Abstract Book

ESTRO 37

S521

Results The results are presented in table 1. Differences in the couch shifts are largely dispersed and large systematic error in mean difference is seen in SI direction between RP and kV-IGRT. Differences in couch shifts between Calypso and kV-IGRT are much smaller. Bland-Altman analysis (fig. 1) shows that the RP do not agree well with kV-IGRT and LOAs are not acceptable, whereas Calypso agrees well with kV-IGRT having acceptable LOAs.

Conclusion Non-rigid approximation demonstrates complex and extensive geometric changes of the vaginal vault target, which are not accounted for in margin recipes using rigid approximation. Contemporary margin recipes and adaptive treatment planning based on non-rigid approximation are required. References 1. van Herk M, Remeijer P, Rasch C, et al. The probability of correct target dosage: Dose– population histograms for deriving treatment margins in radiotherapy. Int J Radiat Oncol Biol Phys 2000;47:1121–1135. PO-0953 Positioning accuracy of two electromagnetic positioning systems in radiotherapy of prostate cancer A. Vanhanen 1 , M. Kapanen 1 1 Tampere University Hospital, Oncology- unit of radiotherapy, Tampere, Finland Purpose or Objective Inter- and intra-fraction motion of the prostate is a well- known phenomenon that has to be corrected for accurate target localization in prostate cancer radiotherapy. This study reports the positioning accuracy of two commercial electromagnetic (EM) positioning systems, RayPilot (Micropos Medical AB, Sweden) (RP) and Calypso (Varian Medical Systems, USA), compared to kilovoltage (kV) image guidance as the golden standard. Material and Methods The RP consists of a wired transmitter which is implanted into the prostate and a detector plate positioned on the treatment couch. Calypso consists of three transponders implanted into the prostate and an electromagnetic array which is set up above the patient. Both systems provide the real-time position of the prostate. RP transmitters as well as three gold markers were implanted into the prostate of 22 patients. RP positioning data was collected throughout the treatment fraction and target localization based on two orthogonal kV images (kV-IGRT) using gold markers as fiducials. Calypso localization and tracking was used for 20 patients and the localization was confirmed by kV-IGRT using three Calypso transponders as fiducials. Couch shifts suggested by the RP at the time points of kV-IGRT were compared to the couch shifts suggested by the kV-IGRT. Similarly, Calypso readings at the time points of kV-IGRT were compared to kV guided couch shifts. The mean and standard deviation of the differences between couch shifts in anterior-posterior (AP), superior-inferior (SI) and left-right (LR) directions were determined. The Bland-Altman method was used to analyze the agreement between the EM systems and kV- IGRT. The 95% limits of agreement (LOA) were determined. Differences larger than ±2 mm were considered unacceptable. A total of 596 RP fractions and 245 Calypso fractions were analyzed. 2. International Commission to Radiation Units and Measure- ments. ICRU report 62: Prescribing, recording, and reporting photon beam therapy (supplements to ICRU report 50). Bethesda: ICRU; 1999.

Conclusion Compared to kV-IGRT, positioning accuracy of Calypso outperformed the RP. RP positioning accuracy was probably affected by the migration of the transmitter caused by pulling forces it was exposed to via transmitter cable and deformations of the prostate caused by varying filling of the rectum and bladder affecting on the relative position between the transmitter and isocenter. Results indicate that Calypso could replace kV-IGRT in the inter- fraction motion management of prostate radiotherapy but positioning with RP should be verified by kV imaging. PO-0954 Fiducial markers and daily kV imaging improve patient setup during proton RT of esophageal cancer R. Apolle 1,2 , M. Rehm 2,3,4 , J. Thiele 2,3 , S. Brückner 5 , J. Hampe 5 , E. Troost 1,2,3,4,6 1 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology – OncoRay, Dresden, Germany 2 OncoRay – National Center for Radiation Research in Oncology, Department of Clinical Radiotherapy, Dresden, Germany 3 University Hospital and Medical Faculty Carl Gustav