Evaluation of CTV to PTV Margin for Head and Neck Cancer with Daily Setup Using MVCBCT Imaging

Purpose: Geometrical inaccuracies of setup using MV-CBCT imaging for patients with head and neck cancer were evaluated. Possibility of modifying of CTV to PTV margin while delineation of such tumors was estimated. Materials and methods: Additional adjustment of simulation kV CT pictures with MV-CBCT images received during the implementation of the online protocol was conducted for 35 patients with head and neck cancer. Offline analysis revealed the discrepancy in the results of online correction and offline analyses. CTV to PTV margin considering daily setup was computed. Results: Obtained values of CTV to PTV margin: 2.4 mm in Vrt, 2.1 mm in Lng and 2.5 mm in Lat. Upward variations from 5 mm margin used in unit were registered for 2 patients. Conclusions: Reduction of CTV to PTV margin up to 2.5 mm is precarious because CTV dose coverage is not reasonable for all patients. Implementation of routine work practice in Off-line Review is required for identification of problem cases and determination of individual CTV to PTV margin.