ESTRO 37 Abstract book

prior to CT vs 0.0 mm (-3.3 to 2.2) for patients fixated at the CT. The figure shows what the RTTs registered for the six parameters in the first 5 fractions in both procedures. The time for the standard procedure ranged from 25 to 45 mins, and 35 to 55 mins for patients immobilized on the CT-couch.

means Mean

0.65º

1.51º 0.68

1.15

standard deviation

Mean percentage of fractions rotation ≥ 3º

3%

21%

4%

15%

Conclusion ProSTEP showed a significantly smaller standard deviation of rotation than Kneefix for gantry 270°/90°. This might indicate that a knee and feet positioning device that is attached to the table in a reproducible way results in a more reproducible positioning of the pelvis. More data of patients making use of the ProSTEP will be collected the next months. EP-2345 Immobilization of head and neck patients on the CT-simulation couch L. Johnsen 1 , J. Gornitzka 2 , S. Krogh 1,3 , J. Johansen 1,3 , C. Rønn Hansen 1,3 , A. Bertelsen 1 1 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 2 Odense University Hospital, Department of Oncology, Odense, Denmark 3 Odense University Hospital, Institute of Clinical Research, Odense, Denmark Purpose or Objective Immobilization of head and neck cancer (H&N) patients with a mask can either be done before planning-CT in a separate procedure outside the CT-room, or directly on the CT-couch. The aim of this study was to investigate differences between the two procedures. Material and Methods Data from 20 H&N patients immobilized on the CT-couch were compared with data from 20 patients immobilized in a separate procedure (standard procedure in our department) prior to planning-CT. All patients were immobilized with a fivepoint Orfit Mask. The patients were undergoing treatment with either, 66-68 Gy/33-34 fractions or 52 Gy/13 fractions. Cone beam CT-scans (CBCT) during the first 5 fractions of radiotherapy were compared between the two groups. Unusual/abnormal patient setup during the treatment course observed by RTTs were collected in the record and verify system and any action categorized as either Re-planningCT, Patient rearrangement, Physicist contacted, Rotations above 3 degrees, Irregular shoulder position. The RTTs were blinded to the immobilization procedure of the individual patient. The reproducibility of the patient positon was assessed by evaluating translational and rotational parameters for rigid image registration between planning CT and CBCT at the first five treatment fractions. For each patient, the systematic and random setup errors were calculated. Differences in median setup errors between patient groups were tested using Mann Whitney Utest. Difference in variance of the distribution of patient mean setup errors was tested using Brown–Forsythe test. A p-value of 0.05 was considered to be statistical significant. Finally, the total time for immobilization and planning-CT was compared for six patients in each group. Results A total of 200 CBCTs were obtained from the two groups. Analysis of the CBCT-CT image registration revealed no Statistic clinical relevant differences between the two groups. Only the median of the longitudinal systematic shift proved to be statistically significant different with a median value of 1.0 mm (-1.2 to 4,2) for patients fixated

Conclusion Performing immobilization either on the CT-couch or in a dedicated room does not reduce preparation time or influence setup accuracy. There was no significant time difference in this study, but there will be an opportunity to release resources in the workforce by immobilizing directly on the CT-couch before planning-CT EP-2346 Reproducibility of patient positioning using Prone Belly-board device A. Lastrucci 1 , Y. Wandael 2 , C. Galeotti 2 , M. Vernaleone 1 , D. Greto 2 , P. Bonomo 2 , G. Simontacchi 2 , L. Livi 1 1 University of Florence, Department of biomedical- experimental and clinical sciences "Mario Serio", Florence, Italy 2 A.O.U. Careggi, Radiotherapy, Florence, Italy Purpose or Objective The prone belly-board positioning (PBBP) significantly allows to reduce the dose to the small bowel compared with the supine position, potentially reducing the toxicity during pelvic 3D-CRT technique. The use of the PBBP can lead to an increase in setup uncertainties which can be counterbalanced by the use of Image-Guided Radiation Therapy (IGRT). The purpose of this study is to demonstrate the reproducibility of the patient setup with PBBP, comparing DRR and EPID images acquired during In this study two radiographers analyzed the EPID images of 25 patients (12 male and 13 female) treated using the PBBP. For each patient, all the the EPID images acquired during the treatment were considered. Overall 100 portal images were analyzed, respectively 50 to Gantry 0° and 50 to Gantry 90° or 270°. The EPID comparison was performed by matching the bone alignment at the pelivc level. Shifts on the three spatial axes: X (Left-Right), Y radiotherapy treatment. Material and Methods