ESTRO 36 Abstract Book

S897 ESTRO 36 _______________________________________________________________________________________________

on the symmetry of the organ.

allowed difference between the thorax wall and boost match is shown in a). The boost CTV to PTV margin required to account for the difference between chest wall and boost match for a given threshold is shown in b). The grey shaded area shows the 90 percent confidence interval obtained by bootstrapping.

Conclusion Rotational errors have to be corrected regardless of translations magnitude. Although rotations don’t impact on CTV V95%, 6DoF corrections allow better PTV’s coverage. Rotational errors could cause considerable dosimetric changes in organs at risk and must be carefully corrected in SBRT to avoid normal tissue complications. An ongoing analysis on setup systems and margin reductions has been planned. EP-1652 A new position verification protocol for breast cancer with integrated boost K.L. Gottlieb 1 , E.L. Lorenzen 1 , J.D. Jensen 2 , M.H. Nielsen 2 , M. Ewertz 2 1 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 2 Odense University Hospital, Department of Oncology, Odense, Denmark Purpose or Objective The use of integrated boost compared to sequential boost allows shortening of the overall treatment time while maintaining the same biologically equivalent dose to the boost region. However when the target is large as in breast cancer treatments there can be challenges in IGRT registrations between the boost volume and total target volume. The present study proposes and evaluates a protocol for daily IGRT using CBCT of breast cancer patients with integrated boost. A threshold is set for the allowed difference between the whole target match and the boost specific match. If the two matches differ by less than the threshold the boost volume and the total volume is treated in the same setup. If the two matches differ by more than the threshold, the total target volume is matched and treated and then secondly an additional CBCT is performed and matched on the boost volume In order to evaluate the match protocol all patients receiving RT during the period 1/1-2016 and 1/7 2016 after breast conserving surgery and with lymph node involvement were retrospectively included in the study. The number of patients eligible for inclusion was 18 (3 was rejected because of missing (2) or faint (1) clips). All patients received daily IGRT with CBCT using XVI from Elekta. CBCT’s for the first ten fractions were included resulting in 180 CBCT. Two matches were performed for each patient: First, a chest wall-match were performed, where CBCT images were registered automatically to the planning CT using a grey value translational match of the thorax wall. Secondly, a boost specific match was performed, where the surgical clips were manually registered to their position on the planning CT. Results The systematic (Σ) and random errors (σ) between the thorax wall and the clips based boost matches are seen in the table. The CTV-PTV margins were calculated based on the systematic and random errors using the Van Herk margin recipe. These numbers are found both using no re- matching and a re-matching if there were more than 5 mm differences between the two matches. In the figure the percentage of rematches needed for a given threshold of which is then treated. Material and Methods

Conclusion The presented protocol can reduce the required CTV to PTV margin for the boost region by re-scanning and re- matching the boost region only for patients where the two regions differ by more than a set threshold. Results are presented that can be used for selecting a threshold with the corresponding required CTV-PTV margin. If e.g. a threshold of 5 mm is used, the required CTV-PTV margin can be reduced from approximately 8mm to 5mm and re- scanning and re-matching will be required in only 5% of the fractions. EP-1653 PolymarkTM fiducial markers migration in Prostate Image Guided Radiation Therapy using CBCT images C. Camacho 1 , I. Valduvieco 1 , J. Sáez 1 , A. Herreros 1 , J. García-Miguel 1 , E. Agustí 1 , C. Castro 1 1 Hospital Clinic i Provincial, Radiotherapy, Barcelona, Spain Purpose or Objective Polymer-based fiducial markers, FM (Polymark TM ) location was analyzed to test the idea that there is no intraseed migration within the prostate, which is fundamental for patient set-up good quality overthe entire course of radiotherapy treatment (RT).