ESTRO 36 Abstract Book

S958 ESTRO 36 _______________________________________________________________________________________________

Techniques", run by the Health Section of the International Atomic Energy Agency (IAEA). The participants of the audit were obliged to irradiate provided dosimetric films, in a slab phantom, for a specific leaf arrangement, producing a pattern of five stripes, commonly called a picket fence. The participants had to programme such a pattern so that the stripes are 5 mm wide and are 3 cm distant between themselves. The Gafchromic EBT2 radiochromic films were placed in a slab phantom close to maximum dose depth. The irradiation was 250 MU per stripe. Results Thirty two Polish radiotherapy centres took part in the audit. They were equipped with various accelerator types and various treatment planning systems. In all cases the 6 MV quality beams were used. The discrepancies between measured and expected stripe positions were in the range 1.2 mm. For particular participants, the leaf position discrepancies were in the range -0,5 mm to 0,5 mm. For particular participants, the mean opening width measured with films for each pair of leafs was between 6 and 8 mm. Conclusion In the audit, the best performance showed the new type multileaf collimators with 120-160 leafs, The worst performance showed collimators MLC80 from Elekta. The results of the audit are very useful for the participants who should carefully investigate the performance of their multileaf collimators. EP-1741 Commissioning of a robotic patient positioning system equipped with an integrated tracking system A. Ableitinger 1 , A. Utz 1 , A. Zechner 1 , S. Vatnitsky 1 , M. Stock 1 1 EBG MedAustron GmbH, Strahlentherapie, Wiener Neustadt, Austria Purpose or Objective Robotic patient positioning systems (PPS) used in clinic must consider weight-induced couch bending and must show high reproducibility and stability to achieve the required positioning accuracy. Extensive commissioning of these robotic systems is therefore crucial. The aim of the current work is to determine the positioning accuracy of the PPS, that is equipped with an integrated optical tracking system. Material and Methods Three different aspects of the PPS were investigated in this study: the basic characteristics including couch bending, reproducibility and stability; the relative spatial deviation in terms of rotation and couch height and the absolute accuracy of the treatment couch. The treatment volume of the PPS has a dimension of 115cm x 50cm x 40cm. The robotic system enables couch rotations of more than 190°, pitch and roll of ±3° and non- isocentric treatment positions. A photogrammetric camera tracks the treatment couch of the PPS via reflecting markers mounted on the bottom side of the couch (see Fig.1). An iterative position correction loop aligns the couch to the prescribed position. The reference instrument was a laser tracker with reflecting probes. Drilling holes near the indexing positions (H4-F9) located laterally along the couch every 14cm served as measurement positions for the evaluation of the basic characteristics and the absolute couch position. For the relative deviation the drift of one measurement point on the couch was evaluated. To determine the bending of the couch it was loaded with six different weights up to 156kg. The reproducible positioning for the same couch position (different axis setting) and the couch stability after 1 hour were measured with the highest payload. The evaluation of the absolute spatial deviation was based on six measurement points being closest to the room

isocenter. They were compared with their expected coordinates for 1020 different robot positions, poses and payloads.

Results The

basic couch-bending, reproducibility and stability were within 0.10mm within the treatment volume (see Fig.2 a, b). The relative spatial deviation was smaller than 0.40mm for rotations ranging up to 200° (see Fig.2 c). For different vertical positions, the couch drifted less than 0.25mm for 2 different loads and rotations. The differences between the prescribed and measured absolute position were evaluated in terms of histograms showing the overall 3D deviation. In 95% of all measurement points the 3D accuracy was better than 0.63 mm (see Fig.2 d). Regarding the weight-induced couch-bending no correlation between the accuracy and payload could be found. characteristics of

Conclusion The results show that the important spatial properties of the patient positioning system are well within the acceptable clinical tolerances. The very high reproducibility of the PPS allows further optimization of the absolute position. The measured datasets serve as new input for a high accuracy calibration. EP-1742 Optimisation and implementation of brain CBCT templates; an institutional pilot study. S. Petkar 1 , N. Lalli 2 , F. Solda' 1 , C. Gillies 2 , S. Moinuddin 1 , N. Fersht 1 1 UCLH NHS Foundation Trust, Radiotherapy, London, United Kingdom 2 UCLH NHS Foundation Trust, Radiotherapy Physics, London, United Kingdom Purpose or Objective Volumetric imaging (CBCT) in brain has facilitated the use of volumetric delivery techniques and reduction of