Abstract Book

S1228

ESTRO 37

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 radiotherapy treatment. Material and Methods 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 (Superior-inferior) and Z (Anterior-Posterior) were calculated. For each patient, the average shift in every direction was obtained. The final result reported are the median value between those determinated by the two radiographers.

Conclusion The use of PBBP, which is usually recommended to displace the bowel bag during pelvic 3DCRT treatments, would require the use of daily IGRT techniques to take into account potential systematic and random setup errors that are inevitably linked to it. Where this is not feasible, it is advisable to follow Center-dependent procedures in order to minimize such uncertainties, such as running daily EPID in the first 5 days( for the systematic errors) and then weekly (for the random ones), in addition to adopting an appropriate Center- specific CTV-PTV margin, calculated through the statistical measurement of systematic and random errors. EP-2347 Positioning of prostate cancer patients by the CBCT: Guide for the staff of Radiotherapy Department A.L. Jussila 1 , A. Määttä 2 , M. Kurttila 2 , K. Marttila-Tornio 2 , K. Paalimäki-Paakki 1 1 Oulu University of Applied Sciences, Special Fields of Health Care, Oulu, Finland 2 Oulu University Hospital, Radiotherapy, Oulu, Finland Purpose or Objective The purpose of the study was to determine how the manual positioning of the prostate cancer patient done by radiation therapists, who use orthogonal X-ray images, differs from the auto-matching, which was done in the offline review and based in the CBCT images taken from the patient during treatment sessions in the Oulu University Hospital (OUH). Material and Methods Firstly a guide was created for the positioning of prostate cancer patients by the CBCT. The positioning guide was pre-tested in the Radiotherapy Department, and completely finalized according to user feedback. Secondly, the data collected in the offline review was analyzed statistically to determine how the auto- matching positioning differs from the manual positioning done by radiation therapists. The research data consisted of prostate cancer patients who were treated with Simultaneous Integrated Boost (SIB) treatment and who also had images taken by the CBCT during treatment sessions. The basic set of the study consisted of 51 patients and the subset was 20 patients containing 40 CBCT images. The results of the study were analyzed statistically per patient-specifically (n = 20) and couch shifts-specifically (n = 40). Results Auto-matching with the region of interest (ROI) and gold fiducial markers produced the best positioning results. In regard to couch shifts, the manual positioning who use orthogonal X-ray images differed significantly statistically (p < 0.05) from the positioning done by auto-matching in the vertical direction (-0.7 mm ± 1.3 mm). Admittedly, less than one millimeter differences are virtually meaningless in radiation therapy. Differences in longitude and lateral were 0.1 mm ± 1.7 mm and -0.2 mm ± 1.0 mm. In the same way, other positioning methods included auto-matching with ROI, and auto-matching with ROI, gold markers and couch rotation. Results also indicated that couch rotation does not improve positioning accuracy. Conclusion The Department of Radiotherapy in the Oulu University Hospital (OUH) has shifted towards the use of Cone Beam Computed Tomography (CBTC) in the positioning of prostate cancer patients in 2016. Daily online review with the use of CBCT enables the safe reduction of the treatment margins without jeopardizing the tumor

Results Table 1

The median shift (± Standard Deviation, SD) along the X was 3,20± 2,06 mm, along the axis Y was 3,61± 2,99 mm and along the axis Z was of 4,25± 2,93 mm. The results obtained are summarized in the table 1. The diagram shows clearly that the most relevant shifts occur in Z direction, where actually the value of the third quartile is of 5,9 mm.