Anatomic Pathology / CALRETININ DIFFERENTIATES SCHWANNOMAS FROM NEUROFIBROMAS

We studied 25 cases of schwannoma and 42 cases of neurofibroma immunohistochemically with antibodies to calretinin and S-100 protein to explore the potential usefulness of calretinin in differentiating schwannomas from neurofibromas. Of 25 schwannomas, 24 (96%) showed moderate to strong staining for calretinin, with the extent of staining ranging from focal to diffuse. In contrast, only 3 (7%) of 42 neurofibromas displayed focal weak to moderate staining with calretinin. All 42 cases of neurofibromas and all 25 cases of schwannomas showed diffuse moderate to strong staining with S-100 protein. Calretinin also labeled mast cells, whose presence was confirmed further by staining for c-kit, which commonly was present in both tumor types in a scattered individual cell pattern easily differentiated from the clustered pattern of neoplastic spindle cells. Taken together, these results indicate that calretinin is detected in almost all schwannomas and in only a small percentage of neurofibromas, suggesting it is a useful marker for differentiating schwannomas from neurofibromas. Although mast cells present in these 2 neoplasms also react with calretinin, the pattern of staining can be distinguished easily from that of neoplastic cells. Schwannomas and neurofibromas are the 2 most common benign neoplasms derived from peripheral nerve.1 Although the early literature suggested that both were pure Schwann cell lesions, subsequent ultrastructural studies2 have shown that schwannomas are composed exclusively of neoplastic cells that simulate the appearance of differentiated Schwann cells, while neurofibromas display a mixture of cell types, including Schwann cells, perineurial cells, and endoneurial fibroblasts.3 Schwannomas typically are well circumscribed and composed of spindle cells organized as cellular areas with nuclear palisading (Antoni A) and paucicellular areas (Antoni B). Neurofibromas are less cellular and not as circumscribed as schwannomas and contain considerable extracellular myxoid material, wavy collagen fibers, and occasional neurites.4 Although these tumors generally are not difficult to differentiate by standard light microscopy, in a small number of cases, they might closely resemble one another. Nuclear palisading is not present in all schwannomas, making some lesions potentially difficult to separate from cellular neurofibromas. Furthermore, schwannomas consisting exclusively of Antoni B areas are sparsely cellular and myxomatous and might mimic the histologic appearance of neurofibromas on H&E staining. The importance in distinguishing these 2 entities lies in the association of some neurofibromas with neurofibromatosis and associated syndromes.5 Immunohistochemical staining for S-100 protein has been used as an adjuvant marker in the differential diagnosis of schwannoma and neurofibroma.6 Although the percentage of positive cells and the intensity of staining usually are higher in schwannomas, S-100 might not reliably distinguish the 2 neoplasms. Calretinin, a calcium-binding protein Anatomic Pathology / ORIGINAL ARTICLE Am J Clin Pathol 2004;122:552-559 553 553 DOI: 10.1309/AGBGTBRJ4W0BC7LN 553 © American Society for Clinical Pathology belonging to the same protein family as S-100,7 is expressed primarily in cells of the central and peripheral nervous systems.8,9 We have previously shown calretinin to be a useful marker for granular cell tumors,10 neoplasms considered to be of schwannian origin,11 suggesting the value of calretinin in identifying Schwann cell lesions. In the present study, we evaluated and compared the expression of calretinin with that of S-100 to determine the usefulness of calretinin in distinguishing these 2 common neoplasms. Materials and Methods Formalin-fixed, paraffin-embedded tissue blocks of schwannomas (25 cases), neurofibromas (42 cases), and nonneoplastic tissue samples containing peripheral nerves (8 cases) were selected from the surgical pathology archives of Montefiore Medical Center, Bronx, NY. All sections were deparaffinized, rehydrated, and quenched with hydrogen peroxide. Antigen retrieval was performed as follows: for S100, calretinin, and c-kit stains, slides were incubated with DAKO Epitope Buffer (DAKO, Carpinteria, CA) in a steam bath at 95°C for 45 minutes. After equilibration in phosphate-buffered saline for 15 minutes, the slides were placed in an autostainer (DAKO) and stained with antibodies to S100 protein (polyclonal, 1:4,000 dilution; DAKO), calretinin (polyclonal, 1:50 dilution; Zymed, San Francisco, CA), and c-kit (polyclonal, 1:50 dilution; DAKO). Immunoreactivity was detected using DAKO EnVision methods (DAKO) according to manufacturer-recommended procedures. For negative control experiments, slides were treated with the same procedure, including antigen retrieval, except for replacement of primary antibodies with a negative diluent (Zymed). Immunoreactivity was evaluated for intensity of labeling (–, 1+, 2+, and 3+) and percentage of cells stained (<25%; 25%-75%, and >75%).