Primary cutaneous follicle center lymphoma -'crosti lymphoma': what can we learn?

In 1951, Crosti reported on seven patients with 'reticulo-histiocytoma of the back' who presented with figurate erythematous plaques and nodules on the back or lateral trunk. Reticulo-histiocytoma of the back was later classified as a primary cutaneous follicle center lymphoma (PCFCL). A definitive diagnosis of the condition is frequently delayed because of a relative lack of clinical symptoms and difficulties in interpretation of the histologic findings. Indeed, a number of primary cutaneous B-cell lymphomas have been mislabeled as pseudolymphomas in the past. We present a case of PCFCL that initially demonstrated predominantly small T lymphocytes on histology. These findings were interpreted as an inflammatory pseudolymphomatous reaction. However, small lymphocytes, whether B or T cells, in early lesions of cutaneous B-cell lymphomas should not automatically be considered 'reactive.' Persistent antigenic stimulation of lymphocytes in a neoplastic process or by an antigen, for example, Borrelia burgdorferi, can lead to transformation and cell division with development of large blast cells. In our patient, the initial scarcity of B lymphocytes also led to further diagnostic difficulties. Although the association of primary cutaneous B-cell lymphoma with Borrelia infection is known, there are still difficulties in differentiating the condition from pseudolymphoma. Such difficulties can in part be ascribed to the morphologic changes such lymphomas can undergo over time. The initially small number of B cells that may be seen at first in PCFCL infiltrates may increase in number in longer-standing lesions. It is also important to recognize that inability to verify monoclonality should not exclude the diagnosis of lymphoma.