P269] Absorbed dose calculation based on CBCT data for head and neck cancer patients

Abstract

Purpose In a radiotherapy magnetic resonance imaging (MRI) only workflow, a synthetic CT (sCT) data set is generated from the MRI data as a substitute to the computed tomography (CT). Since no CT scan will be acquired, no such data is available for quality control (QC) of the sCT. It has been suggested to use kV-cone beam CT (CBCT) data for QC in the head region [1] . The aim of this study is to evaluate the possibility to use CBCT for QC of the sCT for head and neck (H&N) cancer by comparing H&N treatment plans calculated using the CT and CBCT data, respectively. Methods Five patients with one CT and two CBCT data sets each, were evaluated. All CBCT data sets were deformably registered to the CT data, remediating errors originating from repositioning and/or anatomical changes between the imaging sessions. The CBCT and CT data were adjusted to the same field of view (FOV) in the longitudinal direction. A clinical treatment plan was created based on the CT data for each patient. Treatment volumes, organs-at-risk (OAR) and treatment plan were copied from the CT to the deformable registered CBCT. The treatment plan was then recalculated, with the standard CT HU-RED conversion curve. Dosimetric differences between the dose distributions for the treatment plans were calculated and evaluated by comparing dose-volume constrains. Results The mean local difference for the mean absorbed dose to the clinical target volume (CTV) was 0.41% with a maximum and minimum value of 1.37% and 0.15% respectively. The CTV D 98 % criteria had a difference of 0.19% and CTV D 2 % criteria a difference of 0.38%. The planning OAR volume (PRV) of the spinal cord had a difference of 0.39% and the parotid right and left a difference of 0.36% and 0.08%, respectively. Conclusions The dosimetric difference between the dose distributions for treatment plans calculated based on CT and CBCT data were minor. This indicates that CBCT data could be a good candidate for QC of sCT data in an MRI only workflow for H&N cancer patients.