J Smale

An integral quality monitoring system for real‐time verification of intensity modulated radiation therapy

PURPOSE To develop an independent and on-line beam monitoring system, which can validate the accuracy of segment-by-segment energy fluence delivery for each treatment field. The system is also intended to be utilized for pretreatment dosimetric quality assurance of intensity modulated radiation therapy (IMRT), on-line image-guided adaptive radiation therapy, and volumetric modulated arc therapy. METHODS The system, referred to as the integral quality monitor (IQM), utilizes an area integrating energy fluence monitoring sensor (AIMS) positioned between the final beam shaping device [i.e., multileaf collimator (MLC)] and the patient. The prototype AIMS consists of a novel spatially sensitive large area ionization chamber with a gradient along the direction of the MLC motion. The signal from the AIMS provides a simple output for each beam segment, which is compared in real time to the expected value. The prototype ionization chamber, with a physical area of 22 x 22 cm2, has been constructed out of aluminum with the electrode separations varying linearly from 2 to 20 mm. A calculation method has been developed to predict AIMS signals based on an elementwise integration technique, which takes into account various predetermined factors, including the spatial response function of the chamber, MLC characteristics, beam transmission through the secondary jaws, and field size factors. The influence of the ionization chamber on the beam has been evaluated in terms of transmission, surface dose, beam profiles, and depth dose. The sensitivity of the system was tested by introducing small deviations in leaf positions. A small set of IMRT fields for prostate and head and neck plans was used to evaluate the system. The ionization chamber and the data acquisition software systems were interfaced to two different types of linear accelerators: Elekta Synergy and Varian iX. RESULTS For a 10 x 10 cm2 field, the chamber attenuates the beam intensity by 7% and 5% for 6 and 18 MV beams, respectively, without significantly changing the depth dose, surface dose, and dose profile characteristics. An MLC bank calibration error of 1 mm causes the IQM signal of a 3 x 3 cm2 aperture to change by 3%. A positioning error in a single 5 mm wide leaf by 3 mm in 3 X 3 cm2 aperture causes a signal difference of 2%. Initial results for prostate and head and neck IMRT fields show an average agreement between calculation and measurement to within 1%, with a maximum deviation for each of the smallest beam segments to within 5%. When the beam segments of a prostate IMRT field were shifted by 3 mm from their original position, along the direction of the MLC motion, the IQM signals varied, on average, by 2.5%. CONCLUSIONS The prototype IQM system can validate the accuracy of beam delivery in real time by comparing precalculated and measured AIMS signals. The system is capable of capturing errors in MLC leaf calibration or malfunctions in the positioning of an individual leaf. The AIMS does not significantly alter the beam quality and therefore could be implemented without requiring recommissioning measurements.

Two self‐referencing methods for the measurement of beam spot position

PURPOSE Two quantitative methods of measuring electron beam spot position with respect to the collimator axis of rotation (CAOR) are described. METHODS Method 1 uses a cylindrical ion chamber (IC) mounted on a jig corotational with the collimator making the relationship among the chamber, jaws, and CAOR fixed and independent of collimator angle. A jaw parallel to the IC axis is set to zero and the IC position adjusted so that the IC signal is approximately 50% of the open field value, providing a large dose gradient in the region of the IC. The cGy∕MU value is measured as a function of collimator rotation, e.g., every 30°. If the beam spot does not lie on the CAOR, the signal from the ion chamber will vary with collimator rotation. Based on a measured spatial sensitivity, the distance of the beam spot from the CAOR can be calculated from the IC signal variation with rotation. The 2nd method is image based. Two stainless steel rods, 3 mm in diameter, are mounted to a jig attached to the Linac collimator. The rods, offset from the CAOR, lay in different planes normal to the CAOR, one at 158 cm SSD and the other at 70 cm SSD. As the collimator rotates the rods move tangent along an envelope circle, the centers of which are on the CAOR in their respective planes. Three images, each at a different collimator rotation, containing the shadows of both rods, are acquired on the Linac EPID. At each angle the shadow of the rods on the EPID defines lines tangent to the projection of the envelope circles. From these the authors determine the projected centers of the two circles at different heights. From the distance of these two points using the two heights and the source to EPID distance, the authors calculate the distance of the beam spot from the CAOR. Measurements with all two techniques were performed on an Elekta Linac. Measurements were performed with the beam spot in nominal clinical position and in a deliberately offset position. Measurements were also performed using the Flexmap image registration∕ball-bearing test. RESULTS Within their uncertainties, both methods report the same beam spot displacement. In clinical use, a total of 203 monthly beam spot measurements on 14 different beams showed an average displacement of 0.11 mm (σ = 0.07 mm) in-plane and 0.10 mm (σ = 0.07 mm) cross-plane with maximum displacement of 0.37 mm in-plane and 0.34 mm cross-plane. CONCLUSIONS The methods described provide a quantitative measure of beam spot position, are easy to use, and provide another tool for Linac setup and quality assurance. Fundamental to the techniques is that they are self-referencing-i.e., they do not require the user to independently define the CAOR.

SU‐E‐T‐127: Two Dosimetric Methods of Measuring Linac Beam Spot Position

Purpose: Two dosimetric methods of measuring the electron beam spot position of photon beams w.r.t. the Collimator Axis Of Rotation (CAOR) are described. Methods: Method 1 (SNG test) uses a cylindrical ion chamber (IC) (A1SL) in a phantom mounted on a jig co‐rotational with the collimator making the relationship among the chamber, phantom, jaws and CAOR fixed and independent of collimator angle. A jaw parallel to the IC axis is set to zero and the phantom adjusted so the IC signal is 65% of the open field value. cGy/mu is measured every 30 degrees of collimator rotation. The phantom is then translated normal to the IC axis until the signal is 35%. The shift is recorded to calculate sensitivity (%dose/mm). For a radially symmetric spot centred on CAOR, the IC signal is angularly constant. Deviation from constant indicates a displacement from the CAOR and a shape distortion. The displacement and spot shape are calculated using Fourier analysis and penumbra shape (sensitivity). The 2nd method uses a hard wedge instead of the jaw and sensitivity is obtained from wedge profiles. Test measurements were performed on an Elekta 6 MV: the In Plane beam spot position was measured before and after it was moved by changing bending magnet current (Bending Fine). Results: Independent beam spot measurement (Flexmap Image Registration) showed the initial beam spot, projected back to source, 0.29 mm from CAOR, moving to 0.91 mm (delta=0.62 mm) with steering. The SNG test reported the beam spot moving from 0.13mm to 0.92 mm from CAOR (delta=0.79 mm). The hard wedge test gave an initial position of 0.09 mm off axis and a final position of 0.79 mm (delta=0.7 mm) Conclusions: The methods described provide a quantitative measure of beam spot position, not requiring the user to independently define the CAOR. Jason Smale is an Employee of Elekta Canada

SU‐E‐T‐110: Quantitative Image Based Measurement of Electron Beam Spot Position

Purpose: Develop an image based method to measureelectron beam spot position w.r.t the Collimator Axis Of Rotation (CAOR). Methods: Two stainless steel rods, 3 mm in diameter, are mounted in a jig attached to the linaccollimator. The rods lay in different planes normal to the CAOR, one at 159 cm SSD and the other at 70 cm SSD, and are offset 10 and 6 cm from the CAOR respectively. Images of both rods are acquired at multiple coll rotations. At each angle the rods project a tangent to an inscribed circle, the centre of which is calculated. The lower rod is very close the plane of the EPID and the centre of its inscribed circle defines the mechanical CAOR. The centre of the inscribed circle calculated from multiple images of the upper rod will be displaced from the CAOR by some distance proportional to the distance the beam source is from the CAOR. Test measurements were performed on an Elekta 6 MV where the In Plane beam spot position was moved a known distance using steering fine and beam spot position was measured.Results: Independent beam spot measurement (Flexmap Image Registration ball bearing) reported the initial beam spot at 0.29 mm from CAOR, moving to 0.91 mm (delta =0.62 mm) with steering. The rod tests showed the initial beam spot to be .31 mm from the CAOR and 1.17 mm from CAOR after adjustment. Note that in our companion paper we report similar values (initial 0.13mm going to 0.92 mm and initial value of 0.09mm going to 0.79) with our two co‐rotational dosimetric beam spot tests Conclusions: A simple method for measuring beam spot position with respect to CAOR is described. It provides its own internal reference and could be incorporated into routine QA using the EPID. Jason Smale is an employee of Elekta Canada

SU-GG-T-281: Clinical Experience with an EPID-Based Quality Assurance System for Linear Accelerators

Purpose: To report our experience with the implementation and clinical use of an electronic portal imaging device(EPID) based tool for the routine quality assurance (QA) of linear accelerator(Linac) radiation field geometry. Methods and Materials: The development of an EPID‐base QA tool, eQA, for routine QA of Linac radiation field geometry has previously been reported. Implementation of eQA began in 2007 for both Elekta and Varian Linacs in our clinic, with deployment completed by mid‐2008. The eQA is currently used to perform all radiation field geometry QA, including radiation field size, light vs. radiation field congruence, jaw symmetry, MLC leaf position, and gantry, collimator and couch isocentre. The eQA is routinely used to accurately set the lasers to the radiation isocentre and perform MLCcalibrations. Efforts were made to automate much of the eQA functionality, using information in DICOM header tags to automatically identify the type of test and determine associated parameters in the analysis of results. Email summaries of QA results are sent automatically to appropriate staff. Compared to a manual assessment of film‐based measurements, eQA provides reproducible, non‐subjective analysis of the radiation field, reducing the variability in measurements. This has enabled a tightening of the tolerance for radiation field size measurements. Results: The eQA has successfully replaced film for all radiation field geometry tests in our department, and has improved the efficiency of our QA processes. The non‐subjective, reproducible results obtained with eQA have allowed a reduction of radiation field size tolerance from 2 mm to 1 mm. The eQA provides the unique functionality to map the locations of the gantry, collimator and table centre of rotations onto a common reference frame for determining the isocenter sphere. Conclusions: The EPID based software tool has been successfully implemented, improving the efficiency and overall performance of our QA processes.

SU-GG-T-215: Routine Linear Accelerator Electron Beam QA Using EPIDs

Purpose: To investigate the use of Electronic Portal Imaging Devices(EPIDs) for the routine quality assurance (QA) of linear accelerator(Linac)electron beams.Method and Materials: Routine electron beam QA is typically performed using radiographic film. The use of film for beam QA is time consuming and prone to many sources of error. EPIDs have become standard equipment on modern linacs, and their suitability for photon beam QA has been previously established. We have investigated the use of EPIDs for routing electron beam QA, including radiation field size, penumbra, flatness and symmetry. A Varian 2100iX with aSi imager was used. Service mode was used, as the linac does not support imaging with electron beams in clinical mode. Dark and flood field calibration of the EPID was obtained using the electron beams.EPID pixel response linearity with dose was verified for energies 6, 12, and 20 MeV with a 10×10 cm cone by exposing the imager to various MU. For radiation field size and beam flatness/symmetry QA, images were acquired using the 6×6 cm and 20×20 cm cones at energies of 6, 12 and 20 MeV. For comparison, Kodak XV films at a depth of 2 cm were exposed using the same configurations. The SDD for both film and EPID measurements was 105 cm. All films and images were analyzed using MATLAB code. Results: The EPID pixel response is linear with delivered dose. The radiation field size as measured by the EPID agreed with film to within 1.6 mm. Penumbra agreed to within 1.5 mm. It was not possible to obtain accurate flatness and symmetry results, but the EPID can be used for flatness and symmetry constancy. Conclusion:EPIDs can be used for the routine QA of electron beams. Radiation field size results are similar to those obtained with film.

TU-D-M100F-01: A Novel Quality Assurance Monitor for Real-Time Verification of IMRT and IGART

Purpose: To develop an independent treatment verification method which can validate radiation field delivery in real‐time throughout the treatment course. The system is designed to capture common treatmentdelivery errors, and is intended to eliminate the need for pre‐treatment dosimetricquality assurance of intensity modulated radiation therapy(IMRT) and enable the implementation of image guided adaptive radiation therapy.Method and Materials: A monitoring system, termed Integral Quality Monitor (IQM), has been developed that utilizes an area integrated energy fluence monitoring sensor (AIMS) positioned after the final beam shaping device (i.e. multileaf collimator(MLC)) and a signal prediction algorithm, IQM_Calc. The AIMS consists of a novel large area ionization chamber with a gradient oriented along the direction of the MLC motion. The measured signal from the AIMS can be compared in real‐time with the IQM_Calc predicted values. A prototype AIMS has been built with 2 mm thick Aluminum plates, an area of 22 cm × 22 cm and continuously varying electrode separation of 2 to 22 mm. The IQM_Calc uses a modified sector integration of MLC defined apertures and accounts for MLC characteristics such as: rounded leaf ends, transmission, and relative output factor. Testing of the IQM system was performed on Varian and Elekta linear accelerators. Results: Initial results for prostate IMRT fields show an average agreement of 2% between the measured IQM signals and the IQM_Calc results. For a 3 mm simulated MLC leaf positioning error, the signal of a prostate IMRT field changed by 2%. Conclusion: It is demonstrated that the prototype IQM system has the capability of verifying the accuracy of treatmentdelivery in real‐time. The system is also capable of capturing common treatment errors. The IQM system has the potential of playing an important role in the challenging QA environment of modern radiation therapy.

SU‐FF‐T‐156: Development of An Ultra‐Wide Dynamic Range Electrometer

Purpose: To develop a wide range electrometer for measuring the charge from large volume ion chambers, while maintaining reading accuracy and resolution for small chargemeasurements.Method and Materials: Standard commercially available electrometers typically unable to integrate the charge from a large volume ion chamber period without saturating. While it is possible to design an electrometer that can integrate a large charge, readout accuracy and resolution for low charge readings is often compromised. We have developed a unique wide dynamic range electrometer (WDE) design that overcomes these problems by using dual electrometers operating in a switching configuration. Each electrometer is of the familiar integrating capacitor type. The integrating amplifier used is a commercially available integrated circuit designed specifically for use as an integrating electrometer, with on‐chip reset, hold and multiplexing switches. A microcontroller controls the switching, readout and reset of the electrometers such that one electrometer is integrating the ion chamber signal while the other is being read and reset, as well as the communication with PC‐based software. Using this method large chargemeasurements are possible while maintaining the accuracy and resolution of low chargemeasurements. The maximum measurablecharge is limited only by the firmware. The reproducibility and linearity of the electrometer design is comparable to commercially available electrometers.Results: For large ion chamber current measurements of ≈5 uA, the electrometer is able to measure the charge to within an accuracy of 0.2%, compared to a 45% error when using a commercial electrometer. At lower currents, the WDE can accurately measurecharge with a reproducibility standard deviation of 0.06% and a linearity standard deviation of 0.007%, which is comparable to commercial electrometers.Conclusion: A large dynamic range electrometer has been developed that allows measurement of large ion chamber currents while maintaining the accuracy and resolution of small chargemeasurements.

SU‐FF‐T‐137: Development of a Software Tool for Generating Predicted Dose Images From Pinnacle Dose Maps for the Purpose of IMRT Quality Assurance Using PortalVision

Purpose: To develop and evaluate a method of importing the predicted dose maps of patient treatment fields generated by the Pinnacle treatment planning system (TPS) into the dosimetry module of Varians PortalVision electronic portal imaging system (EPID), for the purpose of verifying planned IMRT fields. Method and Materials: PortalVision, when equipped with the dosimetry module, is equipped with tools to perform relative dose comparisons of dose images acquired by the EPID vs. predicted dose images generated by a TPS. Treatment plans were created using the Pinnacle TPS. Dose maps of individual fields were saved as files using Pinnacles Planar Dose Map function. The dose maps were calculated in a geometry equivalent to that of the detector panel. Using in‐house developed software, the dose map files were converted to the correct predicted dose format and imported into the PortalVision software. Predicted dose maps were generated for open square fields of several standard sizes, wedged fields, and IMRT fields. The IMRT fields were taken from typical prostate, and head and neck patients. Fields from ten IMRT patients were evaluated. The treatment fields were delivered using a Cl2100EX accelerator. Point dose measurements were obtained from the acquired dose image and compared with film measurements. The predicted vs. delivered dose distributions were evaluated using the available PortalVision tools. The results were compared against those of films analyzed using RIT. Results: Point dose measurements made with PortalVision in areas of high dose, low dose gradient agreed with film to an average of +/− 3%. Dose Difference and Gamma results agreed with film results to within +/− 3%. Conclusion: This work shows that a predicted dose map from a TPS can be modified and imported into Varians PortalVision software in order to perform IMRT patient QA with accuracy comparable to traditional film methods.

SU‐FF‐T‐197: Filmless Radiation Isocentre Localisation Using An Electronic Portal Imaging Device

Purpose: The increased availability of electronic portal imaging devices(EPID) on linear accelerators provides a capability to replace film with enhance standard commissioning and validation measurements. A series of EPID based measurements have been developed to confirm the coincidence of the radiation and mechanical isocentre with gantry, collimator and table rotation, eliminating the need for traditional star‐shots films and enhancing the information gathered from such tests. Method and Materials: A precision phantom containing a set of radio‐opaque markers was positioned at the mechanical isocentre using a calibrated front pointer. The EPID was positioned at an SID of 140 cm, and a series of images for a 4×10 cm2 where acquired in IMRT mode for different angular rotations of gantry, collimator and treatment table. Images were exported to a MatLab based program which automatically identified the position of the radio‐opaque markers and field edges to determine the relative motion of the isocentre with angle. Results:Measurements were performed with several different models of aSi imager found on Varian and Elekta Precise treatment units, and compared to the results of traditional star shot films. The measurements were consistent, with the radiation isocentre confined to a 2 mm diameter sphere. In addition, the EPID based tests provided additional information on the longitudinal motion of isocentre typically not available from film star shots. Conclusion:EPID based measurements can replace the use of film in geometric verification tests typically performed during commissioning and annual quality assurance. These measurements are at the same time simpler to perform and analyze, while increasing the information extracted from the measurements and providing a reliable, user independent method of monitoring changes to the treatment unit isocentre over the lifetime of the machine.