ESTRO 36 Abstract Book

S890 ESTRO 36 _______________________________________________________________________________________________

significant strong correlation between initial X-rotation and residual Y-translation (figure 1). Small SDs for both rotations and translations (table 1) indicate that patient compensation for rotational corrections is modest. The larger SD values for X and Z are related to significant strong correlations to the initial Y rotation (R=0.77 and 0.68, respectively). No significant correlations were observed between patient-specific parameters and pre or post setup errors. Based the translational systematic and random errors a minimum CTV to PTV margin of 2mm is required (Van Herk et al. IJROBP 2000). However this estimate does not include compensation for rotations of elongated targets.

Conclusion The PTV and body V43 are significantly reduced through margin reduction. For all patients, CTV-E D98 was larger than 43Gy for PTV margins ≥3mm. We notice a considerable advantage for the OARs when decreasing the PTV margin from 8 to 5mm. The advantage for OARs of decreasing the margin further to less than 5mm is significant but less pronounced. In particular, the gain for the OARs is small when moving from 3 to 0 mm while a 3mm margin increases the average D98 by 0.51Gy. EP-1641 Intra-fractional CBCT validation of a 6D couch to facilitate precision RT of head and neck cancer A. Bertelsen 1 , C.R. Hansen 1,2 , M. Nielsen 1 , J. Eriksen 2,3 , N. Gyldenkerne 3 , J. Johansen 2,3 , S.L. Krogh 1 , J. Westberg 1 , C. Brink 1,2 1 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 2 University of Southern Denmark, Department of Clinical Research, Odense, Denmark 3 Odense University Hospital, Department of Oncology, Odense, Denmark Purpose or Objective Small planning margins for head and neck (H&N) cancer patients reduces toxicity and can be assisted by stable fixation equipment and IGRT including online corrections for translations and rotations (6D). This clinical study uses intra-fractional CBCT (iCBCT) to investigate the precision and, accuracy of 6D couch usage, and if the patients compensate for the rotational corrections. Material and Methods H&N patients receiving standard fractionated IGRT were included in this study. The inclusion occurred in a three month period after an upgrade of the 6D couch system (Elekta, Hexapod). After online couch correction based on a pre-treatment CBCT (pCBCT) additional setup verification was performed using iCBCT acquired during single arc VMAT. The residual uncertainty of the 6D couch correction was assessed by comparing the pCBCT and iCBCT registration of the spinal cord. The residual setup error of the target volume ( from the iCBCT) was used to calculate the population-based systematic and random uncertainty given as the standard deviation (SD) of patient mean values and SDs. Correlations between residual errors and initial setup errors as well as patient specifics (age, weight, BMI, performance status (PS)) were tested using Spearmann’s R or Kruskall-Wallis (PS) test. For correlations the standard 5% statistical significance level was Bonferroni corrected. Results In total 44 patients were included resulting in 1174 iCBCTs. On average, the initial setup errors prior to use of the 6D couch were small, but had statistically significant mean values different from zero (table 1). Deviation in Z is due to standard adjustment of the fixation. The X rotation value is likely caused by the Hexapod calibration performed without load on the couch. Large SD was observed in the initial Y rotation. The residual error shows that all setup uncertainties are statistically significant reduced (Leveens test, p<0.001). However, a large systematic residual error of 0.4mm was observed in the Y-direction. This might be related to a

Conclusion The Hexapod system performed well and patients did not seem to compensate for rotational corrections. However due to related uncertainties rotations should be kept at a minimum. The present setup reduces the systematic and random uncertainties which make use of small treatment margins plausible. EP-1642 Patient-specific transperineal ultrasound probe setups for image guided radiotherapy S. Camps 1,2 , F. Verhaegen 3 , P. De With 1 , D. Fontanarosa 2,3 1 Eindhoven University of Technology, Faculty of Electrical Engineering, Eindhoven, The Netherlands 2 Philips Research, Oncology solutions department,