A New Index to Measure Intra-observer Variation in Delineating GTV Primary in Head and Neck Cancer

A critical step in radiotherapy treatment planning process is to accurately locate the tumor and determine its extent. Variability in gross tumor volume (GTV) delineation is a major source of systematic error in conformal radiotherapy. CT scan is the principle source of imaging data used for defining GTV for planning conformal therapy. CT is widely available less costly and compatible with radiotherapy treatment planning systems. It provides information on relative electron densities of various tissues used for those calculations algorithm. CT offers excellent bony details whereas MRI provides better soft tissue resolution, hence complimentary to each other. There is no doubt that MRI helps better visualization of soft tissue structures in head and neck cancer, however there is no agreement on impact of MRI for delineation of GTV primary tumor and its influence on intraobserver variation. It is well established in literature that there is a considerable inter-observer variability in GTV delineation [18]. Hermans et al. [9] showed the inter-observer variability of CT based volume measurements of thirteen laryngeal tumors by five different observers. Both inter-observer variability and to some extent intra observer variability had a statistically significant effect on volume measurement and the most experienced observer obtained the most stable mean tumor volume over all sessions. Various modern imaging modalities like MRI and PETCT are used to reduce the inter-observer variability and thus helping in greater agreement between different observations of the individuals. The aim of this study is to compare the agreement between different intra-observer GTVs in head and neck cancers delineated on CT and MRI on different occasion. We propose a new index to measure the intra-observer variability in delineating GTV in head and neck malignancies.