ESTRO 36 Abstract Book

S505 ESTRO 36 _______________________________________________________________________________________________

average dose, following irradiation with periodically modulated and open 6MV photon fields. Results Survival of normal cells in a 2.5mm striped field was the same as for an open field, but the survival of the cancer cells was significantly lower. However for cancer cell lines in the 5.0mm modulated fields, the response compared to an open field was no longer statistically significant. A mathematical model was developed to incorporate the dose gradients of the spatial modulation into the standard linear quadratic model. Our new extended bystander LQ model assumes spatial gradients drive the diffusion of soluble factors that influence survival through bystander effects. The model successfully predicts the experimental results that show an increased therapeutic benefit. Conclusion We have confirmed that HDMLCs can create spatially modulated fields that increase the therapeutic advantage between normal and cancer cells. Our results challenge conventional radiotherapy practice and propose that additional gain can be realized by prescribing spatially modulated treatments to harness the bystander effect. PO-0912 Short- and long term stability of the isocenter of a three-source Co60 MR guided radiotherapy device Purpose or Objective Recently a 0.35T Co 60 MRIdian system (Viewray Inc., Cleveland) is implemented at our institution. In a similar way as for other image guided radiotherapy techniques, the coincidence of the radiation therapy (RT) and imaging isocenter is of major importance. The purpose of this study is to present a method for daily QA of MR-RT isocenter coincidence and to assess its short- and long term stability using daily film-based isocenter QA. Material and Methods Two pieces of radiochromic film (GafChromic EBT3) are taped to square inlays on the top and the side of a cubic water-filled phantom. The phantom is aligned to the MRI isocenter using MR guided setup relying on three internal cylindrical markers. D. Hoffmans 1 , M.A. Palacios 1 , J.P. Cuijpers 1 1 VU University Medical Center, Radiotherapy, Amsterdam, The Netherlands

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.

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

A treatment plan, consisting of an AP and a lateral square field is delivered. The direction of the lateral field is altered daily between 90 o and 270 o in order to monitor all 3 treatment heads (Head 1 and 2 for the lateral fields respectively, head 3 for the AP field). The films are digitized and the positions of the square fields with respect to the phantom are determined. These data provide a daily measurement of the coincidence of the RT- and MR- isocenters in 3 dimensions. The AP field provides