QA Checks Research Articles

Dose-response dependencies of OSL dosimeters in conventional linacs and 1.5T MR-linacs: an experimental and Monte Carlo study

Objective. This work focuses on the optically stimulated luminescence dosimetry (OSLD) dose–response characterization, with emphasis on 1.5T MR-Linacs. Approach. Throughout this study, the nanoDots OSLDs (Landauer, USA) were considered. In groups of three, the mean OSLD response was measured in a conventional linac and an MR-Linac under various irradiation conditions to investigate (i) dose–response linearity with and without the 1.5T magnetic field, (ii) signal fading rate and its dependencies, (iii) beam quality, detector orientation and dose rate dependencies in a conventional linac, (iii) potential MR imaging related effects on OSLD response and (iv) detector orientation dependence in an MR-Linac. Monte Carlo calculations were performed to further quantify angular dependence after rotating the detector around its central axis parallel to the magnetic field, and determine the magnetic field correction factors, for all cardinal detector orientations. Main results. OSLD dose–response supralinearity in an MR-Linac setting was found to agree within uncertainties with the corresponding one in a conventional linac, for the axial detector orientation investigated. Signal fading rate does not depend on irradiation conditions for the range of 3–30 d considered. OSLD angular (orientation) dependence is more pronounced under the presence of a magnetic field. OSLDs irradiated with and without real-time T2w MR imaging enabled during irradiation yielded the same response within uncertainties. values were determined for all three cardinal orientations. Corrections needed reached up to 6.4%. However, if OSLDs are calibrated in the axial orientation and then irradiated in an MR-Linac placed again in the axial orientation (perpendicular to the magnetic field), then simulations suggest that can be considered unity within uncertainties, irrespective of the incident beam angle. Significance. This work contributes towards OSLD dose–response characterization and relevant correction factors availability. OSLDs are suitable for QA checks in MR-based beam gating applications and in vivo dosimetry in MR-Linacs.

Evaluating Risk of FLASH Experiments on a Clinical LINAC with Failure Mode and Effects Analysis

<h3>Purpose/Objective(s)</h3> Use of clinical linear accelerators in ultra-high dose rate (UHDR) mode can provide a conduit for wider access to UHDR FLASH effects, sparing normal tissue, but care needs to be taken in the use of such systems to ensure errors are avoided. A Failure Modes and Effects Analysis (FMEA) was conducted with a team involved in converting a LINAC between clinical use and UHDR experimental mode for more than one year, following the proposed methods of AAPM (The American Association of Physicists in Medicine) Task Group 100 report. <h3>Materials/Methods</h3> A team of 9 professionals with extensive experience outlined the process map and workflow for analysis, and developed fault trees for potential errors, as well as failure modes that would results. The team scored the categories of severity magnitude (S), occurrence likelihood (O), and detectability potential (D) on a scale of 1 to 10, so that a risk priority number (RPN=S*O*D) could be assessed for each potential failure. <h3>Results</h3> A total of 46 potential failure modes were identified, including 5 with RPN>100, all during converting and experimental use in UHDR mode and none during conversion to clinical mode. These failure modes involved 1) patient set up, 2) gating mechanisms in delivery, and 3) detector in the beam stop mechanism. Identified methods to mitigate errors included 1) use of a checklist post conversion, 2) use of robust radiation detectors, 3) automation of QA and beam consistency checks, and 4) implementation of surface guidance during beam delivery. <h3>Conclusion</h3> The FMEA process was considered critically important in this setting of a new use of a LINAC, and the expert team developed a higher level of confidence in the ability to safely move UHDR LINAC use towards expanded research access.

Implementation of Data-Driven Timelines and Visual Guidance Tools to Improve Radiation Oncology Task Completion: “Plan Readiness”

<h3>Purpose/Objective(s)</h3> The time between physician treatment plan approval and patient treatment initiation is often too short in our current Radiation Oncology practice. This reflects inconsistencies in the timeframe of task completion between CT simulation and treatment plan approval, the first and last steps in a complex, multi-step planning workflow in Radiation Oncology. This poses challenges in terms of the QA and plan checks needed prior to treatment initiation, with potential implications for patient safety. Our goal was to implement a sustainable and clear departmental process that would increase the percentage of plans approved by the attending Radiation Oncologist 1 full business day prior to treatment start for all non-urgent patients. Secondary goals included a reduction in last-minute patient delays and improvement in team member engagement and satisfaction. <h3>Materials/Methods</h3> We established realistic timelines for each task from simulation to treatment initiation, while setting a cutoff time for implementing patient appointment delay when expectations were not met. Treatment site, technique, and complexity-specific timelines were built by our team based on 4 major factors: 1) Clinical Urgency, 2) Task Complexity, 3) Evaluation of "task completion" data from a large cohort of previously treated patients, and 4) Interviews with all team members involved in the treatment planning process. These timelines were generated in a visual task management tool, and were linked as serial or parallel to keep the entire team on schedule for each patient. Prior to implementation, appropriate departmental meetings were held to discuss these changes, with extensive team member feedback utilized to generate the new policy. Timelines were generated that ranged from 5 to 12 working days, with clear goals and guidelines for each involved team member regarding their task(s). Our overarching goal, termed "Plan Readiness", was to achieve an electronically approved treatment plan 1 full working day prior to treatment start for > 90% of our patients. Appropriate exclusions were made for emergencies and special/new procedures in the department. <h3>Results</h3> For the 5 months prior to implementation, our baseline data demonstrated that 71% of plans were electronically approved in our treatment planning system greater than 24 hours prior to treatment start. After implementation of our new "Plan Readiness" process in March 2021, we have been able to stay above 90% pass rate, with a cumulative average of 94.6% over an ongoing 10-month timeframe. <h3>Conclusion</h3> The design and execution of data-driven timelines and visual guidance tools, combined with a team-based approach of communication and open feedback, has resulted in preliminary favorable outcomes in achieving timely "Plan Readiness" in our relatively large, hospital-based Department of Radiation Oncology. Future directions include the use of analytics software and automated data collection to perform more robust assessments.

Experimental validation of daily adaptive proton therapy

Anatomical changes during proton therapy require rapid treatment plan adaption to mitigate the associated dosimetric impact. This in turn requires a highly efficient workflow that minimizes the time between imaging and delivery. At the Paul Scherrer Institute, we have developed an online adaptive workflow, which is specifically designed for treatments in the skull-base/cranium, with the focus set on simplicity and minimizing changes to the conventional workflow. The dosimetric and timing performance of this daily adaptive proton therapy (DAPT) workflow has been experimentally investigated using an in-house developed DAPT software and specifically developed anthropomorphic phantom. After a standard treatment preparation, which includes the generation of a template plan, the treatment can then be adapted each day, based on daily imaging acquired on an in-room CT. The template structures are then rigidly propagated to this CT and the daily plan is fully re-optimized using the same field arrangement, DVH constraints and optimization settings of the template plan. After a dedicated plan QA, the daily plan is delivered. To minimize the time between imaging and delivery, clinically integrated software for efficient execution of all online adaption steps, as well as tools for comprehensive and automated QA checks, have been developed. Film measurements of an end-to-end validation of a multi-fraction DAPT treatment showed high agreement to the calculated doses. Gamma pass rates with a 3%/3 mm criteria were >92% when comparing the measured dose to the template plan. Additionally, a gamma pass rate >99% was found comparing measurements to the Monte Carlo dose of the daily plans reconstructed from the logfile, accumulated over the delivered fractions. With this, we experimentally demonstrate that the described adaptive workflow can be delivered accurately in a timescale similar to a standard delivery.

A New Method and Device for Calibration of High Definition Motion Management System of Gamma Knife ICON

A motion tracking system which monitors patient motion and controls radiation delivery during Gamma Knife radiosurgical treatment is available with the Leksell Gamma Knife ICON machine. Yet the vendor provides no method or tool to calibrate the motion tracking system for commissioning and quality assurance programs. An easy-to-use method and tool is desired to confirm the spatial accuracy and precision of the motion tracking system. To that end, we have developed a new method and device and tested in our center. We developed a system of calibration and QA checks for the ICON high definition motion management (HDMM, also known as intra-fraction motion management) system, which consists of one 3D motion stage set, three stepper motor controllers, three stepper motorized actuators and a set of control software as shown in Figure 1. Measurements for tracking accuracy of the ICON HDMM system were conducted with the motion stage in left-right (X), anterior-posterior (Y) and superior-inferior (Z) directions separately. The HDMM system tracks the position of an infrared marker as a radial distance from a set origin. The motion stage was moved in well-defined displacements DX, DY and DZ for which the radial distance r = (DX2 + DY2 + DZ2)½ was calculated and compared to the HDMM system. The experimental tests were repeated and statistic analysis of the collected data was performed. Figure 1 Motorized QA system. The new motion stage system meets the specified accuracy (50mm/revolution). The mean value of r measured by HDMM was 0.19 mm with maximum standard deviation of 0.06 mm. The new QA system is easy-to-build and easy-to-use. The measurement is repeatable with high accuracy and precision. The ICON HDMM system can track patient motion with accuracy of less than 0.2 mm and precision of 0.06 mm.

Quality assurance tests for the Gamma Knife® Icon™ image guidance system.

IntroductionThe Gamma Knife® Icon™ comes with an image guidance system for tracking patient motion and correcting for inter‐ and intrafractional shifts, mainly used with frameless thermoplastic immobilization. The system consists of a cone‐beam CT (CBCT) and a couch‐mounted infrared camera (IFMM). We report our quality assurance program for Icon's image guidance system.MethodsThe manufacturer‐provided tool is used for daily checks of CBCT positional precision. Catphan® phantom is used for monthly checks of CBCT image qualities (uniformity, contrast to noise ratio (CNR), and spatial resolution) for the two acquisition presets (low‐dose and high‐quality presets). On a semi‐annual schedule, we use a frame tool to check the agreement of CBCT‐based and Frame‐based stereotactic space coordinates by comparing the locations of five attached ball bearings in CT‐sim scans (Frame‐based coordinates determination) and in Icon's CBCT scans. On an annual basis, the accuracy of IFMM, image registration, and delivery‐after‐shift are tested using a translational stage. A weighted CT dose index is measured annually with a pencil chamber in CTDI head phantom.ResultsThe CBCT precision check: 0.12 ± 0.04 mm (maximum deviations average). CBCT image quality: spatial resolution range: [6,7] lp/cm (low), and [7,8] lp/cm (high); uniformity: 12.82 ± 0.69% (low), and 13.01 ± 0.69% (high); CNR: 1.07 ± 0.08 (low), and 1.69 ± 0.10 (high). Agreement of CBCT‐based with Frame‐based stereotactic coordinates range: [0.33, 0.66] mm. Accuracy of IFMM: 0.00 ± 0.12 mm (average) with 0.27 mm (max.); image registration: 0.03 ± 0.06 mm (average) with 0.23 mm (max.); and delivery‐after‐shift: 0.24 ± 0.09 mm (average) with 0.42 mm (max.). CTDI w: 2.3 mGy (low), and 5.7 mGy (high).ConclusionsThe manufacturer‐required QA checks together with additional user‐defined checks are an important combination for a robust quality assurance program ensuring the safe use of Gamma Knife® Icon™'s image guidance and motion management features.

Implementation and evaluation of an Epson V800 Scanner and Gafchromic EBT3 Film for use in routine Linac QA and Patient Specific QA checks

In order to achieve more conformal dose distributions to the patient forms of radiotherapy delivery have become increasingly complex. With this comes an increased chance of error in delivery and an increased need to verify dose distribution using patient-specific QA. The aim of this project was to evaluate the use of Gafchromic EBT3 film and an Epson V800 scanner in linac QA and patient-specific QA checks. Response of EBT3 was found to be linear and accurate within 3% for a dose range of 0–400 cGy with use of the green channel. Effects of change in orientation showed max variation between two orientations of 1%. Change in response of 6% was shown between scans taken one hour and 24 h post-irradiation confirming that all scans should be taken at the same interval post-irradiation. A 1 h interval was chosen. Variation of position showed max change in response laterally of 6% confirming that scans should be performed in the centre of the scanner bed. Variation in number of warm-up scans showed max deviation of 0.4%. Intra-sheet homogeneity was within 0.8% and inter-sheet homogeneity was within 4%. Using these findings a protocol was devised and three basic fields and three IMRT and VMAT patient plans were measured. After normalisation of dose differences between measured and calculated distributions, EBT3 had similar or higher gamma-index passing rates than those achieved by Octavius detectors. In conclusion the method derived was shown to be a suitable alternative to the 2D arrays and reduced time required for film dosimetry.

P100] Calculations of dosimetric parameters from depth dose curve of electron beams, using radiophotoluminescent and thermoluminescent dosimeters

Purpose The aim of this study was to calculate the dosimetric parameters derived from percent depth dose curves by using RPL-GD and TLD-100 detectors for high-energy electron beams. Methods In this study, Monte Carlo simulation code MCNP5 was using to calculate the percent depth dose (PDD) in water phantom for 9, 12, 16 and 20 electron beam energies with utilizing the Radiophotoluminescent (FD-7glass series) and Thermoluminescent (TLD-100) materials. Results We found a good agreement with literature for Monte Carlo calculations parameters derived from the PDD curve such as various ranges R 100 , R 90 , R 80 , R 50 and R p and energies specification such us most probable energy E p , o and mean incident energy E 0 . Conclusion In conclusion, RPL-GD and TLD-100 dosimeters are a very useful detector for routine QA checks for electron therapy [1] .

Dosimetric characterization of a commercial 2-D scintillation detector for quality assurance tests in scanned proton and carbon ion beams

Introduction Pencil beam scanning technique used at CNAO requires beam characteristics to be carefully and periodically checked to guarantee patient safety. Radiochromic films are used within daily QA checks. Purpose This study aimed at characterizing the Lynx PT system (IBA Dosimetry) and assessing its suitability for QA tests, for proton and carbon ion beams. Materials and methods The Lynx PT is a 2D high-resolution dosimetry system consisting of a scintillating screen coupled with a CCD camera, in a compact light-tight box. A dedicated software is used for image acquisition, raw data correction and fast data analysis. Lynx was characterized in terms of short-term stability, image geometrical distortion, uniformity, signal proportionality, response dependence on IRIS and on particle intensity, particle energy and type. Irradiations were performed at the Centro Nazionale di Adroterapia Oncologica (CNAO) for both protons and carbon ions. Measurements with Lynx were compared to those made with EBT3 reference films. The detector response dependence on radiation LET was also investigated. Results Lynx basic characterization results have shown a proportional behavior with delivered number of particles. Strong dependence of the acquired signal on beam energy and particle type was found. No significant differences in the investigated beam parameters measured with Lynx and EBT3 film were observed. The detector also showed good performances in term of image uniformity and geometrical distortion. Conclusion The Lynx PT system appears as a promising tool for measurements for both single pencil beam and scanned beam, enabling fast QA and beam optics optimization checks.

A.198 - Dosimetric characterization of a commercial 2-D scintillation detector for quality assurance tests in scanned proton and carbon ion beams

Introduction: Pencil beam scanning technique used at CNAO requires beam characteristics to be carefully assessed and periodically checked to guarantee patient safety. Radiochromic films are widely used within daily QA checks for pencil beam measurements and determination of dose homogeneity in scanned fields. This study aimed at characterizing the Lynx PT system (IBA Dosimetry) and assessing its suitability for QA tests as compared to EBT3 films, for proton and carbon ion beams.

SU‐E‐J‐199: A Software Tool for Quality Assurance of Online Replanning with MR‐Linac

Purpose:To develop a quality assurance software tool, ArtQA, capable of automatically checking radiation treatment plan parameters, verifying plan data transfer from treatment planning system (TPS) to record and verify (R&amp;V) system, performing a secondary MU calculation considering the effect of magnetic field from MR‐Linac, and verifying the delivery and plan consistency, for online replanning.Methods:ArtQA was developed by creating interfaces to TPS (e.g., Monaco, Elekta), R&amp;V system (Mosaiq, Elekta), and secondary MU calculation system. The tool obtains plan parameters from the TPS via direct file reading, and retrieves plan data both transferred from TPS and recorded during the actual delivery in the R&amp;V system database via open database connectivity and structured query language. By comparing beam/plan datasets in different systems, ArtQA detects and outputs discrepancies between TPS, R&amp;V system and secondary MU calculation system, and delivery. To consider the effect of 1.5T transverse magnetic field from MR‐Linac in the secondary MU calculation, a method based on modified Clarkson integration algorithm was developed and tested for a series of clinical situations.Results:ArtQA is capable of automatically checking plan integrity and logic consistency, detecting plan data transfer errors, performing secondary MU calculations with or without a transverse magnetic field, and verifying treatment delivery. The tool is efficient and effective for pre‐ and post‐treatment QA checks of all available treatment parameters that may be impractical with the commonly‐used visual inspection.Conclusion:The software tool ArtQA can be used for quick and automatic pre‐ and post‐treatment QA check, eliminating human error associated with visual inspection. While this tool is developed for online replanning to be used on MR‐Linac, where the QA needs to be performed rapidly as the patient is lying on the table waiting for the treatment, ArtQA can be used as a general QA tool in radiation oncology practice.This work is partially supported by Elekta Inc.

SU‐E‐J‐53: A Phantom Design to Assist Patient Position Verification System in Daily Image‐Guided RT and Comprehensive QA Measurements

PurposeThis study is to implement a homemade novel device with surface locking couch index to check daily radiograph (DR) function of adaPTInsight™, stereoscopic image guided system (SIGS), for proton therapy. The comprehensive daily QA checks of proton pencil beam output, field size, flatness and symmetry of spots and energy layers will be followed by using MatriXX dosimetry device.MethodsThe iBa MatriXX device was used to perform daily dosimetry which is also used to perform SIGS checks. A set of markers were attached to surface of MatriXX device in alignment of DRR of reconstructed CT images and daily DR. The novel device allows MatriXX to be fit into the cradle which was locked by couch index bars on couch surface. This will keep the MatriXX at same XY plane daily with exact coordinates. Couch height Z will be adjusted according to imaging to check isocenter‐laser coincidence accuracy.ResultsadaPTInsight™ provides robotic couch to move in 6‐degree coordinate system to align the dosimetry device to be within 1.0 mm / 1.0°. The daily constancy was tightened to be ± 0.5 mm / 0.3° compared to 1.0 mm / 1.0° before. For gantry at 0° and couch all 0° angles (@ Rt ARM 0 setting), offsets measured of the couch systems were ≤ 0.5° in roll, yaw and pitch dimensions.ConclusionSimplicity of novel device made daily image guided QA consistent with accuracy. The offset of the MatriXX isocenter‐laser coincident was reproducible. Such easy task not only speeds up the setup, but it increases confidence level in detailed daily comprehensive measurements. The total SIGS alignment time has been shortened with less setup error. This device will enhance our experiences for the future QA when cone beam CT imaging modality becomes available at proton therapy center.

Guidelines for the Statistical Analysis of a Collaborative Study of a Laboratory Method for Testing Disinfectant Product Performance

This paper presents statistical techniques suitable for analyzing a collaborative study (multilaboratory study or ring trial) of a laboratory disinfectant product performance test (DPPT) method. Emphasis is on the assessment of the repeatability, reproducibility, resemblance, and responsiveness of the DPPT method. The suggested statistical techniques are easily modified for application to a single laboratory study. The presentation includes descriptions of the plots and tables that should be constructed during initial examination of the data, including a discussion of outliers and QA checks. The statistical recommendations deal with evaluations of prevailing types of DPPTs, including both quantitative and semiquantitative tests. The presentation emphasizes tests in which the disinfectant treatment is applied to surface-associated microbes and the outcome is a viable cell count; however, the statistical guidelines are appropriate for suspension tests and other test systems. The recommendations also are suitable for disinfectant tests using any microbe (vegetative bacteria, virus, spores, etc.) or any disinfectant treatment. The descriptions of the statistical techniques include either examples of calculations based on published data or citations to published calculations. Computer code is provided in an appendix.

Poster - Thur Eve - 38: Review of couch parameters using an FMEA.

To improve patient safety during positioning, we undertook a systematic review of the processes used by our center to obtain couch positions. We used a Failure Mode and Effects Analysis (FMEA) framework and fifteen different possible failures were identified and rated. The three major failures were 1) Loss of planned couch position and bias from the previous day's couch position, 2) DICOM origin or isocenter is different between two plans (imaging or treatment), and 3) Patient shift in opposite direction than intended. The main effect of these failures was to cause an override of couch parameters. Based on these results, we modified our processes, introduced new QA and software checks and developed new tolerance tables so as to improve system robustness and increase our success rate at catching failures before they can affect the patient. It has been a year since we made these modifications. Based on our results, we have reduced the number of overrides at our center from a maximum of 20.5% to a maximum of 6.3%, with an average at 4% of daily treatments. Our results suggest that FMEA is an effective tool in improving treatment quality that could be used in other centers.

TU‐A‐BRB‐08: A Technique to Determine the Gantry Laser and the Proton Pencil Beam Coincidence Using Computer Radiography Detector for Routine Quality Checks

Purpose: A computed radiography (CR) detector with photostimulable phosphors can detect both direct and indirect ionization radiation, and is even sensitive to photons in the visible range. The purpose of this work is to develop and validate a technique using CR to check both proton radiation versus localization laser coincidence and spot positioning accuracy for magnetically scanned proton pencil beam spots (PPBS). Methods: CR plate is removed from its cassette and set up perpendicular to the incident beam and exposed to an array of PPBS irradiated at predefined coordinates. Upon exposure of the particle radiation, the CR detector is then exposed to the gantry's laser systems. The plate is then put through a digital scanner which then reads the photo stimulated luminescence and converts the signal into a digital image pixel map. MATLAB®‐based analysis software and ImageJ® image processing tools were used to compute the dose gradient from the laser bleaching of the proton exposure of the detector and for quantification of the results of this QA check procedure. The technique further allows quantitative determination of both spot positions, and the relationship between X ray system used for image guided patient setup and the PPBS delivery. Results: Both the coincidence between laser representing the gantry isocenter and center of PPBS along the central axis and spot positions can be checked within 1mm accuracy with the CR detector. Conclusions: We have demonstrated that our technique allows determining the coincidence between the laser and radiation beam. The technique described here is found to be suitable for periodic QA checks of relative positions of the spot center relative to the gantry isocenter and beam central axis. Improvements of this technique by considering the effect of detector's quantum yield and modulation transfer function on the data remain subject of further study.

MO-D-BRB-07: Failure-Mode and Effects Analysis Study for CyberKnife Stereotactic Radiosurgery.

The purpose of this study is to conduct a Failure Modes and Effect Analysis (FMEA) for CyberKnife Stereotactic Radiosurgery to determine the sensitivity of existing QA procedures and determine in which areas new QA procedures needed to be implemented. Members from each professional team providing service for CyberKnife radiosurgery (Medical Physicists, Nurses, Physicians, Radiation Therapists, and Administrators) were interviewed to gather potential failure modes. A patient flow chart was developed from patient consult to conclusion of last treatment. Failure modes were mapped to nodes in the flow charts to identify potential high-risk areas. A matrix was created to correlate existing QA procedures with failure modes to identify failure modes that were not covered by any QA as well as identify the sensitivity of QA procedures to prevent failures. 180 failure modes were identified. Current AAPM QA recommendations were found to focus preferentially on technical failure modes (15%), while the majority of failure modes found are process failures and human errors (85%). Creating a Venn diagram of CyberKnife and Gamma Knife failure modes revealed a large overlap area. The most effective QA checks are checklists for physics second chart review and pre- treatment time-out checklists. Existing checklists were modified and new checklists added to address high-ranked failure modes. New procedure guidelines, e.g. for contouring workflow and add-on simulations, were developed as QC to address clusters of failure modes. An ARIA-CyberKnife DICOM interface is being implemented to resolve failure modes centering around multiple fraction, multiple plan treatments and total dose tracking. This work is the first FMEA study for the CyberKnife stereotactic radiosurgery. It will facilitate medical physicists using the CyberKnife to deliver SRS/SBRT treatments to transition from experience-based technical QA to a comprehensive new quality paradigm including technical, process, and human safety aspects.

MO-D-BRCD-01: Overview of Clinical Dosimetry and Quality Assurance in Proton Therapy.

1. Understand proton beam dosimetry protocols 2. Familiarization with proton dosimetry tools and their limitation 3. Understand the challenges and procedures in proton therapy QA.

SU‐E‐J‐164: Evaluation of Target Localization Accuracy of ExacTrac System for Spine SBRT

Purpose: To assess the positioning accuracy of ExacTrac image guidance system for spine SBRT patients and to quantify the residual errors of ExacTrac calibration in daily QA using an in‐house phantom.Methods: Twelve patients treated with spine SBRT in Siemens Artiste linac using ExacTrac system thus far are included in this study to assess the localization accuracy. The sequence of localization is as following. 1) Patient is aligned to the treatment isocenter using infrared markers. 2) Stereoscopic X‐ray images are taken for all fractions before and after treatment to calculate the setup errors and intrafraction motion. 3) In addition, X‐ray images are acquired whenever the markers showed a deviation of more than 2 mm in any direction. In order to quantitatively verify ExacTrac calibration, an in‐house phantom with high density inserts was developed and was used daily before treatments to acquire residual errors of the system.Results: The initial and final setup errors were calculated using A/P, R/L and S/I shifts of X‐ray localization before and after treatment. The average and standard deviation of initial and final setup errors were 1.00 ± 0.6 mm and 1.6 ± 0.9 mm respectively. Intrafraction motion was found to be 1.2 ± 0.8 mm. It was noted that the final setup error followed initial setup error in that it can be reduced if the initial setup error was kept well below the setup tolerance (2 mm). We observed no significant correlation between deviations shown by infrared markers and subsequent positional shifts of X‐ray images. The average residual error from phantom measurements after daily ExacTrac calibration was 0.2 ±0.11 mm. Conclusions: ExacTrac image guidance system is very useful for accurate target localization of spine SBRT patients. The daily QA checks using our in‐house phantom after ExacTrac calibration provides quantitative assessment of positional uncertainties.

SU‐E‐T‐843: Direct Segment Optimization of Conformal Plans: Achieving Equivalence with Beamlet IMRT

Purpose: Use of inverse planning to improve conformal treatment plan quality has concentrated on optimization of beamlet‐based IMRT plans. The few direct aperture‐based optimization methods are all based on optimizing a beamlet‐based plan, followed by sequencing which identifies the many apertures to be optimized. In contrast, direct segment optimization (DSO), described here, directly optimizes conformal and few‐segment plans using inverse planning. This work compares DSO‐optimized plans versus beamlet IMRT and forward‐planned 3DCRT plans for a series of clinical sites. Methods: Our in‐house beamlet‐based inverse planning system has been enhanced to support DSO with an improved and reimplemented perturbational version of the Edge/Octree calculation algorithm and a multilayer search strategy. DSO optimizes MLC segment shapes, weights and/or jaw positions for multiple field plans (including VMAT). In our inverse planning infrastructure, beamlet, multi‐segment and conformal plans can all be optimized using the same cost functions, dose points, and plan evaluation metrics, making quantitative comparisons between techniques straightforward. Inverse planned DSO 3DCRT plans are compared to forward planned 3DCRT and beamlet IMRT plans for brain, liver, lung, and other sites. Results: For many tested patient geometries, DSO achieves equivalent plan quality to the beamlet IMRT plan optimized with the same cost function, though there are certain situations where IMRT is expected to (and does) perform better. The DSO plans use significantly fewer MU, require less treatment time, are less sensitive to motion, and do not require pre‐treatment IMRT QA checks, so they may be significantly easier to implement in the clinic. Conclusions: Optimization of 3DCRT and multi‐segment plans using DSO results in plan evaluation metrics virtually equivalent to beamlet IMRT plans optimized with the same cost function and beam directions, for many geometries. DSO may potentially lead to important efficiency savings for many clinical protocols.Supported in part by NIH P01‐CA59827

Experience Using DosimetryCheck software for IMRT and RapidArc Patient Pre-treatment QA and a New Feature for QA during Treatment

We have used the DosimetryCheck program with the EPID's on our Varian 2100EX's to perform pre-treatment QA on more than 350 patients, between the last quarter of 2006 and the present. The software uses the EPID measured fluences of the treatment fields to reconstuct the dose distribution in the CT planning model of the patient. Since the dose calculation algorithm, is different from that used by our Eclipse planning system, this provides an independent check of planning accuracy as well as treatment delivery. 2D and 3D dose distributions, point doses, Gamma distributions, DVH statistics and MU calculations can be compared. Absolute differences of Reference Point doses between Dosimetry Check and Eclipse average 1.20%, which is similar to the ionization chamber dose differences of 1.29% for the same patient verification plans. Examples of cases for various treatment sites and delivery modes will be presented. A Special Report in Medical Physics Vol. 37 Number 6 Pg. 2638–2644 from Mans et al at The Netherlands Cancer Institute demonstrated the ability of in vivo EPID dosimetry to detect treatment errors, that escaped other QA checks. A new version of DosimetryCheck awaiting FDA approval, is capable of successfully reconstructing the dose distribution in the patient from the EPID measured exit fluences. This can also be applied to CBCT images providing actual patient dose verification for a treatment session. This should be particularly useful for monitoring hypo-fractionated treatment regimens. Examples of this method will also be presented.