Concerning Appearance, Benign Behaviour: the Pseudolymphoma Spectrum

The spectrum of cutaneous pseudolymphomas (reactive lymphoid hyperplasia, lymphocytoma cutis) is very wide and the histologic differential diagnosis is often difficult. In only a proportion of cases, the aetiology of the process can be elucidated and close clinicopathological correlation is essential to reach the diagnosis. Even with adequate clinical information and detailed microscopic examination, in a proportion of cases the diagnosis of lymphoma cannot be entirely ruled out. In this setting, ancillary techniques including molecular biology studies and further biopsy or biopsies may be necessary over time for the diagnosis to be made. Cutaneous pseudolymphomas can be divided into two categories, T cell and B cell pseudolymphomas. This distinction is artificial, since the infiltrate is frequently composed of both B and T cells. The separation is, therefore, mainly based on the overall pattern of the infiltrate rather than the type of cell that predominates. Infiltrates with a T cell pattern tend to be focal with a predominant perivascular and periadnexal distribution and involvement of the epidermis and/or adnexal structures. Infiltrates with a B cell pattern are more diffuse and nodular and tend to spare the epidermis and adnexal structures. As it will be seen during discussion, this is an oversimplification that mainly serves a didactic process.