Extramedullary myeloid cell tumours localised to the mediastinum: a rare clinicopathological entity with unique karyotypic features.

Extramedullary myeloid cell tumour (EMMT) localised to the mediastinum is a rare manifestation of acute myeloid leukaemia, forming less than 4% of all cases of EMMT. In contrast to other types of EMMT, cytogenetic characteristics of this rare entity are relatively unknown. This report describes a patient with EMMT who had evidence of superior vena cava syndrome and normal peripheral blood counts at diagnosis. The results from an initial biopsy specimen were consistent with a diagnosis of mediastinal large B cell lymphoma. A diagnosis of acute myeloid leukaemia was made three months after initial diagnosis by bone marrow examination. Review of the initial biopsy specimen showed strong positivity for myeloperoxidase, revealing that the patient had been initially misdiagnosed as having large B cell lymphoma. Cytogenetic studies revealed a near triploid and near tetraploid karyotype with structural abnormalities in 12 and three metaphases, respectively. Review of the literature showed that a near tetraploid or triploid karyotype is found in most of the reported cases of mediastinal EMMT. Thus, the presence of a near triploid/tetraploid karyotype and mediastinal EMMT may represent a specific subset of EMMT. The biological relevance of this observation is discussed.