each fraction, intrafraction motion was also quantified through the difference between the pretreatment and posttreatment CBCT registration coordinates. ResultsFor translational registrations, the bony anatomy in pretreatment imaging was outside the 3-mm or 5-mm expansion structure, respectively, for 49% and 15% of fractions on average for patients without a custom headrest and for 48% and 13% of fractions on average for patients with a custom headrest. The addition of rotational corrections reduced these numbers to 21% and 4% and to 28% and 6%, respectively. Intrafraction motion was significantly lower for patients immobilized with the addition of a custom neck cushion: 1.0 ± 0.5 mm compared with 1.8 ± 1.6 mm for patients with the standard headrest only (P = .02). This was reflected in posttreatment positioning accuracy, which was significantly reduced in the case of the standard headrest compared with pretreatment imaging (P values of < .001 to .048). ConclusionsRotational corrections significantly improved pretreatment patient positioning accuracy (P < .001). Intrafraction motion was reduced significantly through the addition of a custom neck cushion and resulted in an increase in posttreatment positioning accuracy for these patients.

Fast three-dimensional superimposition of cone beam computed tomography for orthopaedics and orthognathic surgery evaluation

The aim of this study was to validate a method for fast three-dimensional (3D) superimposition of cone beam computed tomography (CBCT) in growing patients and adults (surgical cases). The sample consisted of CBCT scans of 18 patients. For 10 patients, as the gold standard, the spatial position of the pretreatment CBCT was reoriented, saved as a reoriented volume, and then superimposed on the original image. For eight patients, four non-growing and four growing, the pre- and post-treatment scans were superimposed. Fast voxel-based superimposition was performed, with registration at the anterior cranial base. This superimposition process took 10–15s. The fit of the cranial base superimposition was verified by qualitative visualization of the semi-transparent axial, sagittal, and coronal cross-sectional slices of all corresponding anatomical structures. Virtual 3D surface models of the skull were generated via threshold segmentation, and superimposition errors in the reoriented models and the results of treatment for the treated cases were evaluated by 3D surface distances on colour-coded maps. The superimposition error of the spatial reorientation and for growing and non-growing patients was <0.5mm, which is acceptable and clinically insignificant. The voxel-based superimposition method evaluated was reproducible in different clinical conditions, rapid, and applicable for research and clinical practice.

An Investigation of the Feasibility and Utility of a Low-dose Cone-beam Computed Tomography Scan Protocol for Head and Neck Cancer Patients

IntroductionRoutine use of cone-beam computed tomography (CBCT) scan protocols as part of the image guidance process (image-guided radiation therapy) has become an integral part of the practice of radiation therapists (RTs). Concerns regarding imaging dose as well as increased in-room time for patients led to reluctance among site group members to adopt CBCT for all radical head and neck cancer (HNC) patients at our institution. This investigation set out to assess the feasibility and utility of a revised CBCT scan protocol with the aim of supporting daily CBCT for HNC patients receiving radiation therapy. MethodsThe project was performed in three phases. Phase 1 involved the experimental adjustment of CBCT scan protocol parameters in clinical use for HNC patients at our institution. An Elekta Synergy linear accelerator with kilovoltage CBCT capability and a RANDO head phantom were used for scan acquisition procedures. Image registration using bony anatomy was performed on two image sets generated using the current clinical scan protocol (HNS20) and an experimental modified scan protocol (MHNS20). Image registration results were compared by two investigators. Measurements of scan doses using a metal–oxide–semiconductor field-effect transistor and a Unidose meter were performed. Catphan phantom images were acquired using HNS20 and MHNS20 protocols. In phase 2, ten volunteer RTs performed image registration and matching processes on two image sets performed using HNS20 and MHNS20 protocols. RTs were unaware of the scan protocols used for image acquisition. A threshold of 3 mm was set (the current maximum couch shift allowance in the clinical HNC IGRT protocol) to compare the image registration data from HNS20 and MHNS20. In phase 3, after research ethics board approval, 10 HNC patients consented to the study. Two pretreatment CBCT scans were performed: scan 1 was acquired using MHNS20 protocol, and scan 2 was acquired using the HNS20 protocol. A threshold of 2 mm was set to compare the differences in couch shift data resulting from the image registration of the two image sets. Comparison of HNS20 and MHNS20 based on image registration results was performed. ResultsIn phase 1, radiation doses measured by the investigators on the left optical lens using a metal–oxide–semiconductor field-effect transistor and a Unidose meter indicated that the MHNS20 protocol would result in a lower dose to the left optical lens. In phase 2, shifts of the treatment table to achieve the planned isocentre, which were recorded after the image matching process, were within 3 mm in 80% of the RT procedures. In the y-axis (superior/inferior direction), 100% of the procedures were within 3 mm. In the z-axis (anterior/posterior) and x-axis (lateral), 90% of the procedures were within 3 mm. Qualitative data from a questionnaire completed by RTs after the image matching indicated that 50% of the RTs had no preference between the images sets in terms of visibility of structures. Forty percent of RTs had no preference regarding speed of matching or preference for registration between the image sets. When a preference was indicated, the HNS20 scan protocol was chosen by the RTs. In phase 3, couch shift data recorded after each CBCT scan were compared. All results in all three planes for 10 patients included in the study were within the 2-mm threshold. ConclusionsThe feasibility and clinical utility of a potential lower-dose CBCT scan protocol has been investigated. The modified protocol (MHNS20) produced image data acceptable within current practice using bony anatomy for registration purposes. The MHNS20 protocol also delivered lower doses to the left optical lens and therefore potentially to other pertinent structures. The actual delivered doses to patients during IGRT procedures using the MHNS20 may be different than those measured during this investigation.

Validation of a deformable image registration technique for cone beam CT-based dose verification.

As radiation therapy evolves toward more adaptive techniques, image guidance plays an increasingly important role, not only in patient setup but also in monitoring the delivered dose and adapting the treatment to patient changes. This study aimed to validate a method for evaluation of delivered intensity modulated radiotherapy (IMRT) dose based on multimodal deformable image registration (dir) for prostate treatments. A pelvic phantom was scanned with CT and cone-beam computed tomography (CBCT). Both images were digitally deformed using two realistic patient-based deformation fields. The original CT was then registered to the deformed CBCT resulting in a secondary deformed CT. The registration quality was assessed as the ability of the dir method to recover the artificially induced deformations. The primary and secondary deformed CT images as well as vector fields were compared to evaluate the efficacy of the registration method and it's suitability to be used for dose calculation. plastimatch, a free and open source software was used for deformable image registration. A B-spline algorithm with optimized parameters was used to achieve the best registration quality. Geometric image evaluation was performed through voxel-based Hounsfield unit (HU) and vector field comparison. For dosimetric evaluation, IMRT treatment plans were created and optimized on the original CT image and recomputed on the two warped images to be compared. The dose volume histograms were compared for the warped structures that were identical in both warped images. This procedure was repeated for the phantom with full, half full, and empty bladder. The results indicated mean HU differences of up to 120 between registered and ground-truth deformed CT images. However, when the CBCT intensities were calibrated using a region of interest (ROI)-based calibration curve, these differences were reduced by up to 60%. Similarly, the mean differences in average vector field lengths decreased from 10.1 to 2.5 mm when CBCT was calibrated prior to registration. The results showed no dependence on the level of bladder filling. In comparison with the dose calculated on the primary deformed CT, differences in mean dose averaged over all organs were 0.2% and 3.9% for dose calculated on the secondary deformed CT with and without CBCT calibration, respectively, and 0.5% for dose calculated directly on the calibrated CBCT, for the full-bladder scenario. Gamma analysis for the distance to agreement of 2 mm and 2% of prescribed dose indicated a pass rate of 100% for both cases involving calibrated CBCT and on average 86% without CBCT calibration. Using deformable registration on the planning CT images to evaluate the IMRT dose based on daily CBCTs was found feasible. The proposed method will provide an accurate dose distribution using planning CT and pretreatment CBCT data, avoiding the additional uncertainties introduced by CBCT inhomogeneity and artifacts. This is a necessary initial step toward future image-guided adaptive radiotherapy of the prostate.

Three-dimensional monitoring of root movement during orthodontic treatment

A significant objective of orthodontic treatment is to achieve proper and stable tooth positions that involve not only the crowns, but also their roots. However, the current methods of clinically monitoring root alignment are unreliable and inaccurate. Therefore, the purpose of this study was to develop a methodology that can accurately identify root position in a clinical situation. Pretreatment and posttreatment cone-beam computed tomography (CBCT) and extraoral laser scans of study models of a patient were obtained. Threshold segmentation of the CBCT scans was performed, resulting in 3-dimensional surface models. The pretreatment CBCT teeth were isolated from their respective arches for individual tooth manipulation. These isolated pretreatment CBCT teeth were superimposed onto the posttreatment surface scan depicting the expected root position setup. To validate the accuracy of the expected root position setup, it was compared with the true root position represented by the posttreatment CBCT scan. Color displacement maps were generated to measure any differences between the expected and true root positions. Color map analysis through crown superimposition showed displacement differences of 0.148 ± 0.411 mm for the maxillary roots and 0.065 ± 0.364 mm for the mandibular roots. This methodology has been demonstrated to be an accurate and reliable approach to visualize the 3-dimensional positions of all teeth, including the roots, with no additional radiation applied.

P13.24 * A COMPARISON OF TWO STEREOTACTIC RADIOSURGERY DELIVERY TECHNIQUES IN THE TREATMENT OF BRAIN METASTASIS

The Beatson West of Scotland Cancer Center (BWOSCC) has been treating brain metastasis using LINAC based stereotactic radiosurgery with micromultileaf collimator (Brain lab M3) since 2008. Patients are immobilised using Brainlab fixation and treated with static fixed fields (6MV photon), delivered at conventional dose rates of 400-600 monitor units per minute (MU/min). In August 2012 a TrueBeam STX (Varian Medical Systems) was installed. Patients with localised CNS lesions are now treated with 10MV FFF VMAT. METHOD: Treatment records for 10 for conformal static field patients and 10 VMAT FFF patients were reviewed on Varian Eclipse and Offline Review. Beam on time (BOT), time in room (TIR), and clinical dose rate (CDR) were compared. BOT (minutes:seconds) is the aggregate “beam-on” time for all fields or arcs in a given plan. TIR (minutes:seconds) is measured from the first alignment image to last beam off, inclusive of all pre-treatment imaging studies and/or shifts. CDR (MU/min) is the number of MU divided by the BOT.Static field patients had MV imaging only. VMAT patients had kV imaging and pre-treatment cone beam CT (CBCT). 18 patients had single metastasis, 2 patients had 2 mets. 11 patients were treated with 20Gy, 7: 18Gy and 2: 15Gy. RESULTS: Median Planning Target Volume was 5.41cm3 (min 0.98cm3 max 18.88cm3). 10 patients were treated with 6MV static fields (between 4 and 6 fields). 10 patients with 10MV FFF (All 10 Patients had 2 co-planar) arcs. Beam on time: 6X Static 5.85 (range 4.55-6.6), VMAT FFF 2.60 (range 2.01-3.9). Time In Room: 6X Static 34.17 (range 20.42-58.13) VMAT FFF 17.12 (range 12.42-28.28). Monitor Units: 6X Static 2726 (range 2119-3168) VMAT FFF 6216.5 (range 4472-9340). Clinical Dose Rate: 6X Static 600MU/Min VMAT FFF 2385.5MU/Min. CONCLUSION: VMAT FFF has dramatically reduced the TIR: shorter BOT reduces the likelihood of intrafraction motion. It is envisaged that VMAT FFF treatment times will be reduced further as Radiographer competency and skill increases. This is essential in busy RT departments where treatment slots are at a premium.

Investigation of an adaptive treatment regime for prostate radiation therapy

PurposeChanges in body contour during the course of radiation therapy can compromise the accuracy of treatment delivery. In prostate radiation therapy these changes can result from the daily patient positioning as well as the deformation of the body shape between treatment days and simulation. This study investigates the feasibility of using an online correction method to account for interfraction body contour changes prior to daily delivery of prostate radiation therapy based on cone beam computed tomography (CBCT) imaging. Methods and materialsThis is a retrospective study where 5 prostate patients who received intensity modulated radiation therapy were selected. Patients were selected from a cohort who demonstrated repeated minor (1-2 cm) variations in body contour between planning CT and daily CBCT. Pretreatment CBCT images were used to calculate the “treated” dose and investigate the deviations from prescription caused by changes in body contour. A simple tissue maximum ratio correction was then applied to each field to calculate the “adapted” dose, which accounts for physical changes in path length on the central axis. A comparison was made between the treated and adapted scenarios. ResultsFor this cohort, 22 of the 64 analyzed fractions showed body contour changes ≥1 cm. The treated doses showed deviations from prescribed dose ranging from −2.4% to 1.3% per fraction. After the plan was adapted, 4 of the 5 cases showed an improvement (P < .05) in target dose for the entirety of treatment. The 1 patient not showing improvement had negligible dose change due to contour variation. ConclusionsThis online adaptive strategy is a promising approach to account for changes in body contour during prostate intensity modulated radiation therapy.

Real-time segmentation of multiple implanted cylindrical liver markers in kilovoltage and megavoltage x-ray images

Gold markers implanted in or near a tumor can be used as x-ray visible landmarks for image based tumor localization. The aim of this study was to develop and demonstrate fast and reliable real-time segmentation of multiple liver tumor markers in intra-treatment kV and MV images and in cone-beam CT (CBCT) projections, for real-time motion management. Thirteen patients treated with conformal stereotactic body radiation therapy in three fractions had 2–3 cylindrical gold markers implanted in the liver prior to treatment. At each fraction, the projection images of a pre-treatment CBCT scan were used for automatic generation of a 3D marker model that consisted of the size, orientation, and estimated 3D trajectory of each marker during the CBCT scan. The 3D marker model was used for real-time template based segmentation in subsequent x-ray images by projecting each marker's 3D shape and likely 3D motion range onto the imager plane. The segmentation was performed in intra-treatment kV images (526 marker traces, 92 097 marker projections) and MV images (88 marker traces, 22 382 marker projections), and in post-treatment CBCT projections (42 CBCT scans, 71 381 marker projections). 227 kV marker traces with low mean contrast-to-noise ratio were excluded as markers were not visible due to MV scatter. Online segmentation times measured for a limited dataset were used for estimating real-time segmentation times for all images. The percentage of detected markers was 94.8% (kV), 96.1% (MV), and 98.6% (CBCT). For the detected markers, the real-time segmentation was erroneous in 0.2–0.31% of the cases. The mean segmentation time per marker was 5.6 ms [2.1–12 ms] (kV), 5.5 ms [1.6–13 ms] (MV), and 6.5 ms [1.8–15 ms] (CBCT). Fast and reliable real-time segmentation of multiple liver tumor markers in intra-treatment kV and MV images and in CBCT projections was demonstrated for a large dataset.

The novel use of three-dimensional surface models to quantify and visualise the immediate changes of the mid-facial skeleton following rapid maxillary expansion

BackgroundThe transverse skeletal effects of rapid maxillary expansion (RME) have previously been assessed using cone-beam CT (CBCT). However, to date the majority of studies assess the changes based on two-dimensional slice images, which under utilises the three-dimensional (3D) data captured. This study optimizes the volumetric CBCT data by generating 3D rendered surface models to quantity and visualize the immediate 3D changes of the mid-facial bone surfaces following RME. MethodsThe sample consisted of 14 patients who required RME prior to fixed appliances. Pre-treatment (T0) and immediate post expansion (T1) CBCT images were taken. Following superimposition the mid face was divided into six anatomical regions. A one-sample t-test was used to determine if the differences between the two surfaces were significantly ≥0.5 mm. FindingsAll regions showed a change following RME ≥ 0.5 mm. The maxillary and nasal bones showed 2.3 mm and 2.4 mm expansion respectively, followed by the zygomatic bones (1.4 mm), 2 cases showing asymmetric expansion. ConclusionsThe use of 3D surface rendered models allows quantification and visualisation of 3D changes in the mid-facial skeleton at anatomical sites distant of RME activation. Following activation there can be a pan mid-facial expansion, including not only the maxilla but also the nasal lateral bones and zygomas. The response was highly variable and asymmetric expansion can occur.

Adaptive plan selection vs. re-optimisation in radiotherapy for bladder cancer: A dose accumulation comparison

PurposePatients with urinary bladder cancer are obvious candidates for adaptive radiotherapy (ART) due to large inter-fractional variation in bladder volumes. In this study we have compared the normal tissue sparing potential of two ART strategies: daily plan selection (PlanSelect) and daily plan re-optimisation (ReOpt). Materials and methodsSeven patients with bladder cancer were included in the study. For the PlanSelect strategy, a patient-specific library of three plans was generated, and the most suitable plan based on the pre-treatment cone beam CT (CBCT) was selected. For the daily ReOpt strategy, plans were re-optimised based on the CBCT from each daily fraction. Bladder contours were propagated to the CBCT scan using deformable image registration (DIR). Accumulated dose distributions for the ART strategies as well as the non-adaptive RT were calculated. ResultsA considerable sparing of normal tissue was achieved with both ART approaches, with ReOpt being the superior technique. Compared to non-adaptive RT, the volume receiving more than 57Gy (corresponding to 95% of the prescribed dose) was reduced to 66% (range 48–100%) for PlanSelect and to 41% (range 33–50%) for ReOpt. ConclusionThis study demonstrated a considerable normal tissue sparing potential of ART for bladder irradiation, with clearly superior results by daily adaptive re-optimisation.

Prostate bed motion may cause geographic miss in post‐prostatectomy image‐guided intensity‐modulated radiotherapy

There is little data to guide radiation oncologists on appropriate margin selection in the post-prostatectomy setting. The aim of this study was to quantify interfraction variation in motion of the prostate bed to determine these margins. The superior and inferior surgical clips in the prostate bed were tracked on pretreatment cone beam CT images (n = 377) for 40 patients who had received post-prostatectomy radiotherapy. Prostate bed motion was calculated for the upper and lower segments by measuring the position of surgical clips located close to midline relative to bony anatomy in the axial (translational) and sagittal (tilt) planes. The frequency of potential geographic misses was calculated for either 1 cm or 0.5 cm posterior planning target volume margins. The mean magnitude of movement of the prostate bed in the anterior-posterior, superior-inferior and left-right planes, respectively, were as follows: upper portion, 0.50 cm, 0.28 cm, 0.10 cm; lower portion, 0.18 cm, 0.18 cm, 0.08 cm. The random and systematic errors, respectively, of the prostate bed motion in the anterior-posterior, superior-inferior and left-right planes, respectively, were as follows: upper portion, 0.47 cm and 0.50 cm, 0.28 cm and 0.27 cm, 0.11 cm and 0.11 cm; lower portion, 0.17 cm and 0.18 cm, 0.17 cm and 0.19 cm, 0.08 cm and 0.10 cm. Most geographic misses occurred in the upper prostate bed in the anterior-posterior plane. The median prostate bed tilt was 1.8° (range -23.4° to 42.3°). Variability was seen in all planes for the movement of both surgical clips. The greatest movement occurred in the anterior-posterior plane in the upper prostate bed, which could cause geographic miss of treatment delivery. The variability in the movement of the superior and inferior clips indicates a prostate bed tilt that would be difficult to correct with standard online matching techniques. This creates a strong argument for using anisotropic planning target volume margins in post-prostatectomy radiotherapy.

Plan of the day selection for online image-guided adaptive post-prostatectomy radiotherapy

PurposeTo compare the cone-beam CT (CBCT) soft tissue localization disparity between radiation oncologists (RO) and radiation therapy technologists (RTT) in a novel online protocol of image-guided adaptive radiotherapy to the postoperative prostate bed. MethodUsing the planning CT and pre-treatment CBCTs from the first week of radiotherapy, four adaptive plans of different sizes were derived for each of eight post-prostatectomy patients. Four ROs collectively defined the reference answer, i.e. the plan of the day and isocentre correction for 40 CBCTs taken in weeks 2–6 of treatment for each patient. RTTs were randomly assigned five of these CBCTs; and asked to record their plan of the day selection and isocentre correction. RTT selection and reference answers were compared. The distance between the RTT selection and the reference answer was calculated. ResultsA total of 33 RTTs took part in this study. The average difference in CTV volume (reference answer-RTT selection) was 1.32cm3 (SD 29cm3) overall. The average difference between reference answer and RTT isocentre coordinates was SI 1mm (SD 4.8mm), LR 1.1mm (SD 4.0mm) and AP −0.2mm (SD 3.9mm). Distance of superior 8mm, inferior 6mm, left 4mm, right 2mm, anterior 6mm and posterior 6mm covered 100% of the CTV in 90% of fractions. ConclusionThe difference between RTT and RO selection of adaptive volumes is small and can be accounted for in a clinically acceptable CTV to PTV margin. Adaptive post-prostatectomy radiotherapy is feasible, in the setting of an academic center although at the moment, we have insufficient evidence to suggest that margins can yet be reduced with IGART with the current protocol.

Effect of different breathing patterns in the same patient on stereotactic ablative body radiotherapy dosimetry for primary renal cell carcinoma: A case study

Stereotactic ablative body radiotherapy (SABR) for primary renal cell carcinoma (RCC) targets requires motion management strategies to verify dose delivery. This case study highlights the effect of a change in patient breathing amplitude on the dosimetry to organs at risk and target structures. A 73-year-old male patient was planned for receiving 26Gy of radiation in 1 fraction of SABR for a left primary RCC. The patient was simulated with four-dimensional computed tomography (4DCT) and the tumor internal target volume (ITV) was delineated using the 4DCT maximum intensity projection. However, the initially planned treatment was abandoned at the radiation oncologist's discretion after pretreatment cone-beam CT (CBCT) motion verification identified a greater than 50% reduction in superior to inferior diaphragm motion as compared with the planning 4DCT. This patient was resimulated with respiratory coaching instructions. To assess the effect of the change in breathing on the dosimetry to the target, each plan was recalculated on the data set representing the change in breathing condition. A change from smaller to larger breathing showed a 46% loss in planning target volume (PTV) coverage, whereas a change from larger breathing to smaller breathing resulted in an 8% decrease in PTV coverage. ITV coverage was similarly reduced by 8% in both scenarios. This case study highlights the importance of tools to verify breathing motion prior to treatment delivery. 4D image guided radiation therapy verification strategies should focus on not only verifying ITV margin coverage but also the effect on the surrounding organs at risk.