When radiation is used with curative intent, the radiation volume usually encompasses the detectable tumor and the anatomic areas thought to be at risk for metastatic spread. The International Commission on Radiation Units and Measurements Report No. 50 (1) defines gross tumor volume (GTV) as the gross palpable or visible or demonstrable extent and location of the malignant growth. The same report defines clinical target volume (CTV) as a volume that contains a demonstrable GTV and or is considered to contain (only) microscopic, subclinical extensions at a certain probability level. In clinical practice, the determination of the extent of the CTV is based on the knowledge of the patterns of spread for each specific disease presentation. Additional reliable information can be obtained from patterns-of-failure analysis and necropsy series. For most tumors, the CTV will encompass one or more nodal stations, usually near the primary lesion. Traditionally, the location and boundaries of these nodal stations have been established in reference to anatomic landmarks during the standard simulation setup. Therefore, the radiation oncologist has been specifically trained to determine the boundaries of the different nodal stations on standard two-dimensional radiographs, especially in the anteroposterior and posteroanterior views. With the advent of three-dimensional (3D) virtual clinical target definition, the radiation oncologist faces the challenge of defining the CTV on cross-sectional CT or magnetic resonance images. Unfamiliarity with this new technique can make correlations with the known spatial references difficult to establish. The present nodal atlas is intended to assist radiation oncologists who will use new 3D virtual clinical target definition and treatment planning programs.
