Accuracy study. Elekta Synergy S. High precision radiation therapy using Elekta Synergy S. UMC, Utrecht, Netherlands. Institution: Purpose:

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1 S T E R E O T A C T I C R A D I A T I O N T H E R A P Y Elekta Synergy S High precision radiation therapy using Elekta Synergy S Institution: UMC, Utrecht, Netherlands Purpose: The primary application of Elekta Synergy S at UMC, Utrecht, is frameless intracranial high precision radiation therapy, using multi or single fraction, static or arc techniques. A convergent beam irradiation (3) technique is used to treat patients on this machine, using XVI and 3D VolumeView TM imaging to verify the patient position, correct the table position and then reconfirm the new position. Accuracy study

2 High precision radiation therapy using Elekta Synergy S Elekta Synergy S commissioning team Marjolein Baarda Gijsbert Bol Minique Boere Arther van Bruggen Corine van Es Fred Groen Mark Harms Erick Kouwenhoven Stephen Kwa Rogier Schokker Theo van Soest Ties Timmers Gitta van Vliet Eric Westzaan Hans Welleweerd Andre Wopereis Acceptance tests were performed on the following items to ensure the suitability of Elekta Synergy S for the job in hand. Mechanical/geometrical accuracy tolerance 0.5mm Field definition: strip test/calibration Rotational accuracy of diaphragm/collimator, gantry and table Sources of errors such as mechanical sag or misalignments are addressed. Method and results Beam modulator TM Strip test (1) measurements have been performed on Beam Modulator TM every week since installation to assess the accuracy of the leaf position. So far there has been no structural deviation recorded beyond our tolerance of 0.5mm for both relative positioning error (RPE) and absolute positioning error (APE), consequently no adjustments have been required. Figure 1: (above) set-up of the strip test. (far left) absolute positioning error (in 0.1mm): top average for every leaf pair bottom average for every abutment. relative positioning error (in 0.1mm): top average for every leaf pair bottom average for every abutment.

3 Collimator rotation For acceptance of the diaphragm rotation a film was placed perpendicular to the beam axis on the table. A fieldsize of 0.8 x 20cm was set and 12 beams were irradiated at 15 increments. Figure 2: (far left) a diaphragm rotation film circular profile of the 12 beams (24 peaks) This was performed with the gantry at 0, 90 and 180. The film was scanned into the software at high resolution (0.2mm). The beam axes of the beam were reconstructed using a circular profile (2). The isocenter here is defined as the point with the smallest maximum distance to any beam axis. The isocenter is determined by minimizing this maximum distance with movement of the isocenter. The diaphragm rotation error is the maximum distance found: every beam-axis should hit the tolerance circle. Figure 3: collimator rotation 10MV gantry 0: dev. 0.08mm Gantry rotation To measure the accuracy of the gantry rotation, a film was placed in the trans axial position. A fieldsize of 0.8 x 20 was set and six beams were irradiated at 30 increments from 15 to 165, and a further six beams from 210 to 360. This was done with the diaphragm at 90 and rotation clockwise and, with the diaphragm at 270 and rotation counter-clockwise. Figure 4: (far left) Spoke film with gantry rotation Gantry rotation ccw. 10MV coll. 270: dev. 0.38mm

4 Table rotation A similar test was performed to ensure accuracy of the table position; a film was placed in the coronal position. Table rotations from 90 to +75 were used. To determine the AB error the test was repeated at gantry angles 0 and 180 and to determine the GT error the test was repeated at 90 and 270. Figure 5: (top) table rotation 6MV GT gantry 90/270 : dev. 0.22mm (bottom) table rotation 6MV AB gantry 0/180 : dev. 0.36mm

5 Gantry sag Gantry sag can also be a source of error and the extent of this required investigation. A film was placed on the table in the coronal plane and five strips of 0.8 x 10cm irradiated (extending to, but not over, the center), firstly with the gantry at 0 and then with the gantry at 180. The average distance of the corresponding beam axes was calculated at zero and the measured value was 0.92 and 1.05mm for 6MV and 10MV. Gantry sag was 0.5mm but this averages out with the convergent beam irradiation (CBI) technique to produce a negligible effect, see figure 6b. Figure 6: (above) 6a: gantry sag measurement with film 6b: influence of gantry sag on penumbra of an AP-PA technique Geometrical calibration of XVI A ball bearing test was used to generate the flex map. The flex map corrects for the sag and flex of the XVI panel during gantry rotation. The ball bearing is positioned at the laser intersection isocenter and the exact position of the ball bearing relative to the isocenter is determined using MV beams and iviewgt TM. The ball bearing is then placed in the exact isocenter and the position of the ball bearing is measured on the XVI at all gantry angles. The flex map can be determined from this data. Figure 7: geometrical calibration of XVI

6 XVI image quality The CATPHAN phantom is used to measure the image quality of the Elekta Synergy S kv images on both contrast and detail resolution. Detail >7lp/_cm_ and low contrast value < 2%. Chain test This involved executing a full patient procedure using a phantom and film to record the dose, and aluminum bars to mimic bones. Figure 8 shows the scope of the chain test. The phantom used for the chain test was a polystyrene block 30 x 30 x 10, containing four aluminium bars and a film placed in mid-plane. Figure 8: scope of the chain test Figure 9: chain text phantom in trans axial, coronal and sagittal view The process involved taking a CT scan of the phantom, scan data was transferred to Nucletron PLATO where a CBI plan was created centred on the bars and film, and then the plan data was sent to Elekta Synergy S and the XVI workstation. The phantom was then set-up on the Elekta Synergy S table and a VolumeView TM image was performed. The phantom position was corrected following image registration on the XVI workstation and then the phantom was irradiated using the plan created in PLATO. Finally the film in the phantom was analyzed and by subtracting the dose delivered by the CT and VolumeView TM scans, the CBI positioning error could be determined. Figure 10: chain test - determining the geometrical error Y-profile, error on center = 0.24mm (right) X-profile, error on center = 0.05mm

7 Conclusions The chain test is repeated every month, the strip test every three weeks and XVI ball bearing test and image quality test are performed weekly. Results showed that the maximum deviation from the center, for Beam Modulator TM, was 0.15mm for the diaphragm rotation and 0.40mm for gantry rotation. Table isocentric rotation accuracy was defined as 0.35mm. The chain test demonstrated a geometrical error of less than 1mm. This enabled us to conclude that Elekta Synergy S meets the criteria for high precision radiation therapy as defined by EORTC recommendations. Our future plans are to use Elekta Synergy S to treat brain metastases, meningiomas, pituitary, lung, prostate and some ad-hoc high precision patients not only with static treatment techniques, but also with dynamic techniques. Figure 11: results of spoke films for acceptance Figure 12: chain test results Dynamic techniques are work-in-progress and not available on the current product configuration. Utrecht is an Elekta research site. References 1 Sastre-Padro M. An accurate calibration method of the multileaf collimator valid for conformal and intensity modulated radiation treatments. Phys. Med. Biol Jun 21: 49(12): Treuer H. Hoevels M, Luyken K, Gierich A, Kocker M, Muller RP, sturm V. On isocenter adjustment and quality control in linear accelerator-based radiosurgery with circular collimators and room lasers. Phys. Med. Biol Aug: 45(8): Pastyr O, Hartmann GH, Schlegel W, Schabbert S, Treuer H, Lorenz WJ, Sturm V. Stereotactically-guided convergent beam irradiation with a linear accelerator: localization technique. Acta Neuorochir. (Wien). 1989;99 (1-2): 61-4.

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