S. van der Meer

Integrating a MRI scanner with a 6 MV radiotherapy accelerator: impact of the surface orientation on the entrance and exit dose due to the transverse magnetic field

At the UMC Utrecht, in collaboration with Elekta and Philips Research Hamburg, we are developing a radiotherapy accelerator with integrated MRI functionality. The radiation dose will be delivered in the presence of a lateral 1.5 T field. Although the photon beam is not affected by the magnetic field, the actual dose deposition is done by a cascade of secondary electrons and these electrons are affected by the Lorentz force. The magnetic field causes a reduced build-up distance: because the trajectory of the electrons between collisions is curved, the entrance depth in tissue decreases. Also, at tissue-air interfaces an increased dose occurs due to the so-called electron return effect (ERE): electrons leaving tissue will describe a circular path in air and re-enter the tissue yielding a local dose increase. In this paper the impact of a 1.5 T magnetic field on both the build-up distance and the dose increase due to the ERE will be investigated as a function of the angle between the surface and the incident beam. Monte Carlo simulations demonstrate that in the presence of a 1.5 T magnetic field, the surface dose, the build-up distance and the exit dose depend more heavily on the surface orientation than in the case without magnetic field. This is caused by the asymmetrical pointspread kernel in the presence of 1.5 T and the directional behaviour of the re-entering electrons. Simulations on geometrical phantoms show that ERE dose increase at air cavities can be avoided using opposing beams, also when the air-tissue boundary is not perpendicular to the beam. For the more general case in patient anatomies, more problems may arise. Future work will address the possibilities and limitations of opposing beams in combination with IMRT in a magnetic field.

Patients’ misconceptions about surveillance for hepatocellular carcinoma: Education is needed

of performed surveillance tests), only lower education level was an independent predictor of misconception. Second, patients largely underestimate HCC-related mortality rates. Nearly 50% of patients think that curative treatment is still possible in P40% of HCC cases detected without surveillance (Fig. 1F). Furthermore, many patients believed that surveillance reduces the risk of HCC deaths by P70% (Fig. 1G). Unfortunately,

EP-1506: Evaluation of cranial setup accuracy: a double shell positioning system versus an in-house 3-point mask solution

Purpose/Objective: The aim of this study is to assess the stability of relative gold marker position due to deformation and marker migration for prostate cancer during the treatment course of image-guided VMAT external beam radiotherapy. Materials and Methods: 30 patients with localized prostate cancer, who underwent primary IGRT with implanted gold markers (MPB Marker Kit 1.2x3mm, MPB Scherer Medizinprodukte GmbH, A-Krustetten), were chosen for this study. 27 patients had four and three patients had three implanted markers. The gold marker implantation was carried out one week before the planning CT. The IGRT was carried out with kV-CBCT and orthogonal kV-Imaging (OBI Varian Medical Systems, Palo Alto CA). For this study 739 orthogonal kV-image pairs were evaluated: between 9 and 32 kV image pairs were evaluated per patient. The images were segmented with an edge algorithm (MATLAB R2013a, MathWorks). The centroid of the gold marker was identified to evaluate the gold marker position. The inter-marker distances were determined and compared to those identified in the planning CT. Results: The median marker distance was 23.9 ± 9.7 mm (SD). The marker distance varied from 5.5 to 45.7 mm. For 21 patients the marker distance variations remained under 3 mm. For five patients marker distance variations between 3 and 4 mm were identified. The variation exceeded 4 mm for four patients. For one of them the variation seemed to be the result of either prostate deformation related to organ filling in the planning CT or gold marker migration after the planning CT. In this case the mean distance variation between two markers was 5.7 ± 0.5mm (SD). Overall 640 of 739 IGRT sessions showed deviations in marker distance variation below 3 mm. 67 distance variations were between 3 and 4 mm. In 32 IGRT sessions the inter-marker distance exceeded 4 mm. Conclusions: In most cases the gold marker distance variation seems to be smaller than 3 mm, so that a good image matching can be achieved. If the variation exceeds 3 mm the matching might become more difficult. For patients with larger variation it should be determined if the change is the result of gold marker migration. Furthermore, if necessary, a new planning CT should be performed and the PTV margins appropriately adapted.

PD-0337 PROSTATE IGRT USING QUANTITATIVE 3D ULTRASOUND IMAGING: INTRAMODALITY US VS. MV IMAGING OF IMPLANTED MARKERS

To allow for high precision radiotherapy, imaging prior to a radiation session is performed in Image Guided Radiotherapy (IGRT). Electronic portal imaging (EPI) can visualise bony anatomy and fiducial markers (FM). However, for prostate cancer, visualising the organ itself is more desirable. Ultrasound (US) is a well known widely used diagnostic technique for qualitative imaging of soft tissues such as the prostate. Nowadays a quantitative intramodality 3D US system for radiotherapy image guidance is available (Clarity system, Elekta, Stockholm, Sweden). In this work a comparison was performed between 3D US image guidance and the currently used prostate localization procedure.

TH‐E‐218‐03: On the Significance of Density‐Induced Speed of Sound Variations on Ultrasound‐Guided Radiotherapy

US systems assume that speed of sound (SOS) is constant in human soft tissues (at a value of 1540 m/s), while its actual non homogeneous distribution produces small but systematic errors of up to a few millimeters in the positions of scanned structures. This work aims at showing the effect of SOS aberration on ultrasound guided radiotherapy (US-gRT) as a function of implemented workflow.

WE‐A‐BRA‐03: Towards Real‐Time Ultrasound Image‐Guided Abdominal Radiotherapy

Accurate tumor positioning in stereotactic body radiotherapy of abdominal lesions is often hampered by organ motion and set-up errors. The problems of the daily variation (interfractional motion) in tumor position can be partially corrected with on-line soft-tissue image-guided radiotherapy. However, a greater challenge is to also capture the movement during treatment (intrafractional motion).

529 oral CLINICAL EVALUATION OF A SPEED OF SOUND ABERRATION CORRECTION ALGORITHM IN QUANTITATIVE ULTRASOUND-AIDED IMAGE GUIDED RADIOTHERAPY.

To investigate the clinical impact of a recently introduced speed of sound (SOS) aberration correction algorithm in ultrasound (US) imaging systems for image guided radiotherapy (IGRT). The SOS algorithm was applied to four prostate cancer patients and two liver cancer patients to assess in a real clinical application the extent and spatial distribution of the corrections

1208 poster A NEW SOLUTION FOR IGRT IN SBRT OF LIVER LESIONS, BASED ON 3D US IMAGING COMBINED WITH ACTIVE BREATHING CONTROL

Accurate tumor positioning in SBRT of liver lesions is of major importance, because high dose gradients are delivered in a few fractions. The use of electronic portal imaging (EPI) for verification is not sufficient, because lesion displacement can be independent of bony anatomy. Also Cone Beam CT (CBCT) does not help to visualize the lesions itself, since no iv contrast is used. Therefore, we implemented 3D ultrasound (3DUS) as IGRT tool in SBRT of liver lesions Our A 3DUS device (Clarity, Elekta) was investigated for SBRT of 8 patients on 58 treatment fractions. In 2 patients at 16 treatment fractions the 3DUS was also performed under ABC conditions using the SDX system (DynR, France). The 3DUS and mid-ventilation CT images were fused, the lesion together with surrounding structures were contoured and beams and isocenter . previous results demonstrated that liver deformation and blurring of the US scan as a result of breathing motion, influenced the hampered our image segmentation. As a result we studied the additional value of Active Breathing Control (ABC) during 3D US.

TU‐A‐220‐02: Speed of Sound Aberration Correction in Quantitative Ultrasound‐Aided Image Guided Radiotherapy

To correct speed of sound (SOS) aberrations in ultrasound (US) imaging systems for quantitative image guided radiotherapy (IGRT) applications. US waves travel at different speeds in different human tissues. Conventional qualitative US-based imaging systems assume instead that the SOS has the constant value 1540 m/s, which is an accepted average value for soft tissues. This assumption is a systematic source of errors and image distortion in quantitative US imaging, whereas in qualitative diagnostic imaging it is usually unimportant. Methods: In 3D US IGRT applications, at the simulation stage along with the computerized tomography (CT) scan an US scan is performed, in the same coordinate system. A relationship between the physical density information, provided by the CT scan, and the SOS can be experimentally established and used to transform the CT scan of the patient in a SOS map of the tissues. According to the local values provided by the latter, every US voxel is resized along the radial direction to the dimension correspondent to the distance traveled by sound at the correct speed.