Effect of window level on target volume delineation in treatment planning

In radiotherapy the efficacy of medical imaging is central to the selection and delineation of target volume. Of note is that target volume is intended to be larger than tumour volume, including a geometric margin that accounts for the possible uncertainties in patient set-up. However this reduces potential tissue sparing, irradiating not only the target but also normal tissue. Additionally, features of the object that appear in the 2D image display may be influenced by the appropriate window level selection. This is especially critical for target volume delineation in radiotherapy. The present work seeks to assess the effect of window level selection on feature size in CT, MR and PET images, use being made of a NEMA body phantom and ProSomaTM 3D simulation software. In general, the window level produced discrepancies of up to ±2 mm in all imaging modalities.