Resolving diagnostic impasses

"Findings from routine microscopy using hematoxylin and eosin staining, combined with the clinical evaluation is sufficient to diagnose most skin lesions. However, immunohistochemical evaluation is an additional technique that can be a helpful adjunct in certain situations for identifying the line of differentiation of a cell or tissue type in a sample where the diagnosis based on conventional dermatopathology methods is more difficult. In addition, immunohistochemistry to assess certain biologic parameters, such as proliferative index or expression of tumor-related genes, can sometimes supply useful information for prognostication," according to Dr. Cockerell, clinical professor of dermatopathology, University of Texas Southwestern Medical Center, Dallas.

Immunohistochemistry studies are based on the use of monoclonal or polyclonal antibodies directed against specific antigens that are selectively expressed in certain types of cells, tissues or tumors. Dermatopathologists have a large number of immunohistochemical stains available to them, and they can be categorized into various subgroups based on whether they are markers for identifying epithelial differentiation (keratins and antigens for other glycoproteins), melanocytes, neuroendocrine differentiation, vascular endothelial cells/tissue, hematopoietic differentiation or metastatic tumors.

"Immunohistochemical studies are expensive, and the dermatopathologist must think about the contribution of either a positive or negative result for each stain ordered and weigh that against the cost incurred," Dr. Cockerell says.

Common applications There are a number of situations in dermatopathology where immunohistochemical stains are considered useful for distinguishing between different possible diagnoses.

One of the most common scenarios involves the evaluation of malignant spindle cell neoplasms on sun-damaged skin where the differential diagnosis includes poorly differentiated atypical squamous cell carcinoma, malignant melanoma and atypical fibroxanthoma. The stains most often used for discriminating between those lesions include the epithelial markers keratin and EMA, the melanocytic markers S-100 and HMB-45 and the histiocytic markers CD68 and procollagen 1.

Special stains are also helpful in the diagnosis of Merkel cell carcinoma of the skin where the differential diagnosis includes metastatic small cell carcinoma of the lung and other poorly differentiated basaloid tumors. The stains used in that situation include cytokeratin 20, which is relatively specific for Merkel cell carcinoma, along with other cytokeratins (AE1/AE3 and CAM 5.2) and markers for neural differentiation (neuron-specific enolase and chromogranin).

The panel might also include a marker for thyroid transcription factor-1, which is expressed in thyroid and lung and stains positive in nearly all small cell carcinomas of the lung, but negative in the vast majority of Merkel cell carcinomas.

Immunostaining is also a commonly used technique in diagnosing lymphoid and leukemic infiltrates. The panel of stains used in that situation includes various hematopoietic markers to identify T- and B-cells. When the differential diagnosis includes mycosis fungoides, the tissue specimen is examined for loss of CD2 and CD7 and to assess the CD4:CD8 ratio.

Distinguishing among Pagetoid processes and histiocytic and Langerhans cell infiltrates represent other common uses for immunostaining. In addition, it is applied to diagnose potential primary sites of cutaneous metastases.

Depending on the stains being used, fresh tissue may be preferred for the immunohistochemical studies. For example, a good result may not be obtained using tissue that has been placed in formaldehyde when doing flow cytometry studies and tests for various T-cell markers, such as CD2 and CD7, Dr. Cockerell notes.

"Dermatologists should always maintain good communication with their pathology lab, and clinicians should be aware that there may be cases where increased contact is necessary. For example, if there is a high index of suspicion of lymphoma and immunostaining will likely be necessary, the dermatologist should probably confer with the lab in advance of performing the biopsy to get instructions on proper specimen handling," Dr. Cockerell says.