PBL is an uncommon form of non-Hodgkin lymphoma that classically arises in the oral cavity of HIV-positive patients. PBL is a rapidly progressive lymphoma, and delay in treatment can adversely impact patient survival [7
]. Although prompt and accurate recognition of PBL is imperative, the diagnosis of PBL is not easy due to a variety of contributing factors. First, PBL is rare, and its description as a distinct tumor entity is relatively recent. Prior to its original description in 1997 [3
], most PBLs were likely diagnosed as CD20-negative large B cell lymphomas, intermediate forms of Burkitt lymphoma, or some poorly differentiated non-lymphoid malignancy [1
]. Indeed, even after its description as a distinct entity, Folk et al. [1
] found that up to 40% of cases submitted for pathologic consultation were initially regarded as poorly differentiated carcinomas. Second, PBLs have an unusual immunophenotype that renders them susceptible to pathologic misdiagnosis. PBL is classified as a B cell lymphoma and its cellular morphology resembles immunoblastic diffuse large B-cell lymphoma, yet these tumors fail to consistently and strongly express leukocyte common antigen and the B cell markers CD20 and CD79a [1
]. Diagnosis of PBL requires the use of additional markers to confirm plasmacytic differentiation. PBLs are immunoreactive for the plasmacytic markers CD138 and CD38. Unlike plasmacytomas, most PBLs are EBV related such that in situ hybridization for Epstein-Barr encoded RNA (EBER) is useful in distinguishing between these two plasma cell neoplasms [4
Finally, PBL may be entirely unanticipated when it occurs outside of the oral cavity. Although PBL has a strong predilection for the oral cavity, a growing experience has confirmed that its distribution is not limited to this single site. PBL has also been reported in the nasal cavity, skin, lymph nodes, lung, liver, stomach, anus, heart, and orbit [4
]. Accordingly, PBL should be included in the differential diagnosis of any high grade malignant neoplasm with plasmacytoid features, irrespective of anatomic site. This concept is underscored by our report of a PBL involving the parotid gland—a previously unreported site for PBL. At this site, PBL could be mistaken for poorly differentiated salivary gland carcinoma, metastatic malignant melanoma, or a plasmacytoid variant of a myoepithelial neoplasm. Accurate diagnosis requires some familiarity with this recently described entity, judicious use of immunohistochemical studies including markers for plasmacytic differentiation, and a readiness to include PBL in the differential diagnosis of parotid masses.
In conclusion, our report of PBL of the parotid gland further expands the list of anatomic sites involved by PBL, and it highlights the principle that this aggressive lymphoma must not be eliminated from the differential diagnosis based on extra-oral location, particularly in the setting of an HIV-positive patient. Awareness of this entity and its unusual immunophenotype can prevent misdiagnosis.