Abstract Book

S1200

ESTRO 37

Material and Methods Stability in Hounsfield Units (HUs) over time for one kV- CBCT system as well as variation in HU between six kV- CBCT systems were measured. Phantom measurements were carried out with CIRS Model 062M Electron Density Phantom on the kV-CBCT systems (Varian On-Board Imager TM TrueBeam TM ). The HUs of one kV-CBCT system was also compared to HUs from a Siemens Somatom Definition AS+ CT system. Using 28 CBCT image sets from seven prostate cancer patients, a HU to relative electron density (RED) table was created. Four patients were used for treatment planning and sCT images was generated using the commercial MriPlanner TM software. The CT and CBCT images was rigidly registered towards the sCT, resampled and reshaped. An original treatment plan (RapidArc) were calculated on sCT image using the standard HU-RED table. The plan was transferred onto the CT and CBCT images and recalculated using the same HU-RED table. For the CBCT image, an additional calculation was done using the HU-RED table developed for CBCT (CBCT HU-RED). The difference between the dose distributions was evaluated using clinical dose volume histogram (DVH) criteria. Results The phantom measurements showed that the kV-CBCT system was stable in HU over time (SD < 40 HU for all density plugs in the range of 0.190 - 1.695 RED). All six kV-CBCT systems generated comparable HU values (SD < 70 HU). The variation of HUs between CT and CBCT images was < 60 HU. The CBCT images exhibited larger variation across the field of view compared to CT images. Dose calculation based on CBCT images showed a mean dose difference to PTV of 0.0% (HU-RED CT) and -0.8% (HU-RED CBCT) compared to dose calculations based on sCT image (Fig. 1).

rotational dimensions (rotation, roll, pitch). The misalignments were within three degrees for each rotational parameter while the translational misalignment was within a few millimeters. For all nine test positions, the 6DOF movement required to align the phantom with the CT scan was determined by obtaining two paired kV images on the ET-system and an OBI CBCT- scan. Subsequently the 6DOF movement was performed by the ET-system and the robotic couch. The precision of this movement is within tenth of a millimeter. In four of the nine cases after repositioning of the phantom a second pair of kV images from the ET-system and another CBCT-scan was obtained to verify the couch-movement. The ET X-ray and the OBI-isocenter was calibrated to correspond within half a millimeter. Results For the nine test positions the translational deviations (3D vector) between the ET- and OBI-match ranged from 0.5mm to 1.4mm with an average of 0.8mm and a standard deviation (SD) of 0.3mm. In the same nine positions the deviation for the couch rotations ranged from 0deg to 0.1deg and the deviation for the robotics (roll, pitch) part of the couch movement ranged from 0deg to 0.8deg with an average of 0.2deg and a SD of 0.2deg. In the four cases of verification of the repositioning of the phantom the average of the translational 3D vector between the ET- and OBI-match was 0.3mm with a SD of 0.3mm. The deviations on the rotational parameters for these measurements were 0-0.1deg. The average of the deviations for the robotics parameters was 0.2deg with a SD of 0.1deg. A summary is listed in table 1. Conclusion The correlation between a 6DOF repositioning performed by the ET-system with robotics and verified by a CBCT- scan was tested on a thorax-phantom. The correlation was within tenth of a millimeter for the translational parameters and within tenth of a degree for the rotational parameters. EP-2173 Dosimetric verification of synthetic CT using Cone Beam CT in an MR only workflow of cancer prostate E. Palmér 1,2 , E. Persson 2,3 , P. Ambolt 2 , C. Gustafsson 2,3 , L.E. Olsson 3 1 Lund University, Medical Radiation Physics- Department of clinical sciences, Lund, Sweden 2 Skåne University Hospital, Department of Hematology- Oncology and Radiation Physics, Lund, Sweden 3 Lund University, Medical Radiation Physics- Department of Translational Medicine, Malmö, Sweden Purpose or Objective MR only workflow is based on treatment planning solely from MRI, hence excluding the CT. A synthetic CT (sCT) image is generated from MRI data, replacing the conventional CT image. Using the kV-cone beam CT (CBCT) system we could compare dose distributions calculated on both the sCT and the CBCT images. The aim of this study was to investigate the possibility of using the CBCT image to dosimetrically verify the sCT image of male pelvis.

Conclusion Phantom measurements showed that the kV-CBCT systems were stable in HU over time and relative to each other. Results obtained for one system can therefore be transferred onto all the systems. A correction of HUs was not necessary to obtain sufficiently accurate absorbed dose calculations on CBCT images, indicating that the variation in HUs between one CBCT system and one CT system had no clinical impact. The compared dose distributions based on CBCT and sCT images showed good agreement in terms of absorbed dose accuracy, regardless of which HU-RED table used. This shows that CBCT image can be used to dosimetrically verify the sCT image in an MR only workflow for male pelvis.