What is the difference between involved node and involved site irradiation?

In both involved node and involved site (ISRT), prechemotherapy GTV determines the CTV. However, ISRT accommodates cases in which optimal prechemotherapy imaging is not available to the radiation oncologist. In ISRT, clinical judgment in conjunction with the best available imaging is used to contour a larger CTV (involved node with margins) that will accommodate the uncertainties in defining the prechemotherapy GTV.

There is a recent publication in the Red Journal from the ILROG Group describing this with recommendations.