What term is used to describe a single tumor that involves adjacent sites and is contiguous?

The term that best describes a single tumor that involves adjacent sites and is contiguous is "overlapping tumor." This designation is specifically used in oncology to indicate that the tumor extends beyond its primary site and affects neighboring tissues or organs as a singular mass. Understanding the nature of the tumor's growth pattern is crucial for accurate staging and treatment planning, which is essential in the context of cancer epidemiology and registry purposes.

In contrast, a "focal tumor" usually refers to a tumor that is confined to a specific area without implying extension into neighboring sites. A "tumor focus" may refer more broadly to the site of a tumor but does not carry the connotation of contiguity or adjacency as effectively as "overlapping tumor." Thus, the terminology is critical in defining the extent of disease and impacts clinical decisions and registry documentation.