M van Prooijen

SU‐E‐T‐445: Use of MAPPHAN for Patient Specific VMAT Plan Verification

Purpose: Volumetric modulated arc therapy (VMAT) has been introduced in many clinics. The need for patient specific quality assurance tests for VMAT was reflected in a large number of publications on this subject. Many centers have used the Sun Nuclear Mapcheck diode array for IMRT patient specific QA, and thus the retrofitted MAPPHAN/Mapcheck may appear an economical and trustworthy solution for VMAT. Implementing this phantom was not straightforward. Following manufacturers guidelines for comparing measured and calculated dose distributions, the pass rates were considerably lower than with other volumetric dosimeters. We present a simple method to improve these results to clinically acceptable values. Methods: We commissioned VMAT for two Elekta Infinity accelerators with the Pinnacle3(v9) treatment planning system. For MAPPHAN dose calculation, we used a CT scan of the phantom directly or assigned a density of 1.05 g/cm3 to the whole phantom, as recommended by the manufacturer and tested these on 5 prostate plans. To improve plan/measurement agreement we created an artificial CT scan using two density configurations as possible estimates of the actual phantom. To minimize couch effects, we placed the phantom on 7cm of Styrofoam. Validity of the plans was also verified with the Delta4 phantom (Scandidos) which gave 100% agreement under TG1 19 criteria. Results: Using MAPPHAN as suggested by the manufacturer gave suboptimal results. A literature search revealed that this problem was also identified by others (1). Using an artificial CT representation and raising the phantom improves agreement with plans from (93+/−6)% to (98+/−3)% (TG1 19), approaching Delta4 results Conclusions: Problems with using the MAPPHAN phantom can stem from insufficient knowledge of the phantom composition and treatment couch attenuation. A simple method was developed to improve the results of VMAT patient specific QA.

SU-E-T-252: Safety Systems and Failure Modes and Effects Analysis for a Linear Accelerator - Magnetic Resonance Imager - Brachytherapy System

Purpose: To evaluate and identify safety system concerns and possible failure modes for a multimodality, linear accelerator(linac) — magnetic resonanceimager(MRI) ‐ brachytherapy,radiotherapy system. Methods: A Delphi process is applied to investigate the safety system concerns and possible failure modes in a linac‐MRI‐brachytherapy system. Well established, for instance in the automotive or airline industries, system, design and process failure modes effect analysis can be applied to the design of a multimodality radiation therapy system. Results: Safety design, systems, processes and culture in radiation therapy is of great importance. When implementing new technologies a review of the necessary staff and patient safety may be necessary. To address patient and staff safety concerns a thoughtful design process must be implemented when an MRI is present or near a linac or a brachytherapy system. Each input into a patient specific process map, such as consent, screening, immobilization, planning, image guidance and machine movement coordination must be analyzed for safety and failure modes concerns. Numerous items have been identified as potential failure modes, these items have been classified into the following nine categories: screening, motion, interlocks, imaging, treatment delivery, dosimetry, plan adaptation, mechanical and miscellaneous. A broad range of failure modes have been identified some of these include and range from non‐compatible in‐room finishing to linac performance degradation under repeated small magnetic field influences to improper patient screening and the possibility of machine collisions. Conclusions: An abridged list of suggestions resulting from our safety systems analysis includes a rigorous staff educational and training program, the evaluation of current quality assurance devices used for mechanical and dosimetric tests, an interlocked computerized system for machine translations and rotations, machine servicing protocols and both fire and patient emergency protocols.

SU‐E‐T‐145: Patient Specific Quality Assurance ‐ Not Just Stamp Collecting

Purpose: To assess the feasibility of using MapCheck 2D dose maps to improve quality in radiation therapy. Methods: A MapCheck2 device is used to validate each inverse planned step and shoot IMRT field at our centre. To date over 2000 fields have been measured. It is of interest to use this wealth of data effectively to establish appropriate control limits by applying principals of statistical process control, change planning practice if required as well as evaluate beam model performance. An application has been developed that generates a report comparing the measured 2D dose maps with the planar dose maps exported from the Pinnacle treatment planning system (Philips RadiationOncology Systems, Madison, WI). This application reports mean error and standard deviation for each field within the following three distinct regions of the distribution consistent with AAPMs TG53: inner (80 – 100%), outer (0 – 20 %) and penumbra (20 – 80 %) where the percentages are of the maximum dose in the 2D dose map. Results: Results of the N=2193 measured fields are: inner field mean of the mean error −0.3%, SD 1.0%, outer field mean of mean error −0.1%, SD 1.0%, penumbra mean of mean error 0.6%, SD 2.3%. Control limits have been established. Mean error as a function of the number of control points has been reviewed and more appropriate control point limits established for treatment plan generation. A test suite of patients has been created for validation of new beam models or software releases. Conclusions: This quantity of measured data allows for a thorough evaluation of beam model performance and TPS software version in terms of IMRT delivery. By effectively analyzing the large quantity of measured clinical data, continual quality improvement becomes a less labour intensive task.

SU‐FF‐T‐578: Assessment and Management of Dose Perturbations Due to the Treatment Couch

Purpose: To develop a model which can predict beam intersection with highly attenuating couch elements and determine the impact of the treatment couch on dose distribution. Method and Materials: Our method is based on fusing a CTimage set of the couch with the patient CT data set, importing regions of interest characterizing couch elements and assigning appropriate densities in the TPS. A retrospective study was performed on patient plans that posed difficulties in beam‐couch intersection during setup. The impact of the treatment couch was assessed through a comparison between clinical plans that excluded and included the couch model. The percentage of PTV covered by 95% of the prescribed dose and the mean CTV coverage were compared. Dose compensation strategies for IMRT treatments with beams passing through couch elements were investigated using a four‐field IMRT plan with 3 beams passing through couch elements. Results: The model agrees with positioning measurement to within 1° gantry rotation; prediction of dose is accurate to within 2% (Table 1, Figure 1). Inclusion of the couch resulted in an up to 3% reduction in PTV coverage and an up to 1% reduction in mean CTV coverage. Using film dosimetry, we were able to show that ignoring couch effects can result in plan deviations of 8 ± 3%; including compensation for the presence of the couch reduced the deviations to 2 ± 2% (Figure 2). Conclusion: We have a couch model that can be easily incorporated into planning procedures and which allows full assessment of the impact of the couch on the dose distribution.

SU‐EE‐A2‐02: Verification of Source and Collimator Configuration for Gamma‐Knife® Perfexion TM using Panormaric Imaging

Purpose: To develop a method of verifying the source and collimator configuration of Leksell Gamma Knife® Perfexion™. Method and Materials: The new model of stereotactic radiosurgery machine, Perfexion™, with modified source configuration allows extended reach of targets located in the cranial, neck and cervical regions. The control system allows automatic selection of appropriate built‐in collimator modules eliminating the need of time consuming manual installation of collimator helmets as in the older model of Gamma Knife system. However, the geometric configuration of collimator modules cannot be easily verified. The conventional method of exposing a film at the isocenter plane provides only a composited dose image, which is difficult to interpret in terms of the integrity of each individual source and corresponding collimator system. A method has been developed to capture a panoramic view of 192 Cobalt sources and corresponding images are utilized to verify the integrity and configuration of 192 sources. The images were acquired by exposing Gafchromic films wrapped around the surface of a specially designed 16 cm diameter cylindrical phantom. The phantom was mounted at the isocenter, with its axis aligned along the longitudinal axis of the couch. Depending upon the azimuthal angle of the source location, the shape and size of the source images were calculated and compared with the acquired images. Results: The images allowed clear identification of each of the 192 sources, verifying their integrity and selected collimator sizes. In this presentation the results of the source image alignment with respect to the expected collimator geometry will be described. In addition, the feasibility of relative dosimetric evaluation of the individual sources by the panoramic images will be presented. Conclusion: This method of source/collimator verification by panoramic imaging can provide an enhancement of commissioning and routine quality assurance of the Gamma Knife systems.

Poster — Thurs Eve‐31: Clinical implementation and experience with EPID‐based precision isocentre localization

Modern linear accelerators contain multiple isocentres, defined by the mechanical motions of gantry, collimator and table. Isocentre localization for these motions has been performed using film and manual evaluations which have difficulty in relating the individual motions. To address these limitations, we have developed an EPID based technique to measure the isocentre position for each of the treatment unit motions. This technique uses the projected position of a radio-opaque marker at the isocentre in a series of MV images to determine the motion of the isocentre. This analytical procedure has been implemented in the clinic using a MatLab code to automatically analyze images and determine both the isocentre position and motion about the mean for each of gantry, collimator and table. Results of isocentre measurements for 18 machines from 2 different vendors at 2 separate clinics are reported. These measurements show that while the position of the mean isocentres are contained within a 2mm sphere, combinations of gantry, table and collimator rotations can be found that result in treatment isocentres more than 2mm apart. Results for a treatment unit, which underwent a recent equipment upgrade, are also presented that show a small change in the location of the gantry relative to the table isocentre. The implementation of this of isocentre localization technique has provided important clinical information which can be efficiently completed in less than an hour. This information is an important consideration in monitoring the changes and in assessing the treatment precision that can be obtained.

TH‐C‐M100E‐07: A CT Based Total Body Irradiation Technique Using Intensity Modulated Beams

Purpose: To report experience with a novel total body irradiation (TBI) technique. 3D planning techniques are used to deliver a uniform dose to a patient using a conventional linear accelerator in a standard bunker. Manually segmented intensity modulated fields are employed to provide dose compensation for contour variation, tissue heterogeneity, inverse square law effects and junction dose stability. Methods and Materials: The technique uses a conventional Elekta Synergy linear accelerator together with a custom designed floor couch. The couch, positioned 102.5 cm below the machine isocentre, provides treatment distances near 180 cm SSD. The couch is oriented in the gantry rotation plane, with couch motion along the cranial‐caudal axis enabling a match of beam divergence through patient translations and gantry rotations. Treatment is delivered by a set of 2 to 3 divergence matched abutting fields, with field modulation feathering junctions through 4 cm on the patient. Treatment plans are created using conventional beam models in Pinnacle 7.6C and whole body CT scan data. Independent plans for supine and prone orientations are constructed to deliver a uniform dose at mid‐separation throughout the patient and create a composite uniform dose. Segmentation is used to adjust the dose at mid‐plane, correcting for effects of patient thickness, inverse square law, and lung density. Results: A total of 11 patients have been treated with this new technique. Dosimetry measurements in phantom at extended distance and in‐vivo measurements have demonstrated an accurate dose delivery. Composite AP‐PA dose assessments based on contributions to uniquely identified anatomical points have shown that a dose within 10% of the prescribed dose is achieved throughout the treatment volume. Conclusions: A new TBI technique has been implemented which employs modern imaging and delivery methods to achieve a uniform patient dose. The technique utilizes standard equipment, and does not require specialized bunker design.

SU‐FF‐T‐343: Peripheral Dose Variations with Different IMRT Delivery Systems

Purpose: To obtain an accurate assessment of the relationship between peripheral dose and beam modulation among three commercially available IMRT delivery systems. These systems differ as a consequence of collimator jaw behaviour during IMRT field delivery, with the Varian system using static jaw settings encompassing the fluence from all segments, the Elekta Synergy jaws conforming to individual segment and the Elekta Synergy‐S maintaining a maximum collimated opening for all fields independent of MLC segment shape. Method and Materials: Fields with various degrees of modulation were created using Pinnacle 7.6C to deliver a uniform dose in phantom. Rotated rectangular apertures were used to engage both MLC leafs and jaws in aperture definition. Fields were characterized by an intensity modulation factor (IMF), defined as the ratio of the IMRT MU to that of an open field delivering the same dose. Multiple ionization chamber measurements were made for each delivery system outside the delivered field in solid water at 10cm depth. Results: The Elekta Synergy accelerator delivered the lowest peripheral doses. The Varian produced intermediate peripheral doses and the Elekta Synergy S produced the largest peripheral doses. For an IMF of 8, the peripheral doses 6cm from the field edge were 1.8%, 3.1% and 8.2% for the Elekta Synergy, Varian and Elekta Synergy S, respectively. Conclusion: Differences in peripheral dose levels were measured and compared among a number of available delivery systems for IMRT type treatments. As with all radiation treatments, efforts should be made to minimized the dose to normal tissues. The results of this study extend the information available for balancing treatment benefits against the risk of late side effects and secondary cancer induction, particularly when a choice of delivery platforms is available to a center.

SU-FF-T-184: Dosimetric Verification of a Novel TBI Technique Using Segmented Radiation Fields

Purpose: To evaluate the dosimetric performance of a novel CT planning based total body irradiation (TBI) technique, which utilizes a conventional linear accelerator in a standard bunker and employs segmented beams for dose uniformity. Methods and Materials: An extended distance TBI technique at ∼180 cm SSD has been developed for a conventional linear accelerator using a custom couch aligned along the plane of gantry rotation. Up to 3 abutting fields are used to cover the patient, with divergence matching achieved through a combination of gantry angles and couch shifts. Treatment planning was performed using a full body CT scan and standard beam models within the Pinnacle 7.6C TPS. The dosimetric verification consisted of: (i) pre‐treatment patient specific dose measurements in phantoms and (ii) in‐vivo dosimetry using MOSFET and films. Similar to the process of IMRT verification measurements, treatment plans were transferred to a 40×40×20 cm3 phantom and compared to ionization chamberdose measurements in the same geometry. In‐vivo dosimetry measurements were performed using paired MOSFET detectors (at entrance and exit) at the level of the umbilicus and at the level of the mediastinum. Junction doses were assessed by in‐vivo exit dosimetry using films on the couch. Results:Dose measurements in phantom for 11 clinical TBI patients were found to agree with Pinnacle to within (0.96±0.71)%, with a maximum deviation of 2.08%. The average agreement between the in‐vivo dose measurements and calculated values were found to be within (0.44±4.33)%, with a maximum deviation of 10.7%. No significant dose differences were observed along the junctions. Conclusion: This work validates the accuracy of dose delivery for this novel TBI technique, developed with Pinnacle 7.6C. The segmented fields and the matching of divergent field edges provide adequate dose uniformity throughout the treatment volume.