Introduction

Historically, there has been much confusion regarding the spectrum and classification of malignancies of the monocyte/macrophage system, largely as original classifications were based on morphological appearance alone or with histochemical stains. This approach resulted in misinterpretation of cell lineage in a substantial proportion of cases, especially amongst the lymphomas. More recently, the widespread availability of immunohistochemistry plus cytogenetics to inform and support diagnosis, has established cell lineage in the majority of cases and allowed greater accuracy in classification of these tumours. Based on current knowledge, an appropriate classification is to divide these malignancies between tumours of dendritic cells (follicular dendritic cell (FDC) and interdigitating reticulum cell sarcomas) and tumours of monocyte/macrophage lineage (acute myelomonocytic leukaemia, acute monocytic leukaemia, chronic myelomonocytic leukaemia (CMML) and juvenile myelomonocytic leukaemia (JMML)).

Acute myeloid leukaemia (AML) with involvement of the monocyte lineage has been well defined for many years and classified as AML M4 (myelomonocytic) and M5 (monocytic) under the French–American–British (FAB) system. Originally identified by morphological criteria and cytochemistry, these entities have become better defined by immunophenotyping and the identification of associated cytogenetic changes (see below). Localized tumour masses comprised of these cells are well recognized, and may occur with or without clinically apparent bone marrow disease. In apparently isolated tumour masses, eventual detection of bone marrow involvement is usual in patients who have not received comprehensive systemic therapy.