Morphology

Central to the diagnosis of myeloma is the demonstration of bone marrow infiltration by monoclonal plasma cells. The extent of infiltration typically exceeds 10% although it is well recognized that marrow disease can be patchy and a minority of patients with symptomatic myeloma will have <10% bone marrow plasma cells (BMPC)[1]. The 10% threshold chosen to distinguish myeloma from MGUS is somewhat arbitrary as both disorders form a continuous spectrum and it is therefore essential that the extent of marrow infiltration is correlated with the clinical features before a definitive diagnosis is made. It is recommended that a trephine biopsy be obtained in most patients as it provides a better assessment of the extent of marrow infiltration than even the best quality aspirate smears[2]. It is well recognized that bone marrow aspirate plasma cell counts consistently underestimate the overall level of infiltration[3–6]. An adequate trephine also ensures that a diagnosis can be made when the bone marrow aspirate specimen is of poor quality. Plasma cell cytomorphology varies considerably from patient to patient and a number of groups have proposed classification schema and have also demonstrated that cases with so-called blastic cytology have an inferior outcome[7–9]. However, such morphological classification schemes are poorly reproducible and have been largely superseded as prognostic tools by cytogenetic abnormalities and clinical parameters such as the International Staging System (ISS).