Epidermotropic Cutaneous Lymphoma (Mycosis Fungoides) in an SD Rat

Cutaneous lymphomas can be divided into two major subgroups based on the absence or presence of epitheliotropism. Nonepitheliotropic lymphoma are usually of presumed B-cell origin, as indicated by numerous cell marker studies in human lymphomas. Epitheliotropic lymphoma, which generally refers to mycosis fungoides and its variants, is ofpresumed T-cell origin.) Mycosis fungoides is a tumor of thymus-derived lymphocytes that arises in the skin.5.6 It is slowly progressive, spreading to lymph nodes and internal organs late in the course of the disease.6 Mycosis fungoides has three characteristic histologic features? 1) the cutaneous cell infiltrate is pleomorphic, with plasma cells, eosinophils, and histiocytes accompanying the neoplastic T lymphocytes; 2) many of the neoplastic T lymphocytes have hyperchromatic and convoluted nuclei, and these cells have been named mycosis cells$ and 3) Pautrier’s microabscesses, small intraepithelial clusters of the proliferating tumor cell^,^.^ are present and are considered almost pathognomonic for mycosis fungoides. Diseases resembling mycosis fungoides have been described in the dog,) cat,) calf,’ and Syrian hamster (Mesocricetus a~ratus).~ In laboratory mice, a condition resembling pagetoid reticulosis has been recently described.’ This report presents a case of epidermotropic cutaneous T-cell lymphoma (mycosis fungoides) supported by immunohistochemical staining of CD3 antigen in a rat. A male Sprague-Dawley rat (Charles River Laboratories, Spain), a member of the untreated control group of a longterm (27 months) carcinogenicity study of 400 animals, showed at week 107 of life a circumscribed erythematous patch with scaling in the interscapular region. At 1 15 weeks, two additional patches appeared on the dorsum of the animal. These lesions progressed to smooth, solid plaques that were erythematous, scaly, alopecic, and ulcerated. The animal did not show pruritus nor any other physical alteration, and his body weight was not diminished. Hematologic parameters indicated a slight leucocytosis. Other hematologic parameters were in the normal range of the other rats of the group. The rat was euthanatized at the end of the study (1 17 weeks of age). At necropsy, samples from all organ systems and from the cutaneous lesions were immersion-fixed in 10% buffered formalin and embedded in paraffin. Sections of 4 pm thickness were cut and stained with hematoxylin and eosin (HE).