Immunohistochemical patterns in the differential diagnosis of rhinopharyngeal granulocytic sarcoma.

Granulocytic sarcoma (GS) is a rare extramedullary manifestation of acute myeloid leukemia (AML). GS may develop simultaneously to AML or as a relapse of leukemia, particularly following allogeneic hematopoietic stem cell transplant. Subperiosteal bone, lymph nodes and skin are commonly involved, whereas rhinopharyngeal involvement is less common, with only 14 cases reported in the literature. Due to its rarity, rhinopharyngeal GS may lead to diagnostic pitfalls, particularly when it is poorly differentiated or is without concomitant marrow involvement. Thus, immunohistochemical findings play a key role in diagnosis. The current report describes a case of a 53-year-old female suffering from rhinopharyngeal GS and with a history of AML treated with chemotherapy and radiotherapy, focusing on the importance of the immunohistochemical pattern to assess the right diagnosis. Recent studies have demonstrated that the immunophenotype is of utmost importance for the diagnosis of GS. The high expression of myeloperoxidase (MPO) is common in GS; however, ~30% of GSs do not contain MPO. Therefore, the presence of other markers is required to confirm the diagnosis of GS.