ESTRO 36 Abstract Book

S952 ESTRO 36 2017 _______________________________________________________________________________________________

angle. A consecutive set of 16 images allowed for an analysis giving the distance of the table rotation axis to the collimator rotation axis. We were able to adjust the table position slightly to obtain accuracies necessary for stereotactic application. Results The method was tested, and we found an accuracy (1 SD) of 0.01 mm. Four new Elekta accelerators (Versa HD) were analyzed according the procedure. The main contribution to isocentric inaccuracy for Elekta linacs is the gantry sag. By adjusting the table rotation axis to a position between the collimator rotation axes at gantry 0° and 180°, isocentric accuracy can be optimized. The table presents the results that were obtained. Table: r isoc : the size of the isocenter quantified by the radius of the sphere containing rotation axes when applying several gantry angles at zero table angle. d table- coll : the distance between rotation axis of the table and the rotation axis of the collimator. Linac r isoc (mm) d table-coll (mm) A 0.68 0.20 B 0.53 0.14 C 0.77 0.06 6 0.38 0.01 Figure: an EPID image of a 10x10 cm 2 field and the ball bearing. The field outline and the detected ball are overlayed. Conclusion With our method it is possible to quickly obtain a measure for isocentric accuracy. In combination with table rotation we achieved accuracies better than 0.9 mm, after adjusting the table. EP-1755 Multi-modality end-to-end audit by the ACDS J. Lye 1 , F. Gibbons 1 , M. Shaw 1 , A. Alves 1 , S. Keehan 1 , I. Williams 1 1 Australian Radiation Protection and Nuclear Safety Agency, Australian Clinical Dosimetry Service, Melbourne- Victoria, Australia Purpose or Objective The Australian Clinical Dosimetry Service (ACDS) has commissioned a custom phantom and audit incorporating conformal, IMRT, VMAT, and FFF modalities. The design covers future inclusion of small field and SABR modalities. The vision of the ACDS is to provide a comprehensive suite of audit modalities covering all common clinical practice, ultimately to ensure patient safety and to improve national dosimetry. The ACDS also aims to provide dosimetric information that can be used domestically and globally in the clinical trial setting Material and Methods To ensure efficient delivery of the audit service, all modalities relevant to a facility’s clinical practice are measured in a single audit visit. The incorporation of new audit modalities requires a consideration of phantom design suitable for multiple modalities and limitations on facility and ACDS workload. Classification of new

Figure 1: Transmission through a closed endplate in a 10x10cm^2 applicator for both materials. There is higher transmission through the lead endplate compared to the Cerrobend endplate for all energies but even at the highest energy the difference is only 0.74%. The measured PDDs agree with each other to within 1.2mm for all energies. The discrepancies were observed near the surface of the PDD curves. The output factors measured in solid water using the lead inserts agreed with the commissioning values obtained with the Cerrobend inserts to within ±1% apart from the 6e 10x6cm 2 applicator in which a 1.7% difference in output factor was observed between the lead and Cerrobend. This difference could be due to inaccuracies in the solid water setup as the effective point of measurement of the NACP was estimated to be 1mm. The profiles measured agreed very well, with the largest discrepancies occurring out of field for the higher energies both crossplane and inplane. This is due to the higher transmission through the lead cut-out at higher energies. Conclusion As there was very good agreement between the lead and Cerrobend inserts and cut-outs for all the tests performed, it can be concluded that using the lead cut-outs is dosimetrically similar to the Cerrobend inserts with which the eMC algorithm was configured. EP-1754 Isocentric accuracy of Elekta VersaHD linear accelerators E. Kouwenhoven 1 , J. Van Egmond 1 , J. Van Wingerden 1 , M. De Goede 1 , M. Mast 1 , J. Van Santvoort 1 1 Haaglanden Medical Centre Location Antoniushove, Radiation therapy, The Hague, The Netherlands Purpose or Objective The demands on isocentric accuracy are high when accelerators are used for stereotactic treatments. The determination of the optical or mechanical isocenter is inadequate for this purpose, and instead we aim for a procedure to find the megavoltage isocenter. The radius of the smallest sphere through which all rotation axes pass when various collimator, gantry and table angles are applied, is what we use as a measure to quantify the isocentric accuracy. The purpose of the present study is to (1) give an accurate measure of the size of the region containing the megavoltage isocenter, and (2) establish the distance between the rotation axes of table and collimator, respectively. We developed a highly accurate method to determine the isocentric accuracy of Elekta VersaHD linear accelerators. Material and Methods The tests were performed on all 4 Elekta VersaHD accelerators in our institute. We applied a modified Winston-Lutz test, using a phantom containing a radio- opaque ball, and imaged this phantom onto the EPID using a 10 x 10 cm 2 field. Using the information of a considerable region of the image, it was possible to find the geometry (i.e., ball center and field outline) in much more detail than just subpixel accuracy. Using a set of at least 8 images with various gantry and collimator angles we could accurately obtain the isocentric accuracy per gantry