ESTRO 36 Abstract Book

S501 ESTRO 36 2017 _______________________________________________________________________________________________

information about the alignment along the lateral (X) and the longitudinal (Y) axis. The lateral fields provide the shift in the vertical (Z), and the Y directions. Data were collected daily over a period of 4 months. A linear regression is performed in order to determine any trends in time. Furthermore, the correlation between the two daily values for the shift along the Y-axis is assessed. Results The average 3D vector of the daily shift is found to be 0.8mm (P95 = 1.3 mm). The average misalignments as determined by the individual heads are shown in Table 1.

practice without any guidance. Responses were evaluated by the EAG and used to inform on best practice and identify centres where additional support was required. Results are given here for PTV margins used for metastatic disease. PTV margins are particularly important when treating multiple mets as they can increase the volume of normal brain irradiated and the commissioning criteria requires the total treated volume to be below 20cc, so the choice of PTV margins can impact patient eligibility for treatment. Results All 20 centres responded to the questionnaire with one centre excluded as they were in process of changing equipment. Responses are summarized in Table 1.

The systematic shift in 3D is zero in X and Y direction and 0.2 mm in the Z direction, which is caused by th e vertical shift measured with head 1. No time trend in the shift is observed in any direction as the regression coefficients were not statistically significant different from zero: p=0.39, 0.64 and 0.50 for the X, Y and Z axes respectively. The pearsman correlation coefficient between the Y-shift determined using the two perpendicular fields was very weak and found to be 0.24. Conclusion A method is developed for daily assessment of the coincidence of the MV- and MR-isocenter for an integrated MR-RT unit. The alignment of the MR- to the RT-isocenter is found to be stable during a time period of 4 months. A small systematic shift in vertical direction was found, a star shot measurement confirmed that this was caused by a slight misalignment of Head 1. This misalignment can be compensated by realignment of the MLC leaf positions. The weak correlation in the pair of Y-measurements suggests that the daily misalignment is dominated by random measurement inaccuracies such as the placing markers on the film and rotational setup misalignments of the phantom. PO-0913 A national review of equipment, techniques and PTV margins used for SRS R. Patel 1 , J. Lee 1 , C. Walker 2 , D.J. Eaton 1 1 Mount Vernon Hospital, Radiotherapy RTTQA, Northwood Middlesex, United Kingdom 2 Northern Centre for Cancer Care, Radiotherapy, Newcastle, United Kingdom Purpose or Objective As part of a national commissioning programme, treatment providers were required to complete a SRS quality assurance review in order to benchmark current practice. The process was designed to ensure providers were able to deliver a service in line with parameters developed by a multidisciplinary expert advisory group (EAG). The long term aim of this programme was to progress a system of standardisation and quality improvement of service by promoting consistency and the development of services over time. The short term goals were to highlight any significant variation in practice in order to identify centres that may require further support or mentoring in order to meet nationally agreed parameters. Material and Methods A questionnaire was circulated to 20 centres to establish the equipment, treatment techniques and PTV expansions used to deliver SRS. Centres reported on their current

The most common platform used was the Elekta Gammaknife system. A variety of linacs were used, the majority of those used for the commissioning were specialised units (e.g. Novalis, Truebeam STx) or had been adapted (e.g. fixed cones) for SRS treatments. Centres used a 0, 1 or 2mm margin for brain mets. All Gammaknife centres used a 0mm margin, but other platforms varied depending on the centre as seen in Figure 1. Only four centres used a 2mm expansion for treating brain mets, three of these were non specialised linacs.

Conclusion There is significant variation in the equipment used to treat SRS nationally. A 0mm PTV expansion was the most common for SRS treatment regardless of platform. Gammaknife centres were consistent with their PTV margins, which is based on historical practice but other platforms varied depending on the centre. No system has an end to end accuracy of 0mm, however many centres are choosing to use, which may lead to under-coverage of the target. Following feedback, centres using non-specialised equipment are planning to acquire either stereotactic linacs or upgrades such as the Apex head, with some frameless users acquiring Exactrac systems to reduce uncertainty in patient positioning. These will facilitate margin reduction at centres using 2mm PTV expansions, in line with the ≤1mm recommended by the EAG. PO-0914 Helium Beam Radiography System based on pixelized semiconductor detectors T. Gehrke 1 , G. Arico 1 , S. Berke 2 , J. Jakubek 3 , M. Martisikova 4 1 Heidelberg University Hospital, Radiation Oncology and Radiation Therapy, Heidelberg, Germany 2 German Cancer Research Center, Medical Physics in Radiation Therapy, Heidelberg, Germany