ESTRO 36 Abstract Book

S502 ESTRO 36 2017 _______________________________________________________________________________________________

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

in-line MV EPID. The MPC acquires a series of MV images of an IsoCal phantom, capturing beam properties and mechanical data such as MLC and gantry accuracies. A new MPC test monitors output stability in terms of percent deviation from the baseline data of the actual measured beam. All measured data are automatically processed, analyzed, and displayed for evaluation, thus providing a reliable and fast method for routine machine performance assessment. Independent tests such as star-shots, Winston-Lutz, MLC picket fence patterns and output measurements on a daily basis were employed to benchmark the MPC test results for the prototype system. Results MPC results were collected daily for six months on both the prototype and a TB. The independent tests on the prototype system were repeated weekly to validate the MPC results. A sample comparison of the MPC results for the prototype against independent tests are shown in Table 1. The output stability of the prototype system, as measured with the MPC and a DailyQA TM 3 device, is comparable (Fig. 1), and within 0.5% of independent output measurement for the period shown. All tests performed were within the tolerances allowed by the MPC and agreed in most cases with the result of the independent tests. The prototype system performs as well as the TB system. A summary of MPC test results and comparisons with independent measurements will be shown alongside with the TB MPC results. Table 1

Figure 1

Conclusion The performance of a straight-through linac d elivering 6X- FFF (filter-free) beam with an EPID panel wa s investigated with the MPC testing tool and that method w as validated against independent tests for proof of con cept. MPC is a complete, reliable and quick test suite that monitors the performance of a treatment unit on routine basis.