Paraffin section immunohistochemistry as an adjunct to morphologic analysis in the diagnosis of cutaneous lymphoid infiltrates.

In the skin, separation of selected lymphomas from lymphoid hyperplasia can be challenging. The authors examined 45 cutaneous lymphomas, excluding mycosis fungoides (26 small lymphocytic or mixed tumors; 19 large cell lymphomas), 10 "atypical" lesions, and 40 lesions of presumed lymphoid hyperplasia, comparing morphologic attributes of such proliferations with their immunophenotypes in paraffin sections. The object of this study was to determine whether immunologic data obtained from routinely-processed specimens could be used to further objectify morphologic interpretations. Features favoring lymphoma included a lesional epicenter in the lower dermis or subcutis; poor circumscription of lymphoid aggregates; and dissection of lymphoid cells between collagen bundles. Immunostains included antibodies to CD20, CD43, CD45, CD45RO, CD45RA, CD68, proliferating cell nuclear antigen (PCNA), and MB2. Eleven of 26 small lymphocytic or mixed-cell lymphomas and 3 of 10 "atypical" cases demonstrated an abnormal immunophenotype, including co-expression of CD43 and CD20 or non-physiological CD45RA distribution. In contrast, none of 40 cases with benign features manifested aberrant antigen expression. Thirty-one of 37 cases in which > or = 75% of the cells typed as B lymphocytes showed malignant morphologic features, 5 were "atypical" and possibly lymphomatous, and only one had benign features. PCNA stains showed greater positivity of the lymphoid nuclei in lymphomas, and a labeling index of > 30% was correlated with malignancy in this context. These observations indicate that immunostaining may provide useful adjunctive information in distinguishing benign from malignant cutaneous lymphoid proliferations in paraffin sections.