Radiation Isocenter Research Articles

Estimation of organs doses and radiation-induced secondary cancer risk from scattered photons for conventional radiation therapy of nasopharynx: a Monte Carlo study

We used Monte Carlo modeling to calculate the organs doses due to out-of field photons during radiation therapy of the nasopharynx. A medical internal radiation dose (MIRD)-based mathematical phantom resembling an adult man was modeled by MCNP4C MC code. Three validated models of a cobalt-60 machine, a 6-MV photon beam of a Varian 2300 C/D linac, and a 9-MV photon beam of a Neptun linac were used to simulate the isocentric irradiation of a mathematical phantom with two lateral fields of the nasopharynx. The organspecific dose, effective dose, and cancer risk estimates were obtained. The effective doses for out-of-field radiation were 320, 295, and 248 mSv for the (60)Co beam, 6-MV beam, and 9-MV beam devices, respectively, for a 70-Gy tumor dose. The fatal cancer risks of 1.6%, 1.5%, and 1.2% were estimated for a 70-Gy tumor dose of (60)Co and the 6- and 9-MV photon beams, respectively. Our results regarding the effective dose and cancer risk are in agreement with previously published experimental results on conventional radiation therapy. Further investigation on patients' out-of-field dose to provide more knowledge on various radiotherapy techniques is suggested.

A simple method to quantify the coincidence between portal image graticules and radiation field centers or radiation isocenter

The aim of this study was to develop a computerized method to quantify the coincidence between portal image graticules and radiation field centers or radiation isocenter. Three types of graticules were included in this study: Megavoltage (MV) mechanical graticule, MV electronic portal imaging device digital graticule, and kilovoltage (kV) on-board imaging digital graticule. A metal ball bearing (BB) was imaged with MV and kV x-ray beams in a procedure similar to a Winston-Lutz test. The radiation fields, graticules, and BB were localized in eight portal images using Hough transform-based computer algorithms. The center of the BB served as a static reference point in the 3D space so that the distances between the graticule centers and the radiation field centers were calculated. The radiation isocenter was determined from the radiation field centers at different gantry angles. Misalignments of MV and kV portal imaging graticules varied with the gantry or x-ray source angle as a result of mechanical imperfections of the linear accelerator and its imaging system. While the three graticules in this study were aligned to the radiation field centers and the radiation isocenter within 2.0 mm, misalignments of 1.5-2.0 mm were found at certain gantry angles. These misalignments were highly reproducible with the gantry rotation. A simple method was developed to quantify the alignments of portal image graticules directly against the radiation field centers or the radiation isocenter. The advantage of this method is that it does not require the BB to be placed exactly at the radiation isocenter through a precalibrated surrogating device such as room lasers or light field crosshairs. The present method is useful for radiation therapy modalities that require high-precision portal imaging such as image-guided stereotactic radiotherapy.

Six Degree-of-Freedom Image Guidance for Frameless Intracranial Stereotactic RadioSurgery with kilo-voltage Cone- Beam CT

Abstract Purpose: To investigate localization accuracy for frameless intracranial stereotactic-radiosurgery using 2D orthogonal planar imaging and 3D cone-beam CT (CBCT) in 6-degree-of-freedom (6-DOF). Methods and Materials: In a phantom study, a target ball phantom was used to perform a Winston-Lutz test and to verify coincidence of imaging isocenter and radiation isocenter. A head phantom was placed with pre-defined positions and imaged with CBCT to test imaging accuracy. In a patient study, one hundred patients were included. Patients were initially positioned with a thermoplastic frameless mask system and then aligned with orthogonal planar imaging and CBCT. The setup discrepancies were quantitatively analyzed. Results: Phantom experiments showed discrepancies in root-mean-square were 1.6mm translationally and 0.5° rotationally between CBCT 6-DOF image guidance and the known displacements after deviations from the radiation isocenter are considered. In the patient study, setup displacements between orthogonal planar imaging and CBCT 6-DOF image guidance were 3.2mm translationally and 0.9° rotationally. The positioning of twelve patients was corrected in 6-DOF using CBCT and a robotic couch to reduce translational and rotational discrepancies of 1.4mm and 1.3°, as compared with standard CBCT translational correction. Conclusion: CBCT 6-DOF image guidance offers an explicit view to verify patient positioning in translations and rotations.

TU-D-BRB-08: A CBCT Projection Matrix Method for Radiation and Imaging Isocenter QA

Purpose: To test the use of a cone beam computed tomography(CBCT) projection matrix method for determining the imaging isocenter diameter as a replacement for the traditional gantry star shot for radiation isocenter testing.Method and Materials: The Siemens MVision megavoltage CBCT is calibrated by imaging a geometric reconstruction phantom that contains BBs of various sizes at well characterized positions. This generates several projection matrices, Pq, that define where a point in the reconstruction volume is projected onto the flat panel detector at gantry angle q. The standard reconstruction angles are −90° to 110°. A new protocol, with angles from −30° to 170°, provides information about radiation isocenter from posterior angles. Flat panel positions projected to a plane containing the isocenter are [Uq,Vq] = [0.276(Pq(1,4) − P0(1,4)), 0.276(Pq(2,4) − P0(2,4))]. The room coordinates [xq, yq, zq] = [Uqcos(‐q), Uqsin(‐q), Vq]. The radiation isocenter ellipsoid diameters are (xq max ‐ xq min, yq max ‐ yq min, zq max ‐ zq min), where superscripts max and min refer to maximum and minimum values of the room coordinate, respectively. The maximum diameter is compared to that of a traditional star shot and a Winston‐Lutz type test. Results: Traditional star shots are limited in accuracy due to the subjectivity in the analysis and set‐up error. The maximum radiation isocenter diameters were about 0.8 mm, 0.9 mm and 1.4 mm for the star shot, Winston‐Lutz test and the projection matrix analysis, respectively. The result for the projection matrix includes deviations resulting from flat panel motion and is larger than that of the Winston‐Lutz test, which is unaffected by flat panel motion. Conclusion: The projection matrix method simultaneously checks the stability of the imaging and radiation isocenter, while providing an annual geometric calibration for the CBCT system.

SU‐FF‐T‐284: Accurate Determination of a Linac Radiation Isocenter Using Iterative Analysis of Digitized Star Shot Images

Purpose: To develop an accurate method to measure radiation isocenter for linac with sub‐millimeter isocenter radius. Method and Materials: Conventional star shot film is produced for gantry and collimator radiation isocenters. The pattern of gantry rotation and collimator rotation is produced on the same Kodak XV ready pack film by placing the film at vertical, and then horizontal positions at isocenter using a carefully placed “Iso‐Align” device. The beams can be differentiated easily because of the well define length of testing beams for collimator rotation, and penetrating beams of the gantry rotations. The film was digitized and the image was enhanced to show clearly the beam edges and center line of each narrow beam using a public domain image analysis software. Each beam can be described by a linear equation that was obtained through sampling and linear regression of the beam's central line. Using iterative search algorithms and analytical calculations, the location of the isocenter and its radius were obtained. Because gantry isocenter and collimator isocenter were determined on the same image, we were able to measure the lateral deviation between these two isocenters. The impacts of beam arrangements on isocenter determination were also investigated. Results: We applied this method on a Varian Trilogy machine. A 0.2 mm radius was measured for collimator radiation isocenter. The gantry radiation isocenter was measured to be of 0.3 mm radius. The lateral distance between two isocenters was 0.3 mm. The results are highly reproducible. Conclusion: We established a method to determine accurately the radii of the radiation isocenters of a LINAC. The precision of this method had not been achievable by the traditional analysis of a star‐shot film, but it is necessary for evaluating a linac for high precision radiotherapy.

SU‐FF‐J‐63: A Method for Alignment Test of An X‐Ray Positioning System for Proton Therapy

Purpose: To develop a method to test the alignment of an on‐board x‐ray positioning system for proton therapy. Methods and materials: Each treatment portal at the proton therapy center in Houston (PTC‐H) is equipped with three orthogonal x‐ray imaging devices. We developed a method to test the alignment of imaging devices using a custom built phantom. The phantom is 30 × 30 × 30 cm in size and is positioned on a base plate, which attaches to the robotic patient couch. The phantom position was calibrated against a 2‐mm diameter tungsten ball placed at radiation isocenter. On each side of the phantom, there are radiopaque markers of known sizes and locations. With the measurement of the marker locations in the x‐ray images, we calculated the alignment of the x‐ray source to that of the major axis of the proton machine reference frame. By using points 15 cm away from isocenter, we test the alignment not only with iso center, but also with the reference frame axes to avoid angulations between imaging axis and machine axis. We also derived source‐to‐axis‐distance (SAD), source‐to‐imager distance (SID) for the x‐ray source, and pixel size for the image receptor. The phantom design allowed for alignment of all three imaging systems without couch movement once the phantom was positioned at isocenter. Results: We used this method to test the alignment of imaging positioning system at a proton portal at PTC‐H. We found the measured results to be within 0.4 mm at iso for x‐ray sources alignment at all 3 x‐ray/receptor systems at gantry of 270 degree. The pixel size was within 1%. SAD, SID were within 2% of manufacturer's specifications. Conclusion: We developed a method to align the x‐ray positioning system for a proton gantry to the precision necessary for proton therapy.

SU-FF-T-546: A Novel Template Technique for the Analysis of Alignment Accuracy of a Linac Based SRS System

Introduction The analysis of Winston-Lutz (WL) test for SRS QA is time consuming and subjects to human observation uncertainties. In this work, a novel template technique is described to analyze the low contrast portal images of WL test. Methods/Materials Linac based Steotactic Radiosurgery requires overall end-to-end accuracy of sub-millimeter (<1mm). In our department, a BrainLab® microMLC is utilized to perform SRS procedure. A WL test is performed by imaging a lead ball of size of 5mm in diameter via a Siemens Optivue EPID imager, 1 MU is delivered and a field size of 6mm×6mm is employed for 7 different gantry/table combinations. The acquired EPID images are enhanced with a Mexican Hat Function which is a band limited filter. By using different dilation parameters, either the radiation field or the ball in the field can be enhanced. A template binary image with known radiation isocenter, representing the ideal radiation field of size of 6mm×6mm, is generated. In addition the two dimensional projection of a lead ball is created for feature localization. The defined field and ball template images are then matched with the enhanced EPID images by using normalized cross-correlation method. The location of radiation center and mechanical isocenter can then be determined. The location difference between the beam and mechanical isocenter represents alignment accuracy for the particular field. Results This technique has been test on 151 images. The analysis of 145 images indicated alignment accuracy of better than 1mm. The review of 6 images failed the tolerance (>1mm) and needed to be realigned according to our protocol. The result is agreed with our visual observation. Conclusions By using modern image processing and analysis technique, we developed a template based algorithm to analyze WL test images for SRS quality assurance.

An experimental study on using a diagnostic computed radiography system as a quality assurance tool in radiotherapy

The advent of improved digital imaging modalities in diagnostic and therapy is fast making conventional films a nonexistent entity. However, several radiotherapy centers still persist with film for performing quality assurance (QA) tests. This paper investigates the feasibility of using a diagnostic computed radiography (CR) system as a QA tool in radiotherapy. QA tests such as light field congruence, field size verification, determination of radiation isocentre size, multileaf collimator (MLC) check and determination of isocentric shift for stereotactic radiosurgery (SRS) were performed and compared with film. The maximum variation observed between CR and film was 0.4 mm for field size verification, -0.13 mm for the radiation isocentre size check, 0.77 for MLC check and -0.1 mm for isocentric shift using the Winston Lutz test tool for SRS QA. From these results obtained with the CR it is concluded that a diagnostic CR system can be an excellent cost-effective digital alternative to therapy film as a tool for QA in radiotherapy.

Accuracy of cranial coplanar beam therapy using an oblique, stereoscopic x-ray image guidance system

A system for measuring two-dimensional (2D) dose distributions in orthogonal anatomical planes in the cranium was developed and used to evaluate the accuracy of coplanar conformal therapy using ExacTrac image guidance. Dose distributions were measured in the axial, sagittal, and coronal planes using a CIRS (Computerized Imaging Reference Systems, Inc.) anthropomorphic head phantom with a custom internal film cassette. Sections of radiographic Kodak EDR2 film were cut, processed, and digitized using custom templates. Spatial and dosimetric accuracy and precision of the film system were assessed. BrainScan planned a coplanar-beam treatment to conformally irradiate a 2-cm-diameter x 2-cm-long cylindrical planning target volume. Prior to delivery, phantom misalignments were imposed in combinations of +/- 8 mm offsets in each of the principal directions. ExacTrac x-ray correction was applied until the phantom was within an acceptance criteria of 1 mm/1 degrees (first two measurement sets) or 0.4 mm/0.4 degrees (last two measurement sets). Measured dose distributions from film were registered to the treatment plan dose calculations and compared. Alignment errors, displacement between midpoints of planned and measured 70% isodose contours (Deltac), and positional errors of the 80% isodose line were evaluated using 49 2D film measurements (98 profiles). Comparison of common, but independent measurements of Deltac showed that systematic errors in the measurement technique were 0.2 mm or less along all three anatomical axes and that random error averaged [formula: see text] 0.29+/-0.06 mm for the acceptance criteria of 1 mm/1 degrees and 0.15 +/- 0.02 mm for the acceptance criteria of 0.4 mm/0.4 degrees. The latter was consistent with independent estimates that showed the precision of the measurement system was 0.3 mm (2sigma). Values of Deltac were as great as 0.9, 0.3, and 1.0 mm along the P-A, R-L, and I-S axes, respectively. Variations in Deltac along the P-A axis were correlated to misalignments between laser isocenter and radiation isocenter as documented by daily clinical Lutz tests. Based on results of comparisons of measured with calculated positions of the 80% dose lines along the major anatomical axes, a 1.25, 1.0, and 1.0 mm (0.75, 0.5, and 0.25 mm) gross tumor volume (GTV)-planning target volume (PTV) margin to account for delivery error would be appropriate for the P-A, R-L, and I-S axes, respectively, for an acceptance criteria of 1 mm/1 degrees (0.4 mm/0.4 degrees). It typically took 2 (3) ExacTrac x-ray image sets to achieve and verify acceptance criteria of 1 mm/1 degrees (0.4 mm/0.4 degrees). Our results demonstrated a measurement technique using a CIRS anthropomorphic head phantom with a modified film cassette, radiographic film (Kodak EDR2) with a custom film cutting template, and film dosimetry software has been developed and successfully applied to our clinic. It is recommended that a third party offer this service. Our goal of achieving accuracy of delivery of 1 mm or better in each of the three major anatomical axes was almost, but not quite achieved, not because of the accuracy of the image guidance system, but likely due to inaccuracy of laser isocenter and other systematic errors.

Prostate tumor alignment and continuous, real-time adaptive radiation therapy using electromagnetic fiducials: Clinical and cost-utility analyses

Objective To evaluate the accuracy, utility, and cost effectiveness of a new electromagnetic patient positioning and continuous, real-time monitoring system, which uses permanently implanted resonant transponders in the target (Calypso ® 4D Localization System and Beacon ® transponders, Seattle, WA) to continuously monitor tumor location and movement during external beam radiation therapy of the prostate. Materials and methods This clinical trial studied 43 patients at 5 sites. All patients were implanted with 3 transponders each. In 41 patients, the system was used for initial alignment at each therapy session. Thirty-five patients had continuous monitoring during their radiation treatment. Over 1,000 alignment comparisons were made to a commercially available kV X-ray positioning system (BrainLAB ExacTrac, Munich, Germany). Using decision analysis and Markov processes, the outcomes of patients were simulated over a 5-year period and measured in terms of costs from a payer's perspective and quality-adjusted life years (QALYs). Results All patients had satisfactory transponder implantations for monitoring purposes. In over 75% of the treatment sessions, the correction to conventional positioning (laser and tattoos) directed by an electromagnetic patient positioning and monitoring system was greater than 5 mm. Ninety-seven percent (34/35) of the patients who underwent continuous monitoring had target motion that exceeded preset limits at some point during the course of their radiation therapy. Exceeding preset thresholds resulted in user intervention at least once during the therapy in 80% of the patients (28/35). Compared with localization using ultrasound, electronic portal imaging devices (EPID), or computed tomography (CT), localization with the electromagnetic patient positioning and monitoring system yielded superior gains in QALYs at comparable costs. Conclusions Most patients positioned with conventional tattoos and lasers for prostate radiation therapy were found by use of the electromagnetic patient positioning and monitoring system to have alignment errors exceeding 5 mm. Almost all patients undergoing external beam radiation of the prostate have been shown to have target organ movement exceeding 3 mm during radiation therapy delivery. The ability of the electromagnetic technology to monitor tumor target location during the same time as radiation therapy is being delivered allows clinicians to provide real time adaptive radiation therapy for prostate cancer. This permits clinicians to intervene when the prostate moves outside the radiation isocenter, which should decrease adverse events and improve patient outcomes. Additionally, a cost-utility analysis has demonstrated that the electromagnetic patient positioning and monitoring system offers patient outcome benefits at a cost that falls well within the payer's customary willingness to pay (WTP) threshold of $50,000 per QALY.

SU-GG-T-238: Three Dimensional Characterization of the Radiation Isocenter Using a Single Phantom

Purpose: To provide a quality control procedure that characterizes the radiation isocenter of a linear accelerator simultaneously for collimator, couch, and gantry rotations. The primary question addressed is whether the three rotational axes cross each other within an acceptably small sphere. Method and Materials: A film stack phantom was constructed having four films suitably spaced apart. The phantom was placed in a fixed position on the treatment table, and a single reference frame was defined by registration marks. The combined use of Linac jaws and MLC created small spots on the films for each rotation axis. Special angles were chosen for the different axes so that three dimensional information was retrieved. The film spot coordinates were read using commercially available scanning software, and a computer calculation then rendered the data into a three dimensional display of the axes. Results: The procedure has been used nine times with reasonable results on four different accelerators. Comparisons of the minimum tangent circle radius using this method and standard star shots were favorable. In addition, the orthogonality of the axes and their orientations relative to a reference laser system was reported. Conclusion: This procedure is unique in that it provides, using simple commonly found materials,composite information about the radiation isocenter and all three rotational axes in a single measurement. It seems to offer the confidence needed to become a routinely used tool for the Linac annual check.

TH-D-352-08: Radiation Treatment Machine Mechanical Quality Assurance Check with Megavoltage Portal Imaging Device

Purpose:Treatment machine mechanical check is an important procedure in quality assurance (QA). Linac machines are commonly equipped with electronic portal imaging device(EPID).EPID can be used to perform mechanical QA more accurately and easily. We developed a method to use EPID to measure gantry, collimator and table isocenter runout. Method and Materials: Measurements were performed on an Elekta's ASSEX machine that equips with an a‐Si EPID and kilo‐voltage cone beam CT. A ball‐bearing phantom was position in the radiation isocenter following KV‐MV isocenter calibration procedure. Portal images were taken at various gantry, collimator and table angles. Matlab algorithms were developed to analyze the gantry, collimator and table isocenter positions. Results: Gantry, collimator and table isocenter positions were measured quantitatively with accuracy better than 0.1mm. Results clearly show the discrepancies of the three isocenters. Conclusion:EPID is a useful tool in LINAC mechanic QA. EPID isocenter measurements are more accurate and convenient than traditional approaches.

SU‐GG‐I‐22: Measurement and Correction of Sagging Shifts in KV Cone‐Beam CT From An On‐Board Imaging System

Purpose: To evaluate the effects of sagging on mechanical isocenter accuracy and image quality of kV cone‐beam CT (CBCT) and to develop a pre‐processing algorithm for removing these effects from on‐board‐imaging (OBI). Method and Materials: OBI sagging shifts were measured by scanning a cubical phantom with a 2 mm metal marker. The metal marker was aligned with radiation isocenter and its position was tracked in cone‐beam projections using normalized cross‐correlation image registration algorithm of kV CBCT system. The position of the marker was determined by 650 projections of a CBCT scan over an angular range from 0°–360°. To remove sagging shifts, projections were corrected to reconstruct CBCT images using Feldkamp back projection. The marker center of the cubical phantom in the corrected CBCT reconstructed images was used to determine OBI isocenter. The CATPHAN from phantom lab was used to compare image quality parameters such as blurring, uniformity and linearity for CBCT images before and after OBI sagging shift corrections. Results: We found that sagging affects localization accuracy of the OBI isocenter and image quality in CBCT. The marker position shifts about ± 2 mm over 360° range due to sagging and this produces about 1.4 mm isocenter shift in CBCT reconstructed from projections before correction for sagging. CBCT numbers vary by about 1% in full‐fan scans and up to 3.5% in half‐fan scans. Blurring and spatial distortion artifacts are produced in CBCT before correction of OBI sagging shifts. Conclusion: Sagging of the kV OBI system produces a systematic isocenter shift of CBCT. Further, sagging shifts lead to image artifacts such as spatial distortion, blurring, degradation of soft tissue contrast, position resolution, variation in CBCT number linearity and uniformity. Correction of OBI sagging in cone‐beam projections prior to CBCT reconstruction provides more accurate isocenter localization and improves image quality.

SU‐GG‐J‐37: Assessing Image Guided Radiation Therapy Targeting Accuracy: Coincidence Of Imaging System Isocenter With Treatment Machine Mechanical And Radiation Isocenters

Purpose: The purpose of this study is to introduce a method for quantifying the accuracy of linac and imaging system isocenters and demonstrate its use by measuring the targeting of two kilovoltage (kV) imaging systems, and one gantry mounted megavoltage Electronic Portal Imaging Device (EPID). One is the kV imaging system (OBI) integrated with the linear accelerator which supports radiographic and cone beam CT imaging. The other system is the BrainLab Exac Trac stand alone fixed to the treatment room ceiling and floor. Method and Materials: A cylindrical Lucite phantom was designed and fabricated to perform this study. The cylindrical phantom is 20 cm in diameter and 20 cm long. The phantom contains 13 radio‐opaque fiducial markers. A 5 mm spherical marker is positioned at the center of the cylinder. The phantom was imaged and the CT data sets were imported to two planning systems. Treatment beams were placed and both 3D data sets and isocenter information were transferred to the Linac delivery and imaging workstations. For each imaging system, phantom was initially positioned according to machine cross‐hair. Then images were acquired and measured shifts were applied to precisely position the phantom. Then, Winston‐Lutz test was performed for different gantry and collimator configuration. The radiation beam was collimated by a tray‐mounted 30 mm diameter circular collimator. Results: The radius of spherical volume in which all isocenter intersect or lie was measured. For EPID, OBI, CBCT, and Exac Trac, the radius is: 0.54 mm, 1.4 mm, 1.7 mm and 1.46 mm respectively. From the Winston‐Lutz analysis, the average isocenter deviation from all angles is 1.18 ± 0.28 mm. Conclusion: The error in mechanical, radiation and imaging isocenter is about one millimeter. Regardless what imaging system is used for patient setup, it is important to incorporate this systematic error in any planning margins.

SU‐GG‐T‐441: A New Method of Measuring the Accuracy of Stereotactic Cones and MLC with Gantry and Collimator Rotation

Precision and accuracy are vital for stereotactic radiosurgery and radiotherapy. We set up the patients with lasers and we treat with radiation fields defined by cones or MLC, therefore the lasers must be accurately aligned with the radiation fields. In addition, the manner in which the beam shapers move with gantry and collimator angles must be reproducible and reliable. The standard method of measuring laser alignment and rotational shifts at the Tom Baker Cancer Centre, the Winston‐Lutz test, is only semi‐quantitative, and the results are subject to inter‐observer variation. We have developed a method of digitally detecting therapy X‐Rays and lasers using a 512 by 1024 photodiode array with a 48μm pixel pitch, the Radeye1 from the Rad‐icon Imaging Corporation to which we added optical density filters and an intensifying screen. We took images of the lasers and the radiation fields at various gantry and collimator angles and analyzed them with in‐house developed Matlab tools. For the 4mm diameter cone we measured an apparent head tilt of 0.14 (±0.02) mm and a gantry sag of 0.39 (±0.02) mm. The lasers were within 0.25 (±0.05) mm of the compromise radiation isocentre. For a 6mm by 6mm MLC we found the center of the radiation field varied by 0.58 (±0.02) mm with collimator rotation. The ceiling laser was 0.52 (±0.02) mm from the centre of the MLC rotation. This system can be used to both monitor and improve machine performance, which may lead to improved outcomes for patient treatment.

The use of photostimulable phosphor systems for periodic quality assurance in radiotherapy

The fusion of radiological and optical images can be achieved through charging a photostimulable phosphor plate (PSP) with an exposure to a field of X- or gamma-rays, followed by exposure to an optical image which discharges the plate in relation to the amount of incident light. According to this PSP characteristic, we developed a simple method for periodic quality assurance (QA) of light/radiation field coincidence, distance indicator, field size indicators, crosshair centering, coincidence of radiation and mechanical isocenter for linear accelerators. The geometrical accuracy of radiological units can be subjected to the same QA method. Further, the source position accuracy for an HDR remote afterloader can be checked by taking an autoradiography of the radioactive source and simultaneously an optical image of a reference geometrical system.

Characterization of an add-on multileaf collimator for electron beam therapy

An add-on multileaf collimator for electrons (eMLC) has been developed that provides computer-controlled beam collimation and isocentric dose delivery. The design parameters result from the design study by Gauer et al (2006 Phys. Med. Biol. 51 5987–6003) and were configured such that a compact and light-weight eMLC with motorized leaves can be industrially manufactured and stably mounted on a conventional linear accelerator. In the present study, the efficiency of an initial computer-controlled prototype was examined according to the design goals and the performance of energy- and intensity-modulated treatment techniques. This study concentrates on the attachment and gantry stability as well as the dosimetric characteristics of central-axis and off-axis dose, field size dependence, collimator scatter, field abutment, radiation leakage and the setting of the accelerator jaws. To provide isocentric irradiation, the eMLC can be placed either 16 or 28 cm above the isocentre through interchangeable holders. The mechanical implementation of this feature results in a maximum field displacement of less than 0.6 mm at 90° and 270° gantry angles. Compared to a 10 × 10 cm applicator at 6–14 MeV, the beam penumbra of the eMLC at a 16 cm collimator-to-isocentre distance is 0.8–0.4 cm greater and the depth–dose curves show a larger build-up effect. Due to the loss in energy dependence of the therapeutic range and the much lower dose output at small beam sizes, a minimum beam size of 3 × 3 cm is necessary to avoid suboptimal dose delivery. Dose output and beam symmetry are not affected by collimator scatter when the central axis is blocked. As a consequence of the broader beam penumbra, uniform dose distributions were measured in the junction region of adjacent beams at perpendicular and oblique beam incidence. However, adjacent beams with a high difference in a beam energy of 6 to 14 MeV generate cold and hot spots of approximately 15% in the abutting region. In order to improve uniformity, the energy of adjacent beams must be limited to 6 to 10 MeV and 10 to 14 MeV respectively. At the maximum available beam energy of 14 MeV, radiation leakage results mainly from the intraleaf leakage of approximately 2.5% relative dose which could be effectively eliminated at off-axis distances remote from the field edge by adjusting the jaw field size to the respective opening of the eMLC. Additionally, the interleaf and leaf-end leakage could be reduced by using a tongue-and-groove leaf shape and adjoining the leaf-ends off-axis respectively.

A new approach to quantify the mechanical and radiation isocentres of radiotherapy treatment machine gantries

In this paper a new method is proposed to quantify and reduce the radiation beam position uncertainty due to the radiotherapy treatment machine gantry deflection. A new tool has been designed and manufactured to provide the means to measure the alignment of the collimator axis and of the beam central axis in space, using the NDI Polaris optical tracking system and Gafchromic® films. The tool can be mounted onto the accessory tray of the linacs from different manufacturers. The approach has been demonstrated with measurements of the mechanical isocentre being performed on ten linacs from three major manufacturers at four clinical sites. Measurements of the radiation isocentre were performed on a single linac. The collimator axis trajectory is modelled using a vector-end effector in order to provide more information than standard mechanical assessment methods. The method uses a mathematical optimization technique to calculate the position of the mechanical isocentre and the ‘size’ of the collimator axes intersection locus. Deviations of the collimator axes from the isocentre are expressed in terms of systematic and random error. The effects of measurement uncertainties are evaluated both via simulations and experimentally. The use of optical tracking and optimization techniques combined with an operator-induced measurement error compensation algorithm leads to a faster measurement of the mechanical isocentre (20 min for 24 angles) and eliminates operator-induced uncertainties. The uncertainty of the measurement of the mechanical isocentre was between 40 µm and 70 µm in terms of standard deviation. For some of the linacs assessed, the mechanical isocentre obtained using a standard approach with an adjustable pointer was displaced by over 1 mm from that found with the proposed method. The radii of the collimator axes intersection locus found with the proposed method were between 0.4 mm and 0.72 mm for the linacs assessed. Film measurement revealed a misalignment of the mechanical isocentre and the radiation isocentre by 0.4 mm. The proposed method potentially enables a reduction of the beam position uncertainty. This may be achieved at the planning stage by compensating for the identified systematic collimator axes deviations which were found to be reproducible. The method also creates a potential for using different setup margins independently for each axis and for each gantry angle, calculated specifically for a given linac.

SU-FF-T-390: Study of the Long Term Stability of the Radiation Isocenter of the Cone-Based SRS System On Varian's Trilogy Linac

Purpose: The accuracy of the SRS radiation isocenter is of critical importance since it directly affects the dose distribution in patients undergoing SRS procedures. The cone‐based SRS system on Varian's Trilogy® linac consists of a gantry mount and a set of 13 cones with sizes varying from 5mm–35mm. The system has a rotational isocenter accuracy of better than 0.5mm according to manufacturer's specification. To evaluate the long term stability of the SRS radiation isocenter, we analyzed its off‐set based on the data collected since January 2006. Method and Materials: Before each SRS procedure, the radiation isocenter of the SRS system is checked from different gantry angles to make sure it is within specification. A small steel ball is positioned accurately at the ideal isocenter and a 12mm cone is mounted onto the gantry. Gafchromic® films are then exposed at five angles (240°, 270°, 0°, 90°, and 120°) along the most commonly used arcs in our treatment plan. A template of isocentered circles and an Edmund Optics® magnifier are then used to measure the off‐set of circular radiation field from the center of the steel ball. This method can provide an accuracy of 0.1mm and a total of 23 measurements are analyzed in this study. Results: During the period of more than one year, the average off‐sets of the isocenter at the 5 angles are 0.31±0.09mm (240°), 0.27±0.06mm(270°), 0.48±0.08mm(0°), 0.21±0.07mm (90°), and 0.33±0.09mm(120°). We observed that they all fall within the 0.5mm specification of the system. We further observed that the offset at 0 gantry angle (AP) is the largest among the five due to mechanical tilt of the gantry head. Conclusion: The radiation isocenter of Varian Trilogy® cone‐based SRS system is very stable and can provide sub‐millimeter accuracy required for the SRS procedures in the long term.

MO‐E‐AUD‐01: Automated CBCT QA for Image‐Guided Radiation Therapy

Purpose: The use of image‐guided patient positioning requires fast and reliable Quality Assurance (QA) methods to ensure the megavoltage (MV) treatment beam coincides with the integrated kilovoltage (kV) imaging and guidance system. This study describes an automated and comprehensive QA procedure to monitor the coincidence of the mechanical, radiation and imaging isocenters using cone‐beam CT (CBCT) and planar X‐ray imaging. Method and Materials: The On‐Board Imaging (OBI) system consists of a kV x‐ray tube and an amorphous‐silicon flat panel imaging detector which are attached to a medical linear accelerator. A Penta‐Guide phantom (Modus Medical Devices Inc. London, Ontario, Canada) with five internal markers and external markers for isocenter position and field sizes was imaged using the CBCT, kV and MV capabilities. The markers' location and size can be automatically determined using our graphical software system. The accuracy of CBCT imaging is assessed by comparison of the extracted marker positions and sizes against the phantom specifications. The coincidence of the imaging and dosimetric isocenters are tested by a similar analysis of the markers extracted from the kV and MV images. Additional tests are also performed such as isocenter stability with gantry angle, image size, collimator angle and size, and gantry angle. Results: The test was performed on all four IGRT‐enabled machines available in our institution. The coincidence between the mechanical, radiation and imaging isocenter are within 1 mm for all four accelerators. Isocenter stability with gantry angle was also within 1 mm. The acquisition of the images took ∼ 5 min, and the automated software analysis took less than 1 min. Conclusion: Our automated image analysis may be used as a daily QA procedure because it is completely automated and uses a single phantom setup.