ESTRO 36 Abstract Book

S230 ESTRO 36 _______________________________________________________________________________________________

addition, we found that irradiation is inducing CSC marker and CSC properties in a dose- and time-dependent manner. This irradiation-induced CSC-plasticity was attributed to the modulation of the histone methylation code. Within the present study we analyzed a panel of secreted cytokines and their corresponding cytokine receptors in the radioresistant prostate cancer sublines, in a s.c. xenotransplantation model, in ex vivo irradiated primary prostate cancer biopsies and in blood samples of prostate cancer patients during the course of radiotherapy and found, for example, the CXCR4-CXCL12 signaling to be involved in the CSC maintenance and the induction of prostate cancer radioresistance. Conclusion Our studies suggest that the combination of irradiation with cytokine signaling modulation, especially the CXCR4- CXCL12 signaling, may increase the cytotoxic effects of irradiation in prostate cancer cells. The expression profiling of proteins involved in the cytokine signaling can be used to predict clinical outcome of prostate cancer patients after radiotherapy. OC-0437 Scatter imaging: promising modality for image guided ablation radiotherapy for lung cancer patients J. Chu 1 , G. Redler 1 , G. Cifter 1 , K. Jones 1 , J. Turian 1 1 Rush University Medical Center, Department of Radiation Oncology, Chicago IL, USA Purpose or Objective Early stage lung cancers can be effectively treated by stereotactic ablation radiation therapy (SABR). Successful treatment requires hypofractionation and large dose per fraction (up to 20 Gy) while maintaining a high level of accuracy (≤1.0mm). By imaging the photons that are Compton-scattered out of the treatment beam, real-time, non-invasive monitoring of the tumor location may be possible. To assess the potential of this modality, we have obtained scatter images of static and movable tumor phantoms, and calculated images from CT-based Monte Carlo simulations. Material and Methods Compton scatter is a natural by-product of external beam radiation therapy. The scattered radiation contains information about the patient anatomy and the transient tumor location. An embedded tumor in a Quasar respiratory motion phantom (Modus Medical Devices Inc.) was programmed to move linearly over 2.5cm. While irradiating the embedded tumor using a 6MV Varian TrueBeam linear accelerator, experimental scatter images were measured with a Varian PaxScan flat panel detector and a pinhole collimator. Tumor centroid locations were then measured from various scatter images and compared with the expected values. Monte Carlo N-Particle (MCNP) code was used to simulate scatter images from phantoms and patient CT images using 10 - 1000MU, or 0.5 – 50 second time scales. The quality of the images was assessed to determine their potential for tumor localization during treatment. Results The measured tumor centroid locations agreed with the expected values to within 1mm, which is adequately accurate for clinical tumor tracking. Lung tumor phantom images showed excellent signal and contrast. The contrast-to-noise ratio ranged from 3.4 to 15.1 for scatter images acquired with 0.5 to 50s. The attached figures below show CT and simulated scatter images (corresponding to the red shaded region in CT) for both inhale and exhale breathing phases. The scatter images clearly show variation of tumor and diaphragm locations for two breathing phases. Other pertinent anatomical Proffered Papers: New technologies for imaging and therapy

structures, such as chest wall, heart, and lung are also clearly visible. Conclusion This study has demonstrated the feasibility of using scatter imaging to track lung tumor movement during SABR treatments. The potential benefits may include real- time, good quality image guidance without additional radiation. We are optimistic that improvements in the detector and collimation system, will lead to better image quality, more accurate tumor localizations, and possible 3D reconstruction of patient’s anatomy within the irradiated region.

OC-0438 The impact of a 1.5 T MR-Linac fringe field on neighbouring linear accelerators. T. Perik 1 , J. Kaas 1 , F. Wittkamper 1 1 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective In our institution a clinical prototype of the MR- Linac(MRL)(Elekta AB, Stockholm, Sweden) was installed in an existing treatment room. The MRL, which has a field strength of 1.5 T, is neighboured by 3 clinical Elekta accelerators at a distance (Isocenter MRL to gun linac) of 7.5 and 5.5 and 11 meters. The peripheral magnetic field outside of the magnet core of the MRL, the fringe field, may influence the beam steering of accelerators in adjacent treatment rooms. This influence for a pre- clinical prototype was described by Kok et al. in 2009. The aim of this study is to investigate the influence of the significantly reduced fringe field of the clinical prototype on the beam steering of its neighbouring accelerators. Material and Methods A STARCHECK MAXI detector array (PTW Freiburg, Germany) was mounted on all the neighbouring accelerators with a frame that puts the array on isocentre height. An inclinometer was attached to the gantry to acquire a gantry rotation signal. For every available energy, two 360 degree arcs (clockwise and counter clockwise) were irradiated with a 40x40 field. A measurement of beam profile was acquired in movie mode at a frame rate 2.5 Hz. Beam symmetry (IEC) was determined for every frame. These measurements were done before and after ramping up the MRL magnet, and a 3 rd time after adjusting the look-up tables (LUT) which correct the beam steering by applying a gantry-angle dependent current to the steering coils (2R and 2T). These LUTs were adjusted using the accelerator internal monitor chamber. Results A change in beam symmetry as a function of gantry angle, before and after ramping the magnet, of up to 4% (Linac A) and 1% (Linac B) is observed, causing beam symmetry on both linacs to be out of tolerance (IEC 102%). Linac C did not show any significant change. Figure 2 shows the LUT before ramping the magnet (pre) and after adjustment (post) and the difference for Linac A for the 10 MV beam. After adjustment of the beam steering on Linac A and B, the symmetry was within tolerance for all gantry angles. Adjusting the LUTs took 1.5 hours per linac. Conclusion