ESTRO 2022 - Abstract Book

S345

Abstract book

ESTRO 2022

Conclusion We have developed a framework to evaluate the impact of ROI/DOF definitions on CBCT based patient positioning from image, geometric and dosimetric aspects. The results indicated that ROI selection only marginally influences the dosimetric result for proton treatment of skull-based and H&N tumours. However, enabling the 6DOF daily positioning offset calculation is important to reduce residual position uncertainties.

PD-0403 Assessment of residual setup errors of clinical target volumes for head and neck radiotherapy

K. Ng Wei Siang 1 , S. Both 1 , E. Oldehinkel 1 , J. Langendijk 1 , D. Wagenaar 1

1 University of Groningen, University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands Purpose or Objective To assess the residual non-rigid setup errors due to anatomical changes of the clinical target volumes (CTVs) after online verification for clinical head and neck cancer (HNC) proton therapy patients. Materials and Methods Eleven HNC patients treated with simultaneous integrated boost – 70Gy RBE and 54.25Gy RBE to the primary CTV (CTV70) and elective CTV (CTV54.25 ), respectively – were retrospectively analysed to determine the residual non-rigid setup errors. Using an in-house validated deep convolution neural network (DCNN) architecture daily cone beam CTs (CBCTs) were converted to synthetic CTs (sCTs). The sCTs have comparable quality to the CTs, validated against weekly verification CTs acquired during the course of treatment. The CTV was propagated from the original CT where planning was made to the daily sCTs using a hybrid deformable image registration (RaySearch). These auto-propagated CTVs (dirCTVs) were also reviewed by the HNC radiation oncologist (pCTVs) (see Figure 1). We focused here only on the region above the cricoid cartilage where the CBCT quality is good. This ensured high quality derived sCTs for contour review. The original CTV was first uniformly expanded in discrete steps of 1 mm. The CTVs of the daily sCTs were co-registered and intersected with the expanded planning CTV to determine the volume overlap. For CTV70 we determined for the patient group the interpolated shell distance at which 95% of daily CTVs have at least 98% volume overlap with the planning CTV + d r . The elective CTV54.25, was examined at 95% volume overlap with the planning CTV + d r . Both pCTVs and dirCTVs were evaluated, and the dice similarity coefficient (DSC) were compared.