Differential diagnosis of malignant and benign cutaneous lymphoid infiltrates: a study of 57 cases in which malignant lymphoma had been diagnosed or suspected in the skin.

Clinical and histopathologic findings were reviewed in 57 cases in which there had been a diagnosis or suspicion of malignant lymphoma in the skin other than mycosis fungoides. Patients were included only if 1) extracutaneous tissue satisfactory for lymphoma classification was available, or 2) there was no evidence of extracutaneous involvement after a minimum of five years. Thirty-five patients had systemic lymphoma; in eight of these that disease first became manifest in the skin. There were two patients with typical lymphomatoid papulosis and one with a somewhat similar but solitary lesion which did not recur after excision; we have termed the latter a "solitary lymphomatoid papule." The remaining 19 cases without extracutaneous involvement were regarded as benign lymphoid hyperplasia. Clinically, the cutaneous lesions of malignant lymphoma were difficult to distinguish from those of lymphoid hyperplasia; only lymphomatoid papulosis was distinctive. Histologically, it was found that a definite diagnosis of malignant lymphoma (other than mycosis fungoides) could be made in the skin when three criteria were fulfilled: 1) exclusive presence or marked preponderance of medium-sized or larger lymphocytes, 2) absence of preferential involvement of the upper dermis, and 3) absence of extensive epidermal infiltration. These criteria were met in 21 cases of lymphoma. In both cases of lymphomatoid papulosis, the infiltrate was predominantly composed of large lymphocytes but was primarily limited to the upper dermis. The "solitary lymphomatoid papule" contained a similar cell population and involved the full thickness of the dermis; however, it displayed a prominent intraepidermal component. Fourteen cases of malignant lymphoma and all cases of lymphoid hyperplasia were characterized by a mixture of lymphocytes of varying size or a predominance of small lymphocytes. Nine cases of lymphoid hyperplasia exhibited at least one of three findings not observed in lymphoma; these findings were 1) germinal centers, 2) marked paucity of medium-sized lymphocytes in comparison to both small and large lymphocytes, and 3) sharp margination between adjacent groups of small and large lymphocytes. Aside from one case of small lymphocytic lymphoma which showed deep subcutaneous extension, the remaining cases (13 malignant and 10 benign) could not be reliably differentiated from each other by skin biopsy findings alone and were thus considered histologically indeterminate. The 13 lymphoma cases in this group were all of the small lymphocytic or follicular types; the failure of follicular lymphoma to exhibit a follicular pattern in the skin was a major impediment to its recognition in that location. The cutaneous infiltrates in all cases of large lymphocytic, Burkitt's, and lymphoblastic lymphoma were diagnostic of malignancy.