While the prevalence of PET imaging has made the CRu category obsolete, the CRu component of the 1999 IWG-NHL criteria continues to be a source of misinterpretation.
- CRu was originally intended for those curable lymphomas, such as Diffuse Large B-Cell Lymphoma and Hodgkin's Lymphoma, in which a large mediastinal mass or retroperitoneal mass becomes residual scar tissue and thus may never fully regress to normal size.
- The CRu category was created to allow for single large masses which may completely respond with no residual lymphomatous tissue remaining, even though a measurable band of fibrous scar tissue may remain present.
- CRu was never intended for groups of multiple large nodes that did not meet the normalization requirement.
- CRu is not meant to represent a higher level of partial remission, but instead is meant to represent a complete remission that is lacking confirmation by biopsy.
- CRu was intended for a residual lymph node only and never for intraparenchymal organ nodules or other extranodal sites as these manifestations of disease must become absent to be considered normal.
With the publication of the 2007 IWG-NHL criteria, the category of CRu has become obsolete and should no longer be used to assess response in lymphoma trials. This also applies to trials where PET is either not available or not consistently used.