Abstract
Poorly differentiated malignant neoplasia arising within the head and neck region may originate from diverse sources. We report a case of a cytologically undifferentiated malignant neoplasm clinically presenting as masses involving thyroid and parotid. Although PAX8 was immunoreactive and thus worrisome for anaplastic thyroid carcinoma, the tumor was eventually proven to represent PAX5 positive diffuse large B-cell lymphoma expressing cross-reactivity with polyclonal PAX8 antibody. Cross-reactivity between commercially available polyclonal PAX8 and PAX5 immunostains has been described in the literature but is not widely known, and it is a potential pitfall for making a misdiagnosis. This distinction can have importance in selection of subsequent clinical therapy and should be considered in choice of immunohistochemical stains for diagnostic purposes.
Highlights
Differentiated malignant neoplasms may be diagnostically challenging, especially in the head and neck region
PAX genes are further subdivided into subgroups on the basis of structural similarity with Subgroup II including PAX2, PAX5, and PAX8 [6]
Morgan et al reported reactivity of polyclonal PAX8 antibody in nonneoplastic B cells and mature B-cell neoplasms, but not with monoclonal PAX8 antibody within these tissues, and proposed cross-reactivity of the polyclonal commercially available PAX8 antibody with B-cell marker PAX5 [5]. They further cautioned that the diagnosis of B-cell lymphoma should be considered in cases of polyclonal PAX8 positive and epithelial marker negative neoplasia of unknown primary origin
Summary
Differentiated malignant neoplasms may be diagnostically challenging, especially in the head and neck region. Familiarity with the developmental embryology of head and neck anatomical structures as well as the unique molecular expression of common malignancies is necessary in interpreting immunohistochemistry (IHC) to pinpoint a specific diagnosis when confronted with an undifferentiated tumor. Knowledge of specific immunostain molecular targets and cross-reactivity of selected antibodies can be critical to avoid misinterpretation of overlapping immunoreactivity in unrelated diagnostic entities. More recent reports have noted cross-reactivity of commercially available polyclonal PAX8 immunostain antibodies with another pairedbox transcription factor, PAX5 [5]. We present a case of a 71-year-old woman presenting with poorly differentiated thyroid and parotid masses initially interpreted as PAX8-immunoreactive but subsequently determined to be cross-reactive with PAX5, changing the diagnosis and therapeutic options
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