Cutaneous pseudolymphoma: an unusual presentation of a deep subcutaneous thigh mass.

Cutaneous pseudolymphoma is considered to be a benign (reactive) cutaneous lymphoid infiltrate; the term designates reactive diseases of the skin that histologically mimic cutaneous lymphoma. We report a case in which a 63-year-old female presented with a 5-month history of a progressive skin eruption and an enlarging subcutaneous mass following a presumed insect bite. Excisional biopsy showed this to be a pseudolymphoma extending from the dermis into the subcutaneous tissue. A number of pathological features that distinguish pseudolymphoma from cutaneous lymphoma, including histology, immunophenotype, and immunogenotype, are reviewed. The case herein challenges previous beliefs that pseudolymphoma is confined to cutaneous involvement and indicates that the process can involve deeper tissues. The final criterion for distinguishing benign from reactive processes is biological behavior. Since the depth of invasion in cutaneous pseudolymphoma has not previously been appreciated, the patient will need to be carefully examined periodically until the biological behavior of the process has been determined.