ESTRO 36 Abstract Book

S506 ESTRO 36 _______________________________________________________________________________________________

3 Czech Technical University in Prague, Institute of Experimental and Applied Physics, Prague, Czech Republic 4 German Cancer Research Center DKFZ, Department of Medical Physics, Heidelberg, Germany Purpose or Objective (1) Purpose: In highly conformal radiotherapy, like ion beam radiotherapy, inter- and intrafractional monitoring of the target is desirable. Due to the steepness of the rising part of the Bragg curve, ion beam radiography can in principle provide high resolution of the traversed tissue thickness. Ion beam radiography is furthermore attractive due to its potential to measure the stopping power of the tissue directly. However, currently there is no detection system for clinical imaging of patients. Helium ions as the imaging modality provide the advantages of decreased multiple scattering in comparison to protons and lower biological effectiveness than the carbon ions. Material and Methods (2) Methods: Plastic phantoms contain ning 1mm deep step-like inhomogeneities were imaged with helium ion beams at the Heidelberg Ion Beam Therapy facility in Germany. To register the radiation, a system of 5 parallel layers, based on the semiconductor pixelized detector Timepix, which was developed by the Medipix Collaboration at CERN, was used. Two layers in front of the phantom enabled us to measure the position and direction of incoming helium ions. Another pair of detection layers, located behind the phantom, registered the outgoing particles and an additional layer was used to measure their energy loss and to identify the ion type. Synchronization of all the five detector layers enabled us to associate the outgoing particles to the incoming ones. To build the image of the phantom, we used the measured information about the transversal position of the incoming and outgoing particle, their direction and type (He or H). Results (3) Results: With this system we imaged a 1 mm step in a 160 mm thick PMMA phantom. Spatial resolution below 2 mm was reached when the inhomogeneity was located in the phantom, while resolution below 1 mm was achieved in the cases where the step was located at the front or at the end of the phantom. Hereby we have shown that the information about flight direction of the incoming and outgoing ion, together with the capability to identify them and thus select solely helium ions, enables to improve the spatial resolution by a factor of more than three. Conclusion (4) Conclusion: We have shown experimentally that helium beam radiography reaches in simple phantoms spatial resolution in the region which is attractive for highly conformal radiotherapy. In the presentation the results obtained with helium beams as the imaging modality will be compared to proton-based imaging. PO-0915 Performance study of a prototype straight- through linac delivery system with an EPID assembly R. Scheuermann 1 , C. Kennedy 1 , D. Mihailidis 1 , J. Metz 1 1 University of Pennsylvania, Radiation Oncology, Philadelphia, USA Purpose or Objective To study and expand the use of the Machine Performance Check (MPC) tool in monitoring the continuous operational performance of a prototype delivery system composed of a straight-through-linac and an in-line MV portal imaging panel (Proof-of-Concept). Material and Methods The MPC, as implemented in TrueBeam (TB), is an integrated self-check tool that assures that critical machine performance are within specifications, e.g. mechanical accuracy and radiation output. As adapted to the prototype straight-through linac delivering 6X-FFF (filter-free) beam, the automated tests are based on its

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 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