External Beam Treatment Planning Research Articles

Clinical feasibility of a commercially available MRI-only method for radiotherapy treatment planning of the brain.

Advancements in deep-learning based synthetic computed tomography (sCT) image conversion methods have enabled the development of magnetic resonance imaging (MRI)-only based radiotherapy treatment planning (RTP) of the brain. This study evaluates the clinical feasibility of a commercial, deep-learning based MRI-only RTP method with respect to dose calculation and patient positioning verification performance in RTP of the brain. Clinical validation of dose calculation accuracy was performed by a retrospective evaluation for 25 glioma and 25 brain metastasis patients. Dosimetric and image quality of the studied MRI-only RTP method was evaluated by a direct comparison of the sCT-based and computed tomography (CT)-based external beam radiation therapy (EBRT) images and treatment plans. Patient positioning verification accuracy of sCT images was evaluated retrospectively for 10 glioma and 10 brain metastasis patients based on clinical cone-beam computed tomography (CBCT) imaging. An average mean dose difference of Dmean = 0.1% for planning target volume (PTV) and 0.6% for normal tissue (NT) structures were obtained for glioma patients. Respective results for brain metastasis patients were Dmean = 0.5% for PTVs and Dmean =1.0% for NTs. Global three-dimensional (3D) gamma pass rates using 2%/2 mm dose difference and distance-to-agreement (DTA) criterion were 98.0% for the glioma subgroup, and 95.2% for the brain metastasis subgroup using 1%/1 mm criterion. Mean distance differences of <1.0 mm were observed in all Cartesian directions between CT-based and sCT-based CBCT patient positioning in both subgroups. In terms of dose calculation and patient positioning accuracy, the studied MRI-only method demonstrated its clinical feasibility for RTP of the brain. The results encourage the use of the studied method as part of a routine clinical workflow.

Clinical Evaluation of an Automated Treatment Planning Framework for Radiation Oncology

<h3>Purpose/Objective(s)</h3> Automation of radiotherapy treatment planning increases consistency, provides a safety net around error prevention, and reduces planning time. The objective of this study was to validate a scalable framework to automate the creation of VMAT and IMRT treatment plans using commercial planning systems. <h3>Materials/Methods</h3> A total of 35 centers across Australia were used for the study and the disease sites consisted of Urinogenital, Lung, Breast, Head and Neck, Neurological, GI and Gynecological. A scalable framework for external beam treatment planning was implemented for two commercial treatment planning systems (TPS), Eclipse (Varian Medical Systems, Palo Alto, CA) and Monaco (Elekta AB, Stockholm, Sweden). The framework received CT images, target volumes, normal tissue structures, and prescription data from dosimetrists. Information was parsed into the system, validated against configuration settings, the desired treatment protocol template selected, optimization parameters adjusted, and dose calculation completed. The framework controlled the TPS through the Application Programming Interface (API). The dosimetry team evaluated each automated plan by comparison of the plan metrics to the prescription and assessment of the dose distribution. The plans were scored in three areas: 1) clinician approved with no intervention, 2) minimal intervention with minor adjustments, and 3) manual replanning. <h3>Results</h3> Since Aug. 2021, 4972 plans (36% of total plans) have been successfully processed across 35 treatment centers with 111 clinicians. 487 plans were rejected by the framework due to error. The system automated plans for GUI (1822), Lung (629), Breast (745), Head and Neck (65), Neurological (96), GI (568) and Gynecological (163). Out of these plans, 23% were clinician approved with no user intervention, 41% required minimal intervention to achieve clinician approval, and 36% required additional optimization structures and/or cycles of optimization to achieve clinician approval. The average time between clinician target volume delineation and plan approval reduced by 25% from 1.6 to 1.2 days. In Feb. 2022, the framework successfully processed 970 plans (41% of total plans), rejecting 126 (5%) due to error. Evaluation between systems showed 27% of Eclipse plans and 43% of Monaco plans were processed successfully. Of these plans, 19% (Eclipse) and 23% (Monaco) required no intervention while 25% (Eclipse) and 46% (Monaco) required minimal intervention. Analysis of dosimetrist productivity showed an increase of 16% since Aug. 2021 while the average time saved per plan was 85 minutes. Analysis of adverse and near- miss dosimetry related events showed a 6% reduction in reported incidents. <h3>Conclusion</h3> The framework for automated planning achieved a 60% clinician approval rate for plans with minimal or no intervention. Automation can result in more consistent planning and timesaving's for expert operators. A 6% reduction in recorded dosimetry related errors was achieved.

MRI-only brain radiotherapy: Assessing the dosimetric accuracy of synthetic CT images generated using a deep learning approach

PurposeThis study assessed the dosimetric accuracy of synthetic CT images generated from magnetic resonance imaging (MRI) data for focal brain radiation therapy, using a deep learning approach. Material and methodsWe conducted a study in 77 patients with brain tumors who had undergone both MRI and computed tomography (CT) imaging as part of their simulation for external beam treatment planning. We designed a generative adversarial network (GAN) to generate synthetic CT images from MRI images. We used Mutual Information (MI) as the loss function in the generator to overcome the misalignment between MRI and CT images (unregistered data). The model was trained using all MRI slices with corresponding CT slices from each training subject’s MRI/CT pair. ResultsThe proposed GAN method produced an average mean absolute error (MAE) of 47.2 ± 11.0 HU over 5-fold cross validation. The overall mean Dice similarity coefficient between CT and synthetic CT images was 80% ± 6% in bone for all test data. Though training a GAN model may take several hours, the model only needs to be trained once. Generating a complete synthetic CT volume for each new patient MRI volume using a trained GAN model took only one second. ConclusionsThe GAN model we developed produced highly accurate synthetic CT images from conventional, single-sequence MRI images in seconds. Our proposed method has strong potential to perform well in a clinical workflow for MRI-only brain treatment planning.

Stereotactic radiosurgery with MLC-defined arcs: Verification of dosimetry, spatial accuracy, and end-to-end tests.

PurposeTo measure dosimetric and spatial accuracy of stereotactic radiosurgery (SRS) delivered to targets as small as the trigeminal nerve (TN) using a standard external beam treatment planning system (TPS) and multileaf collimator‐(MLC) equipped linear accelerator without cones or other special attachments or modifications.MethodsDosimetric performance was assessed by comparing computed dose distributions to film measurements. Comparisons included the γ‐index, beam profiles, isodose lines, maximum dose, and spatial accuracy. Initially, single static 360° arcs of MLC‐shaped fields ranging from 1.6 × 5 to 30 × 30 mm2 were planned and delivered to an in‐house built block phantom having approximate dimensions of a human head. The phantom was equipped with markings that allowed accurate setup using planar kV images. Couch walkout during multiple‐arc treatments was investigated by tracking a ball pointer, initially positioned at cone beam computed tomography (CBCT) isocenter, as the couch was rotated. Tracks were mapped with no load and a 90 kg stack of plastic plates simulating patient treatment. The dosimetric effect of walkout was assessed computationally by comparing test plans that corrected for walkout to plans that neglected walkout. The plans involved nine 160° arcs of 2.4 × 5 mm2 fields applied at six different couch angles. For end‐to‐end tests that included CT simulation, target contouring, planning, and delivery, a cylindrical phantom mimicking a 3 mm lesion was constructed and irradiated with the nine‐arc regimen. The phantom, lacking markings as setup aids was positioned under CBCT guidance by registering its surface and internal structures with CTs from simulation. Radiochromic film passing through the target center was inserted parallel to the coronal and the sagittal plane for assessment of spatial and dosimetric accuracy.ResultsIn the single‐arc block phantom tests computed maximum doses of all field sizes agreed with measurements within 2.4 ± 2.0%. Profile widths at 50% maximum agreed within 0.2 mm. The largest targeting error was 0.33 mm. The γ‐index (3%, 1 mm) averaged over 10 experiments was >1 in only 1% of pixels for field sizes up to 10 × 10 mm2 and rose to 4.4% as field size increased to 20 × 20 mm2. Table walkout was not affected by load. Walkout shifted the target up to 0.6 mm from CBCT isocenter but, according to computations shifted the dose cloud of the nine‐arc plan by only 0.16 mm. Film measurements verified the small dosimetric effect of walkout, allowing walkout to be neglected during planning and treatment. In the end‐to‐end tests average and maximum targeting errors were 0.30 ± 0.10 and 0.43 mm, respectively. Gamma analysis of coronal and sagittal dose distributions based on a 3%/0.3 mm agreement remained <1 at all pixels. To date, more than 50 functional SRS treatments using MLC‐shaped static field arcs have been delivered.ConclusionStereotactic radiosurgery (SRS) can be planned and delivered on a standard linac without cones or other modifications with better than 0.5 mm spatial and 5% dosimetric accuracy.

Adding Value to Physician Peer Review of External Beam Treatment Plans

Radiation Oncology professional societies recommend individual physician peer review as part of a Quality Improvement Program. The American College of Radiology (ACR) recommends that case-specific peer review should be performed by another radiation oncologist(s) on a weekly basis. (1) The review should include at a minimum a presentation of new patients and give attention to indications for radiation therapy, target definition, and dose. The American Association of Physicists in Medicine (AAPM) also recommends new patient planning conferences attended by radiation oncologists, radiation therapists, dosimetrists and medical physicists. (2) These standards are also used for program accreditation. (3,4,5) The objective of this study is to report findings of contour, field and plan review as part of weekly physician peer review conference. As part of our quality improvement program, contour, field and plan review was included in our weekly physician peer review conferences. Each external beam (EBRT) case was independently reviewed by a staff radiation oncologist in a conference format with radiation oncologists, medical physicist(s), dosimetrist(s), radiation therapist(s) and radiation oncology nurse(s) in attendance. In most cases, contours and fields were reviewed after simulation and before the final treatment plan. Treatment plans were reviewed before the first treatment or during the first week of treatment. During 2017, 205 EBRT cases (52% IMRT, 8% SBRT, 40% other) were reviewed. Seventy-four (36.1%) cases received comments from the reviewing radiation oncologist. In 44.6% of the cases, the comments were given only during contour/field review, 41.9% received comments only during plan review and 13.5% during both contour/field and plan reviews. Comments resulted in contours modified for 29 cases, fields modified for 6 cases, and dose or fractionation modified for 7 cases. Only 10 (4.9%) of all EBRT cases reviewed needed a new plan. Eight out of 10 of these cases were IMRT, which required more time and effort resulting in a possible delay in treatment. Twenty-seven cases had comments that did not result in any change, but added value to the case. A physician peer review of all EBRT patients was found to be very useful in providing quality care to our patients with minimal time commitment from the radiation oncologists. Having the contour/field review after simulation and before the treatment plan is completed, has resulted in <5% replan rate for all EBRT cases. If only plan review was incorporated, then an estimated 20% of the EBRT treatment plans would be suboptimal and in need of a change once the patient starts treatment.

RayStation: External beam treatment planning system

RaySearch Laboratories is a world leader in the field of advanced software and creator of the RayStation treatment planning system for radiation therapy. The aim with RayStation is to deliver an unmatched user experience and leading functionality. Unique features described here include multiatlas based autosegmentation for contouring, deformable registration with 2 different algorithms, multicriteria optimization, Plan Explorer, fallback planning, ultrafast computation speed, and 4-dimensional (4D) adaptive radiation therapy. RayStation can be used to plan for electrons and photons on traditional linacs, for protons on various delivery systems, and for Accuray's helical TomoTherapy system. This paper describes some of these modalities, with reference to clinical cases and including descriptions of the impact on workflow.

Hendee's Radiation Therapy Physics, 4th ed. TPawlicki, DJScanderbeg &amp; GStarkschall. New Jersey: Wiley‐Blackwell, 2016. Hardcover 352. pp. Price: $159.95. ISBN: 978‐0‐470‐37651‐5.

This is the 4th edition of a book on the physics of radiation therapy that covers basic principles and new technologies in the field and how these apply to the clinical practice. It is a well-referenced and easy to read text. The text is designed as an alternative or supplement to other well-established books in the field of radiotherapy physics. In addition, the authors have made an extra effort to provide a concise summary of the objectives for every chapter. They even set aside a chapter on patient safety and quality improvement. This article is protected by copyright. All rights reserved.

SU‐F‐T‐527: A Novel Dynamic Multileaf Collimator Leaf‐Sequencing Algorithm in Radiation Therapy

Purpose:A novel leaf‐sequencing algorithm is developed for generating arbitrary beam intensity profiles in discrete levels using dynamic multileaf collimator (MLC). The efficiency of this dynamic MLC leaf‐sequencing method was evaluated using external beam treatment plans delivered by intensity modulated radiation therapy technique.Methods:To qualify and validate this algorithm, integral test for the beam segment of MLC generated by the CORVUS treatment planning system was performed with clinical intensity map experiments. The treatment plans were optimized and the fluence maps for all photon beams were determined. This algorithm started with the algebraic expression for the area under the beam profile. The coefficients in the expression can be transformed into the specifications for the leaf‐setting sequence. The leaf optimization procedure was then applied and analyzed for clinical relevant intensity profiles in cancer treatment.Results:The macrophysical effect of this method can be described by volumetric plan evaluation tools such as dose‐volume histograms (DVHs). The DVH results are in good agreement compared to those from the CORVUS treatment planning system.Conclusion:We developed a dynamic MLC method to examine the stability of leaf speed including effects of acceleration and deceleration of leaf motion in order to make sure the stability of leaf speed did not affect the intensity profile generated. It was found that the mechanical requirements were better satisfied using this method.The Project is sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

Evolution of Dose Calculation Algorithms for External Beam Treatment Planning.

Evolution of Dose Calculation Algorithms for External Beam Treatment Planning.

Assessing the Dosimetric Accuracy of Magnetic Resonance-Generated Synthetic CT Images for Focal Brain VMAT Radiation Therapy.

The purpose of this study was to assess the dosimetric accuracy of synthetic CT (MRCT) volumes generated from magnetic resonance imaging (MRI) data for focal brain radiation therapy. A study was conducted in 12 patients with gliomas who underwent both MR and CT imaging as part of their simulation for external beam treatment planning. MRCT volumes were generated from MR images. Patients' clinical treatment planning directives were used to create 12 individual volumetric modulated arc therapy (VMAT) plans, which were then optimized 10 times on each of their respective CT and MRCT-derived electron density maps. Dose metrics derived from optimization criteria, as well as monitor units and gamma analyses, were evaluated to quantify differences between the imaging modalities. Mean differences between planning target volume (PTV) doses on MRCT and CT plans across all patients were 0.0% (range: -0.1 to 0.2%) for D(95%); 0.0% (-0.7 to 0.6%) for D(5%); and -0.2% (-1.0 to 0.2%) for D(max). MRCT plans showed no significant changes in monitor units (-0.4%) compared to CT plans. Organs at risk (OARs) had average D(max) differences of 0.0 Gy (-2.2 to 1.9 Gy) over 85 structures across all 12 patients, with no significant differences when calculated doses approached planning constraints. Focal brain VMAT plans optimized on MRCT images show excellent dosimetric agreement with standard CT-optimized plans. PTVs show equivalent coverage, and OARs do not show any overdose. These results indicate that MRI-derived synthetic CT volumes can be used to support treatment planning of most patients treated for intracranial lesions.

SU‐E‐T‐605: Performance Evaluation of MLC Leaf‐Sequencing Algorithms in Head‐And‐Neck IMRT

Purpose:To investigate the efficiency of three multileaf collimator (MLC) leaf‐sequencing algorithms proposed by Galvin et al, Chen et al and Siochi et al using external beam treatment plans for head‐and‐neck intensity modulated radiation therapy (IMRT).Methods:IMRT plans for head‐and‐neck were created using the CORVUS treatment planning system. The plans were optimized and the fluence maps for all photon beams determined. Three different MLC leaf‐sequencing algorithms based on Galvin et al, Chen et al and Siochi et al were used to calculate the final photon segmental fields and their monitor units in delivery. For comparison purpose, the maximum intensity of fluence map was kept constant in different plans. The number of beam segments and total number of monitor units were calculated for the three algorithms.Results:From results of number of beam segments and total number of monitor units, we found that algorithm of Galvin et al had the largest number of monitor unit which was about 70% larger than the other two algorithms. Moreover, both algorithms of Galvin et al and Siochi et al have relatively lower number of beam segment compared to Chen et al. Although values of number of beam segment and total number of monitor unit calculated by different algorithms varied with the head‐and‐neck plans, it can be seen that algorithms of Galvin et al and Siochi et al performed well with a lower number of beam segment, though algorithm of Galvin et al had a larger total number of monitor units than Siochi et al.Conclusion:Although performance of the leaf‐sequencing algorithm varied with different IMRT plans having different fluence maps, an evaluation is possible based on the calculated number of beam segment and monitor unit. In this study, algorithm by Siochi et al was found to be more efficient in the head‐and‐neck IMRT.The Project Sponsored by the Fundamental Research Funds for the Central Universities (J2014HGXJ0094) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

Comparing the Use of the Full Dose-Volume Histogram (DVH) Curves With Point Dosimetric Indices to Characterize External Beam Treatment Plans for Endometrial Cancer

Comparing the Use of the Full Dose-Volume Histogram (DVH) Curves With Point Dosimetric Indices to Characterize External Beam Treatment Plans for Endometrial Cancer

WE-G-16A-01: Evolution of Radiation Treatment Planning

WE-G-16A-01: Evolution of Radiation Treatment Planning

SU-E-T-577: Commissioning of a Deterministic Algorithm for External Photon Beams

Purpose: We report commissioning results for a deterministic algorithm for external photon beam treatment planning. A deterministic algorithm solves the radiation transport equations directly using a finite difference method, thus improve the accuracy of dose calculation, particularly under heterogeneous conditions with results similar to that of Monte Carlo (MC) simulation. Methods: Commissioning data for photon energies 6 – 15 MV includes the percentage depth dose (PDD) measured at SSD = 90 cm and output ratio in water (Spc), both normalized to 10 cm depth, for field sizes between 2 and 40 cm and depths between 0 and 40 cm. Off-axis ratio (OAR) for the same set of field sizes was used at 5 depths (dmax, 5, 10, 20, 30 cm). The final model was compared with the commissioning data as well as additional benchmark data. The benchmark data includes dose per MU determined for 17 points for SSD between 80 and 110 cm, depth between 5 and 20 cm, and lateral offset of up to 16.5 cm. Relative comparisons were made in a heterogeneous phantom made of cork and solid water. Results: Compared to the commissioning beam data, the agreement are generally better than 2% with large errors (up to 13%) observed in the buildup regions of the FDD and penumbra regions of the OAR profiles. The overall mean standard deviation is 0.04% when all data are taken into account. Compared to the benchmark data, the agreements are generally better than 2%. Relative comparison in heterogeneous phantom is in general better than 4%. Conclusion: A commercial deterministic algorithm was commissioned for megavoltage photon beams. In a homogeneous medium, the agreement between the algorithm and measurement at the benchmark points is generally better than 2%. The dose accuracy for a deterministic algorithm is better than a convolution algorithm in heterogeneous medium.

SU‐E‐T‐596: A Comprehensive Comparison of Plan Quality: Non‐Coplanar IMRT Vs. RapidArc and Conventional IMRT

Purpose: Careful selection of beam number and orientation is of critical importance for external beam treatment planning. The utilization of well‐chosen non‐coplanar beams could allow the generation of high‐quality plans. This study is to evaluate the quality of the IMRT plans generated with the non‐coplanar (NCP) beams selected with our in‐house program by comparing it with coplanar (CP) IMRT and RapidArc (ARC) plans for prostate and pancreatic cancer treatments. Methods: Ten patients with prostate and pancreas cancer treated previously were selected for this study. A set of 13 non‐coplanar beams was selected with our in‐house program based on the CT data of one patient and was used to generate NCP plans for the ten cases. For each case we also generated an eight‐beam CP and a double‐arc ARC plan. All plans were generated with the Varian Eclipse TPS. For individual cases, the same dose‐volume constraints and weightings were used for all three plans. Results: All plans provided conformal and complete converge of the PTV and led to clinically acceptable doses to the organs‐at‐risk. The NCP plans show the best quality in terms of PTV Dmean, Dmax and HI, while the ARC plans are best in terms of PTV Dmin and CI. For the prostate cases, the NCP plans show the best V40 and V60 for rectum and bladder, and best D2% for the right‐femoral head, while the ARC plans show the best left‐femoral head D2%. For the pancreas cases, the NCP plans show the best sparing of the left‐and right‐kidney as well as the spinal cord. However, the NCP plans lead to slightly worse Dmean and D1/3 to the liver than the CP plans. Conclusion: We have shown that the non‐coplanar beams selected with our program allow the generation of high‐quality IMRT plans, by comparing them with the conventional IMRT and RapidArc plans.

Improved dose–volume histogram estimates for radiopharmaceutical therapy by optimizing quantitative SPECT reconstruction parameters

In radiopharmaceutical therapy, an understanding of the dose distribution in normal and target tissues is important for optimizing treatment. Three-dimensional (3D) dosimetry takes into account patient anatomy and the nonuniform uptake of radiopharmaceuticals in tissues. Dose–volume histograms (DVHs) provide a useful summary representation of the 3D dose distribution and have been widely used for external beam treatment planning. Reliable 3D dosimetry requires an accurate 3D radioactivity distribution as the input. However, activity distribution estimates from SPECT are corrupted by noise and partial volume effects (PVEs). In this work, we systematically investigated OS-EM based quantitative SPECT (QSPECT) image reconstruction in terms of its effect on DVHs estimates. A modified 3D NURBS-based Cardiac-Torso (NCAT) phantom that incorporated a non-uniform kidney model and clinically realistic organ activities and biokinetics was used. Projections were generated using a Monte Carlo (MC) simulation; noise effects were studied using 50 noise realizations with clinical count levels. Activity images were reconstructed using QSPECT with compensation for attenuation, scatter and collimator–detector response (CDR). Dose rate distributions were estimated by convolution of the activity image with a voxel S kernel. Cumulative DVHs were calculated from the phantom and QSPECT images and compared both qualitatively and quantitatively. We found that noise, PVEs, and ringing artifacts due to CDR compensation all degraded histogram estimates. Low-pass filtering and early termination of the iterative process were needed to reduce the effects of noise and ringing artifacts on DVHs, but resulted in increased degradations due to PVEs. Large objects with few features, such as the liver, had more accurate histogram estimates and required fewer iterations and more smoothing for optimal results. Smaller objects with fine details, such as the kidneys, required more iterations and less smoothing at early time points post-radiopharmaceutical administration but more smoothing and fewer iterations at later time points when the total organ activity was lower. The results of this study demonstrate the importance of using optimal reconstruction and regularization parameters. Optimal results were obtained with different parameters at each time point, but using a single set of parameters for all time points produced near-optimal dose–volume histograms.

Measured vs simulated portal images for low MU fields on three accelerator types: Possible consequences for 2D portal dosimetry

As external beam treatment plans become more dynamic and the dose to normal tissue is further constrained, treatments may consist of a larger number of beams, each delivering smaller doses (or monitor units, MU), in, e.g., volumetric modulated arc therapy (VMAT). Electronic portal imaging devices (EPID) may be used to verify external beam treatments on integrated fractions as well as in a more time dependent manner such as field by field. For treatment verification performed during a fraction (e.g., individual fields or VMAT control points), the lower limit of EPID measurement capability becomes important. The authors quantified the signal and timing accuracy of EPID images for low MU intensity modulated radiotherapy (IMRT) and conformal fields. EPID images were collected from three different vendor's accelerators for low MU fields and compared to expected images. Simulations were performed to replicate the EPID acquisition pattern and to enhance the understanding of EPID readout schemes. Large discrepancies between observed and predicted images were noted due to an under-response to single low MU fields. It is shown that a variability of up to 37% can be observed for low MU fields in clinically used EPID acquisition modes and that the majority of this variability can be accounted for by the readout scheme, integration, and timing of EPID acquisitions. Simulations have confirmed the causes of the discrepancies. The occurrence and extent of the variation has been estimated for clinical settings. Incorrect absolute EPID signals collected for low MU fields in external beam treatments will negatively affect quantitative applications such as individual field based EPID dosimetry, typically appearing as an underdose, unless corrections to currently employed EPID readout schemes are made.

SU-E-I-13: Evaluation of CT Number with Changing Scan Parameters for Nucletron Simulix Evolution CBCT.

To quantify the variation in CT number generated by the Simulix Evolution CBCT with changes in scan length and phantom thickness. Three phantoms were used in this study: CIRS Model 610 AAPM CT Phantom, Gammex 467 Tissue Characterization Phantom, and Catphan 600 phantom. The AAPM Phantom was used to assess the variation of HU with phantom thickness. Scans were acquired with two field size settings (full- and half-beam) with and without a 3.5 cm thick ring. The Catphan and Gammex phantoms were used to assess the Simulix's capability of producing a consistent CT-to-ED conversion table with different scan lengths, ranging from 1 cm (very thin) to 20 cm (clinical use). The data were also compared to data acquired with our in-house CT Sim (GE HiLite LightSpeed 16 slice). The AAPM phantom scans with and without the ring yielded an average difference in HU of 145 HU (full-beam) and 74 HU (half-beam) for each of five inserts. The HU for Cortical Bone (SB3) [largest Gammex electron density insert; 1.69] ranged from 923 to 1170 HU for the 4 cm and 1 cm scan lengths, respectively. The HU for Teflon [largest Catphan electron density insert; 1.867] ranged from 657 to 951 for the 20 cm and 1 cm scan lengths, respectively. The HU for air in Catphan ranged from -749 to -905, and the HU for LDPE [electron density 0.944] ranged from -82 to -42, for the 20 cm and 1 cm scan lengths, respectively. Results show a large variability in the calculated CT number with differences in phantom thickness, as evidenced by the results with the AAPM phantom. In addition, there appears to be a dependence on scan length, attributed to increased scatter contribution. Further tests will be done to evaluate the appropriateness of the use of the Simulix CBCT unit for heterogeneity corrected external beam treatment planning. The author has received no funding during the course of this research.

WE‐C‐214‐06: Patient Safety Improvement through Incident Learning in Radiation Treatment ‐ Four Years Experience

Purpose: To describe results from our comprehensive incident learning system gained over 4 years. Methods: An incident learning system specifically tailored to a radiation treatment program and based on published principles was implemented in our large academic cancer centre in late 2006. In the adopted system, every incident, whether or not there is a resulting direct impact on a patient treatment, is recorded, investigated to determine basic causes and strategies developed to correct system weaknesses. Our centre operates 10 megavolt accelerators and treats nearly 4,000 patients per year. Incidents are categorized into one of 5 types and classified according to 4 levels of severity. Interventions introduced as part of the learning from these reports have included changes in staffing levels in more vulnerable areas and several human factor error reduction strategies. Results: Our data are derived from a total of 2,221 incidents reported during the four years of operation, with 98.1% classified as either minor or potential. Of the actual incidents reported, we have seen a significant decline from 117 in 2007 to 84, 62 and 52 in the subsequent three years (chi‐squared non‐parametric test). The actual incident rate at the point of treatment delivery has decreased from 0.095% in 2006 to 0.022% in 2010. During this period, the system provided rapid feedback to monitor several new initiatives including a new external beam treatment planning system, three different types of image guidance hardware and software and several new techniques such as stereotactic body radiosurgery and volumetric modulated arc therapy. Conclusions: We report a sustained significant increase in the safety of treatment through the use of root cause analysis. Our results indicate that an effective incident learning system implementation will strongly encourage the reporting of potential incidents as a proactive means of enhancing safety and quality.

Novel technique for simulation and external beam treatment planning for obese patients

PurposeIn computed tomography (CT)-guided radiation treatment planning, an accurate assessment of a patient's external contour is needed to determine the number of monitor units appropriate to treat a target volume to the prescribed dose. When obese patients are imaged, even using a large-bore CT scanner, lateral tissues frequently fall outside the field of view of the CT scanner, which makes it difficult to accurately calculate the dose needed for lateral or oblique treatment fields. We aimed to develop a technique to capture the external contour of patients in whom traditional simulation techniques would fail to capture part of the anatomy. Methods and MaterialsThe following technique was used in multiple patients. The patient was centered on the CT couch, and an isocenter was placed. The initial scan was obtained with the isocenter at the central position on the table. Two subsequent scans were obtained with the patient first shifted right on the table and then shifted left. The 3 CT scans were fused using a CT-CT cross-correlation algorithm. The accuracy of the scans was reviewed manually. Using the fused scans, lateral tissues were contoured on the primary scan and assigned a density of 1 g/cm3. ResultsWith this technique, each patient's entire body contour was available on a single fused scan. The availability of the entire body contour on the scan allowed for treatment planning with a 360-degree freedom for external beam placement. ConclusionsThis novel approach of fusing multiple CT scans allows for the external anatomy of obese patients to be completely captured. The increase in available beam angles made possible by this approach can optimize external beam treatment planning, including planning for intensity-modulated radiation therapy.