Atypical lymphoid proliferations: the pathologist's viewpoint.

Lymphoid proliferations are traditionally thought to be either benign conditions (reactive hyperplasia and lymphadenitis) or malignant lymphomas. However, not all lymphoid lesions at present can be precisely placed into one of these categories. Therefore, in addition to these two extremes, there also exist a third group of lymphoid proliferations - the atypical lymphoid proliferations (AtLP). AtLP is a descriptive term used when it is not possible for the pathologist to differentiate between the benign and the malignant nature of a given lymphoid infiltrate. AtLP represent biologically indeterminate lesions that have some worrisome clinicopathologic features but cannot be interpreted as malignant lymphomas using all criteria currently available. They have some likelihood for subsequent transformation into lymphomas, and therefore AtLP occupy a middle ground between benign and malignant lymphoid proliferations. Nevertheless, sometimes AtLP are not necessarily premalignant and may very well represent a fully benign situation mimicking malignancy. In the author's opinion, when confronted with a challenging lymphoproliferative lesion, the pathologist should marshall all resources available to interpret it as precisely as possible and therefore place it into one of the two categories: unequivocally benign condition or malignant lymphomas. The resources should include immunohistochemical and molecular studies, obtaining expert opinion and rebiopsy. However, if clinical, morphologic and molecular findings are not sufficient for diagnosis of a benign condition versus lymphoma, the descriptive term AtLP can be used. In the author's opinion, the use of this descriptive term AtLP may obviate the need to force some lymphoid proliferations (equivocal lesions or mimickers of lymphomas) into either the benign or malignant categories.