[Gross tumor volume (GTV) : Basics, methods, registration, limitations].

CLINICAL ISSUE: Gross tumor volume (GTV) denotes the macroscopic tumor which as the central target volume needs to be correctly identified for successful radiotherapy. STANDARD RADIOLOGICAL METHODS AND METHODICAL INNOVATIONS: In precision radiotherapy, GTV is outlined on 3D tomographic images. The basis is computed tomography (CT), which is often supplemented by additional diagnostic information, e. g. magnetic resonance imaging (MRI) and positron emission tomography (PET). New developments like dual-energy CT, functional MRI and specific PET tracers facilitate a continuously better differentiation between tumor and surrounding normal tissue. ACHIEVEMENTS: The concept of GTV is a central part of radiotherapy and the basis of radiation treatment planning. Studies regarding the interobserver variability are performed in order to analyze the impact of different imaging modalities, interventions and observer qualifications, and to deduce steps to constantly improve the practical realization. Each tumor entity presents specific challenges which are demonstrated here using examples.