Elekta Oncology Systems Research Articles

Machine Log File and Calibration Errors-based Patient-specific Quality Assurance (QA) for Volumetric Modulated Arc Therapy (VMAT).

Dose reconstructed based on linear accelerator (linac) log-files is one of the widely used solutions to perform patient-specific quality assurance (QA). However, it has a drawback that the accuracy of log-file is highly dependent on the linac calibration. The objective of the current study is to represent a new practical approach for a patient-specific QA during Volumetric modulated arc therapy (VMAT) using both log-file and calibration errors of linac. A total of six cases, including two head and neck neoplasms, two lung cancers, and two rectal carcinomas, were selected. The VMAT-based delivery was optimized by the TPS of Pinnacle^3 subsequently, using Elekta Synergy VMAT linac (Elekta Oncology Systems, Crawley, UK), which was equipped with 80 Multi-leaf collimators (MLCs) and the energy of the ray selected at 6 MV. Clinical mode log-file of this linac was used in this study. A series of test fields validate the accuracy of log-file. Then, six plans of test cases were delivered and log-file of each was obtained. The log-file errors were added to the corresponding plans through the house script and the first reconstructed plan was obtained. Later, a series of tests were performed to evaluate the major calibration errors of the linac (dose-rate, gantry angle, MLC leaf position) and the errors were added to the first reconstruction plan to generate the second reconstruction plan. At last, all plans were imported to Pinnacle and recalculated dose distribution on patient CT and ArcCheck phantom (SUN Nuclear). For the former, both target and OAR dose differences between them were compared. For the latter, γ was evaluated by ArcCheck, and subsequently, the surface dose differences between them were performed. Accuracy of log-file was validated. If error recordings in the log file were only considered, there were four arcs whose proportion of control points with gantry angle errors more than ± 1°larger than 35%. Errors of leaves within ± 0.5 mm were 95% for all arcs. The distinctness of a single control point MU was bigger, but the distinctness of cumulative MU was smaller. The maximum, minimum, and mean doses for all targets were distributed between -6.79E-02-0.42%, -0.38-0.4%, 2.69E-02-8.54E-02% respectively, whereas for all OAR, the maximum and mean dose were distributed between -1.16-2.51%, -1.21-3.12% respectively. For the second reconstructed dose: the maximum, minimum, and mean dose for all targets was distributed between 0.0995~5.7145%, 0.6892~4.4727%, 0.5829~1.8931% separately. Due to OAR, maximum and mean dose distribution was observed between -3.1462~6.8920%, -6.9899~1.9316%, respectively. Patient-specific QA based on the log-file could reflect the accuracy of the linac execution plan, which usually has a small influence on dose delivery. When the linac calibration errors were considered, the reconstructed dose was closer to the actual delivery and the developed method was accurate and practical.

Investigating the impact of collimator size variation on the single beam radiation of Gamma Knife PerfexionTM based on Monte Carlo simulation

The Gamma Knife PerfexionTM (Elekta Oncology Systems) is a specialized stereotactic radiotherapy device that uses 192 Cobalt-60 radioactive sources. A dynamic collimator system enables three circular field sizes of 4 mm, 8 mm, and 16 mm diameter to be delivered. This study investigates the dosimetric characteristics of the three different sized collimated beams using Monte Carlo (MC) simulations. The MC simulation made use of previously calculated phase space from a Monte-Carlo model of the encapsulated Cobalt-60 sources. The collimators were modelled using the FLATFILT component module of the BEAMnrc user code. The output of the single beam radiation was stored in a circular scoring plane with a radius of 1.4 cm at source surface distance (SSD) 17.20 cm in the phase space file. The simulation parameters were used 2.1 x 109 particles, electron cut off energy (ECUT) of 0.7 MeV, and photon cut off energy (PCUT) of 0.01 MeV. To investigate the physical full width at half maximum (FWHM) and penumbra width (80-20%), the single beam profiles and mean energy were obtained using BEAMnrc/BEAMDP code. Based on the results that there was an increase of the full width at half maximum (FWHM) with the size of the collimator on the beam profile and the mean energy. The FHWM of the energy fluence profile for the collimator 4, 8, and 16 mm are 2.22, 3.90, and 7.80 mm, respectively. The FHWM of the fluence profile for the collimator sizes 4, 8, and 16 mm are 1.87, 3.87, and 7.80 mm. The FHWM of mean energy as a function of position for the collimator sizes 4, 8, and 16 mm are 2.56, 4.20, and 7.94 mm, respectively. The maximum mean energy for the collimators decreases as much as 0.0185 MeV.

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

TU‐D‐202‐02: Make Margins Simple: Use Real‐Time Target Tracking

Respiratory motion has long been recognized as an important factor affecting the precision of radiotherapy. After the introduction of the 4D CT to visualize the respiratory motion in 3D, the internal target volume (ITV) has been widely adopted as simple method to take the motion into account in treatment planning and delivery. The ITV is generated as the union of the CTVs as the patient goes through the respiratory cycle. Many issues have been identified with the ITV. In this session three alternatives for the ITV will be discussed: 1) An alternative motion‐inclusive approach with better imaging and smaller margins, called mid‐position CT. 2) The tracking approach and 3) The gating approach.The following topics will be addressed by Marcel van Herk (“Is ITV the correct motion encompassing strategy”): Magnitude of respiratory motion, effect of motion on radiotherapy, motion encompassing strategies, and software solutions to assist in motion encompassing strategies. Then Paul Keall (“Make margins simple: Use real‐time target tracking”) will discuss tracking with: clinical drivers for tracking, current clinical status of tumor tracking, future tumor tracking technology, and margin margin challenges with and without tracking. Finally Daniel Low will discuss gating (“Gating is the best ITV killer”): why ITV in the first place, requirements for planning, requirements at the machine, benefits and costs.The session will end with a discussion and live demo of motion simulation software to illustrate the issues and explain the relative benefit and appropriate uses for the three methods. Learning Objectives: Explain the 4D imaging and treatment planning process. Summarize the various approaches to deal with respiratory motion during radiotherapy Discuss the tradeoffs involved when choosing one of the three discussed approaches. Explain in which situation each method is the best choiceResearch is partly funded by Elekta Oncology Systems and the Dutch Cancer Foundation; M. van Herk, Part of the research was funded by Elekta Oncology Systems and the Dutch Cancer Foundation

TU‐D‐202‐03: Gating Is the Best ITV Killer

Respiratory motion has long been recognized as an important factor affecting the precision of radiotherapy. After the introduction of the 4D CT to visualize the respiratory motion in 3D, the internal target volume (ITV) has been widely adopted as simple method to take the motion into account in treatment planning and delivery. The ITV is generated as the union of the CTVs as the patient goes through the respiratory cycle. Many issues have been identified with the ITV. In this session three alternatives for the ITV will be discussed: 1) An alternative motion‐inclusive approach with better imaging and smaller margins, called mid‐position CT. 2) The tracking approach and 3) The gating approach.The following topics will be addressed by Marcel van Herk (“Is ITV the correct motion encompassing strategy”): Magnitude of respiratory motion, effect of motion on radiotherapy, motion encompassing strategies, and software solutions to assist in motion encompassing strategies. Then Paul Keall (“Make margins simple: Use real‐time target tracking”) will discuss tracking with: clinical drivers for tracking, current clinical status of tumor tracking, future tumor tracking technology, and margin margin challenges with and without tracking. Finally Daniel Low will discuss gating (“Gating is the best ITV killer”): why ITV in the first place, requirements for planning, requirements at the machine, benefits and costs.The session will end with a discussion and live demo of motion simulation software to illustrate the issues and explain the relative benefit and appropriate uses for the three methods. Learning Objectives: Explain the 4D imaging and treatment planning process. Summarize the various approaches to deal with respiratory motion during radiotherapy Discuss the tradeoffs involved when choosing one of the three discussed approaches. Explain in which situation each method is the best choiceResearch is partly funded by Elekta Oncology Systems and the Dutch Cancer Foundation; M. van Herk, Part of the research was funded by Elekta Oncology Systems and the Dutch Cancer Foundation

TU‐D‐202‐01: Is ITV the Correct Motion Encompassing Strategy?

Respiratory motion has long been recognized as an important factor affecting the precision of radiotherapy. After the introduction of the 4D CT to visualize the respiratory motion in 3D, the internal target volume (ITV) has been widely adopted as simple method to take the motion into account in treatment planning and delivery. The ITV is generated as the union of the CTVs as the patient goes through the respiratory cycle. Many issues have been identified with the ITV. In this session three alternatives for the ITV will be discussed: 1) An alternative motion‐inclusive approach with better imaging and smaller margins, called mid‐position CT. 2) The tracking approach and 3) The gating approach.The following topics will be addressed by Marcel van Herk (“Is ITV the correct motion encompassing strategy”): Magnitude of respiratory motion, effect of motion on radiotherapy, motion encompassing strategies, and software solutions to assist in motion encompassing strategies. Then Paul Keall (“Make margins simple: Use real‐time target tracking”) will discuss tracking with: clinical drivers for tracking, current clinical status of tumor tracking, future tumor tracking technology, and margin margin challenges with and without tracking. Finally Daniel Low will discuss gating (“Gating is the best ITV killer”): why ITV in the first place, requirements for planning, requirements at the machine, benefits and costs.The session will end with a discussion and live demo of motion simulation software to illustrate the issues and explain the relative benefit and appropriate uses for the three methods. Learning Objectives: Explain the 4D imaging and treatment planning process. Summarize the various approaches to deal with respiratory motion during radiotherapy Discuss the tradeoffs involved when choosing one of the three discussed approaches. Explain in which situation each method is the best choiceResearch is partly funded by Elekta Oncology Systems and the Dutch Cancer Foundation; M. van Herk, Part of the research was funded by Elekta Oncology Systems and the Dutch Cancer Foundation

TU‐D‐202‐00: Rest In Peace ‐ ITV

Respiratory motion has long been recognized as an important factor affecting the precision of radiotherapy. After the introduction of the 4D CT to visualize the respiratory motion in 3D, the internal target volume (ITV) has been widely adopted as simple method to take the motion into account in treatment planning and delivery. The ITV is generated as the union of the CTVs as the patient goes through the respiratory cycle. Many issues have been identified with the ITV. In this session three alternatives for the ITV will be discussed: 1) An alternative motion‐inclusive approach with better imaging and smaller margins, called mid‐position CT. 2) The tracking approach and 3) The gating approach.The following topics will be addressed by Marcel van Herk (“Is ITV the correct motion encompassing strategy”): Magnitude of respiratory motion, effect of motion on radiotherapy, motion encompassing strategies, and software solutions to assist in motion encompassing strategies. Then Paul Keall (“Make margins simple: Use real‐time target tracking”) will discuss tracking with: clinical drivers for tracking, current clinical status of tumor tracking, future tumor tracking technology, and margin margin challenges with and without tracking. Finally Daniel Low will discuss gating (“Gating is the best ITV killer”): why ITV in the first place, requirements for planning, requirements at the machine, benefits and costs.The session will end with a discussion and live demo of motion simulation software to illustrate the issues and explain the relative benefit and appropriate uses for the three methods. Learning Objectives: Explain the 4D imaging and treatment planning process. Summarize the various approaches to deal with respiratory motion during radiotherapy Discuss the tradeoffs involved when choosing one of the three discussed approaches. Explain in which situation each method is the best choiceResearch is partly funded by Elekta Oncology Systems and the Dutch Cancer Foundation; M. van Herk, Part of the research was funded by Elekta Oncology Systems and the Dutch Cancer Foundation

SU‐E‐J‐74: Impact of Respiration‐Correlated Image Quality On Tumor Motion Reconstruction in 4D‐CBCT: A Phantom Study

Purpose:To investigate the effects of scanning parameters and respiratory patterns on the image quality for 4‐dimensional cone‐beam computed tomography(4D‐CBCT) imaging, and assess the accuracy of computed tumor trajectory for lung imaging using registration of phased 4D‐CBCT imaging with treatment planning‐CT.Methods:We simulated a periodic and non‐sinusoidal respirations with various breathing periods and amplitudes using a respiratory phantom(Quasar, Modus Medical Devices Inc) to acquire respiration‐correlated 4D‐CBCT images. 4D‐CBCT scans(Elekta Oncology Systems Ltd) were performed with different scanning parameters for collimation size(e.g., small and medium field‐of‐views) and scanning speed(e.g., slow 50°·min−1, fast 100°·min−1). Using a standard CBCT‐QA phantom(Catphan500, The Phantom Laboratory), the image qualities of all phases in 4D‐CBCT were evaluated with contrast‐to‐noise ratio(CNR) for lung tissue and uniformity in each module. Using a respiratory phantom, the target imaging in 4D‐CBCT was compared to 3D‐CBCT target image. The target trajectory from 10‐respiratory phases in 4D‐CBCT was extracted using an automatic image registration and subsequently assessed the accuracy by comparing with actual motion of the target.Results:Image analysis indicated that a short respiration with a small amplitude resulted in superior CNR and uniformity. Smaller variation of CNR and uniformity was present amongst different respiratory phases. The small field‐of‐view with a partial scan using slow scan can improve CNR, but degraded uniformity. Large amplitude of respiration can degrade image quality. RMS of voxel densities in tumor area of 4D‐CBCT images between sinusoidal and non‐sinusoidal motion exhibited no significant difference. The maximum displacement errors of motion trajectories were less than 1.0 mm and 13.5 mm, for sinusoidal and non‐sinusoidal breathings, respectively. The accuracy of motion reconstruction showed good overall agreement with the 4D‐CBCT image quality results only using sinusoidal breathings.Conclusion:This information can be used to determine the appropriate acquisition parameters of 4D‐CBCT imaging for registration accuracy and target trajectory measurements in a clinical setting.

Impact of a breathing-control system on target margins and normal-tissue sparing in the treatment of lung cancer: experience at the radiotherapy unit of Florence University

In lung cancer, a high radiation dose to the target area correlates with better local control but is frequently counterbalanced by a higher risk of lung toxicity. Several methods exist to coordinate respiratory motion in lung radiotherapy. We aimed to investigate the impact of a breathing-control system on irradiated volumes and dosimetric parameters in three-dimensional conformal radiotherapy (3D-CRT) and stereotactic radiotherapy (SRT) treatments. Twelve patients were scheduled for radical radiotherapy: five for SRT and seven for 3D-CRT. For each patient, in addition to the free-breathing computed tomography (CT) scan, four additional sets of CT slices were acquired using the Active Breathing Coordinator device (ABC, Elekta Oncology Systems Ltd., UK). The volumes acquired with the ABC device were significantly smaller than the free-breathing volumes [23 % reduction of planning tumour volume (PTV), p = 0.002]. ABC allowed a reduction of all dosimetric parameters [2.28 % reduction of percentage volume of lung treated to a dose of ≥ 20 Gy (V20), p = 0.004; 10 % reduction of mean lung dose (MLD), p = 0.009]. Significant differences were found both in SRT and in 3D-CRT, in peripheral and apical lesions. In our experience, ABC has the potential to reduce lung toxicity in the treatment of lung cancer; alternatively, it can allow the prescribed dose to be increased while maintaining the same risk of lung toxicity.

Feasibility study of patient positioning verification in electron beam radiotherapy with an electronic portal imaging device (EPID)

The purpose of this study is to demonstrate the feasibility of verification and documentation in electron beam radiotherapy using the photon contamination detected with an electronic portal imaging device. For investigation of electron beam verification with an EPID, the portal images are acquired irradiating two different tissue equivalent phantoms at different electron energies. Measurements were performed on an Elekta SL 25 linear accelerator with an amorphous-Si electronic portal imaging device (EPID: iViewGT™, Elekta Oncology Systems, Crawley, UK). As a measure of EPID image quality contrast (CR) and signal-to-noise ratio (SNR) are determined. For characterisation of the imaging of the EPID RW3 slabs and a Gammex 467 phantom with different material inserts are used. With increasing electron energy the intensity of photon contamination increases, yielding an increasing signal-to-noise ratio, but images are showing a decreasing contrast. As the signal-to-noise ratio saturates with increasing dose a minimum of 50 MUs is recommended. Even image quality depends on electron energy and diameter of the patient, the acquired results are mostly sufficient to assess the accuracy of beam positioning. In general, the online EPID acquisition has been demonstrated to be an effective electron beam verification and documentation method. The results are showing that this procedure can be recommended to be routinely and reliably done in patient treatment with electron beams.

Extraction of electron beam dose parameters from EBT2 film data scored in a mini phantom

Quality assurance of medical linear accelerators includes dosimetric parameter measurement of therapeutic electron beams e.g. relative dose at a depth of 80% (R₈₀). This parameter must be within a tolerance of 0.2 cm of the declared value. Cumbersome water tank measurements can be regarded as a benchmark to measure electron depth dose curves. A mini-phantom was designed and built, in which a strip of GAFCHROMIC® EBT2 film could be encased tightly for electron beam depth dose measurement. Depth dose data were measured for an ELEKTA Sl25 MLC, ELEKTA Precise, and ELEKTA Synergy (Elekta Oncology Systems, Crawley, UK) machines. The electron beam energy range was between 4 and 22 MeV among the machines. A 10 × 10 cm² electron applicator with 95 cm source-surface-distance was used on all the machines. 24 h after irradiation, the EBT2 film strips were scanned on Canon CanoScan N670U scanner. Afterwards, the data were analysed with in-house developed software that entailed optical density to dose conversion, and optimal fitting of the PDD data to de-noise the raw data. From the PDD data R₈₀ values were solved for and compared with acceptance values. A series of tests were also carried out to validate the use of the scanner for film Dosimetry. These tests are presented in this study. It was found that this method of R₈₀ evaluation was reliable with good agreement with benchmark water tank measurements using a commercial parallel plate ionization chamber as the radiation detector. The EBT2 film data yielded R₈₀ values that were on average 0.06 cm different from benchmark water tank measured R₈₀ values.

Conformal stereotactic radiosurgery therapy: plan evaluation methods and results

The purpose of our study was the objective evaluation of micro-multileaf collimator (mMLC)-based stereotactic radiosurgery treatment plans. Forty-seven patients, 71 lesions received static beam conformal stereotactic radiosurgery treatment in our institute between November 2005 and June 2008. Target volume and organs at risk were outlined on a MRI-CT image fusion basis. BrainSCAN 5.31 system (BrainLAB AG, Heimstetten, Germany) was used for treatment planning, Elekta Presice TS linear accelerator (Elekta Oncology Systems Ltd, Crawley, UK) and BrainLAB m3 mMLC were used for treatment delivery. An invasive head frame, mounted to the treatment table, was used with four screws for patient head fixation. Treatment plans were analysed with objective parameters, such as conformal index (COIN), homogeneity index (HI), coverage index (CI) and healthy tissue relative overdose factor (HTOF) tools. x2 tests were performed between COIN, HI and the geometrical parameters of the target volume (lesion volume - LV, lesion-organ distance - LOD, lesion deformity index - LDI). Mean value of COIN, HI, HTOF and CI was 0.52 (SD 0.13), 1.16 (SD 0.1), 0.88 (SD 0.53), and 0.94 (SD 0.11), respectively. COIN significantly correlated with (p<0.001 in all three cases), while HI was independent of LV, LOD, LDI (p=0.94; 0.14 and 0.72). COIN is similar, HTOF is less than data from the literature. According to our results geometrical parameters of the target volume (size, location, deformation) significantly influence the COIN, but they have no effect on HI.

Quality Assessment of Frameless Fractionated Stereotactic Radiotherapy Using Cone Beam Computed Tomography

A quality assessment of intracranial stereotactic radiotherapy was performed using cone beam computed tomography (CBCT). Setup errors were analyzed for two groups of patients: (1) those who were positioned using a frameless SonArray (FSA) system and immobilized with a bite plate and thermoplastic (TP) mask (the bFSA group); and (2) those who were positioned by room laser and immobilized using a TP mask (the mLAS group). A quality assurance phantom was used to study the system differences between FSA and CBCT. The quality assessment was performed using an Elekta Synergy imager (XVI) (Elekta Oncology Systems, Norcross, GA) and an On-Board Imager (OBI) (Varian Medical Systems, Palo Alto, CA) for 25 patients. For the first three fractions, and weekly thereafter, the FSA system was used for patient positioning, after which CBCT was performed to obtain setup errors. (1) Phantom tests: The mean differences in the isocenter displacements for the two systems was 1.2 ± 0.7 mm. No significant variances were seen between the XVI and OBI units (p~0.208). (2)Patient tests: The mean of the displacements between FSA and CBCT were independent of the CBCT system used; mean setup errors for the bFSA group were smaller (1.2 mm) than those of the mLAS group (3.2 mm) (p < 0.005). For the mLAS patients, the 90th percentile and the maximum rotational displacements were 3° and 5°, respectively. A 4-mm drift in setup accuracy occurred over the treatment course for 1 bFSA patient. System differences of less than 1 mm between CBCT and FSA were seen. Error regression was observed for the bFSA patients, using CBCT (up to 4 mm) during the treatment course. For the mLAS group, daily CBCT imaging was needed to obtain acceptable setup accuracies.

TH-C-201C-04: Breathing Motion and Deformation of Pancreatic Cancer Patients and Its Effect on Planning Dose

Purpose: Generate a deformable model of the pancreas and surrounding anatomy and evaluate the dosimetric impact of respiration motion on the pancreas and the surrounding organs‐at‐risk (OARs) during radiation therapy for pancreatic cancer.Methods and Materials: A multi‐organ biomechanical model‐based deformable image registration platform (MORFEUS) characterize exhale to inhale breathing motion from the 4DCT treatment planningimages of seven pancreatic cancer patients. The deformed pancreas and OARs were compared to their respective volumes contoured on the inhale images using a volume overlap criteria (DICE). The dose delivered to the OARs during free breathing was calculated using MORFEUS and compared to the static planning dose.Results: The DICE using MORFEUS for the pancreas duodenum left and right kidneys was (mean±SD) 0.79±0.06 0.71±0.08 0.89±0.05 and 0.88±0.06 respectively. The effect of breathing on the max and mean dose was small however differences were seen in the dose to a percentage of the volume. For the duodenum 4 patients had an average dose difference of 1.9±0.9 Gy for 60–90% volume (max 4 Gy). Similarly for the stomach 6 patients had an average dose difference of 1.8±1.1 Gy for 20–50% of the volume (3 patients had > 3 Gy). One patient had 1.8 Gy difference to 40% of the bowel volume. Conclusion: A deformable model of the pancreatic canceranatomy was developed and shown to provide good volume overlap. Due to the overlap of the CTV with the OARs the dosimetric effect of breathing on the overall max dose was small however changes of more than 1 Gy were observed in the duodenum stomach and bowel to a percent of the volume which may have clinical significance especially in these patients treated with combined radiosensitizers. Research sponsored by NIH 5RO1CA124714‐02 Elekta Oncology Systems and K. Brock is supported by a Cancer Care Ontario Research Chair.

SU-DD-A4-04: Comparison of Dose Deposition from KV X-Rays during CBCT Scans between Two Commercially Available On-Line Imaging Systems

Purpose: Accurate dosimetry of the additional dose from kV cone‐beam CT(CBCT)image‐guidedradiation therapy(IGRT) is becoming increasingly important for radiationoncologists to make decisions on IGRT frequency in regards to the additional dose to radiosensitive organs, especially skin. In this study, the skindoses and dose profiles during CBCT scans within an anthropomorphic Rando Phantom were compared for two commercially available systems: X‐ray Volumetric Imager (XVI, Elekta Oncology Systems, Crawley, UK) and On‐Board Imager (OBI, Varian Medical Systems, Palo Alto, CA). Method and Materials:CBCT scans were acquired using the XVI (v. 4.2) on an Elekta Synergy linac (Elekta, Crawley, UK). Doses from three clinical scan protocols Head and Neck S20F0, Chest M20F1, and Prostate M10F1 were compared to doses from similar protocols on OBI (v. 1.4) system using the same 2D reference dosimetry protocol that employs XR‐QA GAFCHROMIC™ film. Results: Our results show that the skindose of 4.5 ± 0.2 cGy from the Prostate M10F1 protocol on XVI, compared to 3.7 ± 0.09 cGy from Pelvis protocol on the OBI system, has higher values. Measured skindose from XVI Head and Neck S20F0 protocol ranged from 0.44 ± 0.01 mGy to 1.58 ± 0.03 mGy, which is significantly lower than the dose from OBI Standard and High Quality head protocols. Dose profiles for the three XVI protocols are heavily dependant on gantry angle sweep during the CBCT acquisition.Conclusions: The skindose from the XVI system is higher for the pelvis and thorax, but is much lower for the head CBCT scans compared to OBI protocols. Given the ability to manipulate the preset parameters, lower doses may be achieved with the XVI system by lowering the total mAs for a given scan, while taking into consideration the image quality reduction.

MO‐EE‐A3‐06: First Clinical Results of 3D Verification of Volumetric‐Modulated Arc Therapy (VMAT) of Prostate and Lung Cancer Patients Using EPID Dosimetry

Purpose: To evaluate the first clinical results of 3D verification of volumetric‐modulated arc therapy (VMAT) of prostate and lung cancer patients using EPID dosimetry, both pre‐treatment and in vivo. Method and Materials: EPID images obtained during VMAT delivery using gantry‐angle resolved data acquisition are converted to 3D dose distributions inside a phantom or patient using a back‐projection model. In addition to verification of the dose at the isocentre, 3D gamma analysis is performed to compare planned and EPID‐reconstructed 3D VMAT dose distributions. Gamma evaluation parameters are mean and maximum (1%) gamma and % gamma &lt; 1. Results: VMAT plans of 10 prostate and 10 lung cancer patients, verified by means of 3D EPID dosimetry, have been analysed. The average difference of the dose at the isocentre was for prostate cancer VMAT (−0.2±1.6)% and (+0.4±1.6)%, pre‐treatment and in vivo, respectively, and for the hypofractionated lung cancer VMAT plans (+2.0±1.2)% and (−0.7±2.3)%. 3D gamma evaluation showed almost the same results for pre‐treatment and in vivo verification of the 3D dose distribution for prostate cancer VMAT. For instance, the average of the mean gamma values of all patients is (0.42±0.12)% and (0.48±0.06)% for the pre‐treatment and in vivo results, respectively. Larger deviations have been observed for lung cancer VMAT verification; e.g. the average of the mean gamma values of all patients is (0.41±0.03)% for pre‐treatment verification, and (0.68±0.27)% for the in vivo results. Conclusion: The results of this study show that EPID dosimetry is a valuable tool for 3D verification of VMAT delivery, both pre‐treatment and in vivo. More clinical data are needed to assess acceptance criteria for 3D gamma evaluation of VMAT verification. Conflict of Interest: Research sponsored by Elekta Oncology Systems Corporation.

An organ and effective dose study of XVI and OBI cone‐beam CT systems

The main purpose of this work was to quantify patient organ doses from the two kilovoltage cone beam computed tomography (CBCT) systems currently available on medical linear accelerators, namely the X‐ray Volumetric Imager (XVI, Elekta Oncology Systems) and the On‐Board Imager (OBI, Varian Medical Systems). Organ dose measurements were performed using a fiber‐optic coupled (FOC) dosimetry system along with an adult male anthropomorphic phantom for three different clinically relevant scan sites: head, chest, and pelvis. The FOC dosimeter was previously characterized at diagnostic energies by Hyer et al. [Med Phys 2009;36(5):1711–16] and a total uncertainty of approximately 4% was found for in‐phantom dose measurements. All scans were performed using current manufacturer‐installed clinical protocols and appropriate bow‐tie filters. A comparison of image quality between these manufacturer‐installed protocols was also performed using a Catphan 440 image quality phantom. Results indicated that for the XVI, the dose to the lens of the eye (1.07 mGy) was highest in a head scan, thyroid dose (19.24 mGy) was highest in a chest scan, and gonad dose (29 mGy) was highest in a pelvis scan. For the OBI, brain dose (3.01 mGy) was highest in a head scan, breast dose (5.34 mGy) was highest in a chest scan, and gonad dose (34.61 mGy) was highest in a pelvis scan. Image quality measurements demonstrated that the OBI provided superior image quality for all protocols, with both better spatial resolution and low‐contrast detectability. The measured organ doses were also used to calculate a reference male effective dose to allow further comparison of the two machines and imaging protocols. The head, chest, and pelvis scans yielded effective doses of 0.04, 7.15, and 3.73 mSv for the XVI, and 0.12, 1.82, and 4.34 mSv for the OBI, respectively.PACS number: 87.57.uq

SU-FF-I-96: Motion Adaptation for Registering Daily Online CBCT Images to Planning CT Images

Purpose: Perform online image guidance (IGRT) and motion assessment using deformable registration by adapting patient liver motion models. Materials and Methods: 4DCT and daily 4DCBCT images were obtained for 15 patients treated under a stereotactic‐body radiotherapy protocol. Finite element models of patient liver respiratory motion were generated using a biomechanical‐model based platform, MORFEUS from 4DCT images. Motion was assessed using a Navigator channels (NC) technique. Five NCs, rectangular regions of interest, were placed on two images at corresponding spatial locations to define left (LR), anterior‐posterior (AP), and superior‐inferior (SI) liver edges. To determine interfraction liver/tumour positioning, the 4DCT exhale and daily 4DCBCT exhale images were used, while intrafraction liver/ tumour motion used exhale and inhale 4DCBCT images. The NC determines the 1D displacement between both images. This 1D motion adapts the MORFEUS‐based liver motion model. NC adaptation accuracy was evaluated by comparing the results with MORFEUS registration. Results: NC refinement has been performed on 8 patients to date in less than 2 mins each. Inter‐fraction motion of the liver exceeded 1 cm. Inter‐fraction NC technique accuracy (absolute mean (SD)) was 0.15 (0.11), 0.16 (0.10), and 0.15 (0.09) cm, in the LR, AP, and SI directions, respectively. Intra‐fraction motion exceeded 1.5cm. NC technique accuracy was 0.14 (0.10), 0.15 (0.08), and 0.18 (0.09) cm, in the LR, AP, and SI directions, respectively. The accuracy of the NC technique to identify the center of mass shift of the tumour for intra‐fraction motion was 0.16 (0.15), 0.14 (0.12), and 0.15 (0.10) cm, in the LR, AP, and SI directions, respectively. Conclusion: NC adaptation technique can accurately and efficiently integrate deformable registration into the IGRT process to account for inter and intra‐fraction motion. Research sponsored by the National Cancer Institute of Canada‐Terry Fox Foundation, Elekta Oncology Systems, and NIH 5RO1CA124714‐02.

SU‐FF‐I‐13: Organ Doses From Cone‐Beam CT in Radiation Therapy

Purpose: Daily use of cone‐beam computed tomography (CBCT) for patient position verification can lead to a substantial dose to a large volume of normal tissue outside of the planned target volume. The purpose of this work is to quantify this dose for the two kilovoltage CBCT systems currently available on medical linear accelerators, namely the X‐ray Volumetric Imager (XVI®, Elekta Oncology Systems) and the On‐Board Imager (OBI®, Varian Medical Systems). Method and Materials: An adult male anthropomorphic phantom was constructed from three tissue equivalent materials representing lung, bone, and soft tissue. To perform dose measurements within the phantom, a new fiber optic coupled (FOC) dosimeter was also developed and characterized. A total of three clinically relevant scan sites were investigated for each machine: head and neck, chest, and pelvis. For each scan site, the dose to a total of 25 organs was measured based on recommendations by the International Commission on Radiological Protection (ICRP) Publication 103 for the evaluation of effective dose. Results: The dose to organs inside the CBCT scan volume was found to be as high as 30 mGy. Due to the large field‐of‐view (FOV) of CBCT and associated radiation scatter, a substantial dose to organs outside of the scan volume was also observed and in some cases this dose was nearly 50% of the dose to organs inside of the scan volume. Conclusions: Results indicate that daily CBCT imaging in a high fraction therapy regimen can result in a substantial dose to organs well outside the planned target volume and should be taken into consideration during the treatment planning and delivery process.

An empirical method for lag correction in cone‐beam CT

Image lag degrades image quality in cone-beam CT (CBCT), resulting in contrast reduction, lack of CT number accuracy and uniformity, and skin-line artifacts. The magnitude of such degradation and the extent to which imaging performance can be improved by means of a lag correction method were investigated. Measurements were performed using a radiotherapy CBCT guidance system (Elekta Synergy XVI, Elekta Oncology Systems, Atlanta, GA), for which the imaging system is based upon a RID1640-AL1 flat-panel imager (Perkin Elmer, Wiesbaden, Germany). Image lag and its relationship to various parameters including signal magnitude, photon energy, and frame number were investigated, and an empirical lag correction method was developed to manage lag artifacts. The correction method was simply the subtraction from the current frame by previous frames weighted by the temporal response function. The CatPhan 500 phantom (The Phantom Laboratory, Salem, NY) within an irregularly shaped body annulus was used to demonstrate the magnitude of artifacts with and without lag correction. CBCT images after correction demonstrated improvement in skin-line reconstruction, CT number accuracy, image uniformity, and contrast-to-noise ratio. Lag artifacts can be reduced by means of algorithmic correction of the projection images. Lag correction is most important for all shapes of objects having contrast inserts. For circular/cylindrical objects, lag correction does not improve the skin-line artifact but can improve low contrast visibility adjacent to high contrast objects.