Signet ring cell lymphoma. A rare morphologic and functional expression of nodular (follicular) lymphoma.

Nodular lymphomas and diffuse lymphomas of corresponding cellular composition have been shown to arise from follicular center cells. This paper describes a rare functional and morphological expression of malignant lymphomas arising from follicular center cells, namely, immunoglobulin production, an observation for which no detailed description or nanlysis is available in the literature. Furthermore, the unusual signet ring-like appearance of the lymphoma cells, which is due to retention of immunoglobulins within the cytoplasm, may result in an erroneous interpretation of metastatic adenocarcinoma or liposarcoma. Therefore, we are presenting a detailed analysis of light microscopic, histochemical, immunocytochemical, and ultrastructural observations. The lymphomas of all seven patients in our series showed nodular growth patterns; in all but one, diffuse areas were also observed. Five of the lymphomas were classified as poorly differentiated lymphocytic type and two as mixed cell type, according to Rappaport's classification. In four of the seven patients, the majority of the neoplastic cells had a clear vacuolated cytoplasm, and in three of these cases, a few of the neoplastic cells showed immunoperoxidase positivity for monoclonal IgG. This group in particular closely simulated metastatic carcinoma composed of so-called signet ring cells. In the remaining three cases, most of the neoplastic cells contained PAS-positive, Russell body-like monoclonal IgM. Ultrastructurally, the monoclonal IgG appeared as even-sized electron-dense spherules or irregular electron-dense clumps, while the monoclonal IgM appeared as membrane bound, homogeneous, electron-dense material. The implications of these findings and the morphologic features which are helpful in the identification of these lymphomas are discussed.