ESTRO 37 Abstract book

S104

ESTRO 37

times for each volunteer and a total of 185 sets of images were obtained. The 1 st MRI of each volunteer was used as a reference. By rigid image registration of 1 st MRI and the other MRI, shifts were recorded and setup reproducibility was investigated in terms of systematic, random error and 3-dimension vector length (3DVL). The results were compared with a previous clinical reproducibility study which was done in Tomotherapy machine using Mega Voltage Computed Tomography (MVCT) for position verification. Unpaired t-test and F test were used to compare the results from the two studies. Shifts of internal structures in the repeated setup were also investigated in this study. Seventeen head and neck structures were contoured in all the acquired MRI images. Shifts of center of mass for each structure were calculated. Results Calculated systematic errors, random errors, 3DVL and the previous study result were demonstrated in table 1. All 6D (Lateral, longitudinal, vertical, roll, pitch, yaw) systematic errors were very close to zero and neglectable. Random errors were also small and all below 1 mm except in longitudinal direction which had 1.05mm error. In statistical tests with previous study result, all compared errors were significantly smaller in MRI simulator study (p<0.05), except for lateral systematic error (p= 0.867). The structures shift in the repeated setup procedures in terms of systematic error, random error and 3DVL mean were shown in table 2. The head structures (brainstem, eyes, mandible, C1 and C2 vertebral body) were more stable than the neck and shoulder structures (C3 to T3 vertebral body, head of humerus). The reproducibility of the structures at head region was good (3DVL mean < 2mm, random error in all directions < 1.5mm) in MR simulator environment. However, for structures in the neck and shoulder region, the position shifts in the repeated setup were relatively large, especially for the heads of humerus (3DVL mean up to 4.3mm, random error up to 4.1mm in longitudinal direction).

need better immobilization devices and careful setup to improve the position reproducibility. OC-0196 Reproducibility of the liver in stereotactic radiotherapy using abdominal compression. D. Zwiers 1 , S. Koekoek 1 , A. Bouwhuis-Scholten 1 , H. Piersma 1 , E. Van Dieren 1 1 Medisch Spectrum Twente, Radiotherapy, Enschede, The Netherlands Purpose or Objective In stereotactic treatment of a liver laesion, accurate positioning is essential. This is usually done using an abdominal compression band. However, patient comfort limits the amount of compression and residual movement and positioning error may occur. Hence, it is standard to use CBCT a priori and posteriori, to judge treatment positioning accuracy. Due to a limited field of view of the CBCT, the whole liver usually cannot be imaged. In such a case, the accuracy cannot be judged fully. The aim of this study is to develop a new method to judge the partially imaged liver and to determine the reproducibility of the liver positioning in abdominal compression. Material and Methods In total 24 patients treated in our institution were included in this retrospective study. All patients had been imaged a priori and a posteriori, using Cone Beam CT (CBCT) with standard abdominal settings (on a Varian TrueBeam), and an abdominal compression band (Orfit Industries) was applied. For this study, it was assumed that liver contour reproducibility was directly related to target reproducibility. For each patient, the reproducibility of the liver contour was determined by comparing the differences in center of mass (CMS), CMS-vector and DICE similarity coëfficiënt between the CBCT and planning CT. Using set limits, results were classified as poor, sufficient, or excellent. When liver was only partially imaged (75% of cases), contours of the plan CT were shortened to match CBCT length, allowing all patients to be analysed similarly. The suitability of this method was determined by comparing, for those patients for whom the entire liver was imaged, the DICE and CMS-vector between whole-liver and shortened-liver approach. To track intrafractional differences, the CMS and DICE were compared and the delta-CMS was determined. Finally, the reliability of SmartAdapt was determined by comparing the DICE, CMS and CMS-vector of the automatic contours of SmartAdapt with a manually drawn liver contour. Results The liver DICE, CMS and CMS-vector were sufficient to excellent, excluding one patient. CMS-vectors between both methods did not significantly differ (p=0.600). The DICE was significantly better for the shortened liver approach (p=0.039). For the DICE and CMS, no significant intrafractional differences were found (p>0.244). The correlation between the duration of the irradiation and the DICE was weak (r=0.053). The manual liver contour was significantly improved in terms of DICE, CMS and CMS-vector (p<0.0001). Conclusion The new method is valid and ensures reliable measurements of all liver stereotactic irradiations. The reproducibility of the liver contour in abdominal compression between fractions and during the treatment is sufficient to excellent, indicating that stereotactic treatment the tumor in the liver has been performed accurately.

Conclusion The reproducibility of head and neck setup in MR Simulator environment is good and at least no worse than the clinical result in tomotherapy treatment room. Position of head structures in the repeated setup procedures were also reproducible. Structures in neck and shoulder region which were comparably unstable may

Poster Viewing : Poster viewing 4: Planning applications and optimisation algorithms