We reviewed 41 cases reported by a referring pathologist / cytopathologist to have a plasma cell neoplasm. All cytological specimens were reviewed by a joint-panel of cytologists, and all histopathological specimens were reviewed independently by a joint-panel of histopathologists. Where the diagnosis of a MRD was in question, we carried out further histochemistry and / or immunohistochemistry prior to inclusion or rejection: toluidine blue, astra blue, lysozyme, fascin, Melan A, BLA36, cytokeratin, MHC II, CD79a, CD3 and immunoglobulin heavy (α, γ and μ) and light chains (λ and κ). Four cases with both κ and λ light chain immunolabelling were excluded from further analysis as these were interpreted as reactive plasma cell accumulations rather than neoplastic (MRD) lesions. Eleven other cases were found to have been mis-diagnosed as MRD (n=4 mast cell tumours, n=4 histiocytic tumours, n=2 poorly-differentiated round cell tumours, n=1 lymphoma (non-Ig secreting)) (Mellor et al 2008). Note that overall, there was good concordance between cytology and histopathology in the morphological categorization of MRD in our series.

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Mis-diagnosed Fig. 1: Originally diagnosed as a cutaneous plasmacytoma by the referring pathologist on the basis of examination of an HE section, this astra blue stain reveals an alternate round cell tumour diagnosis: mast cell tumour. This case was excluded from our case series.

Mis-diagnosed Fig. 2: Originally diagnosed as a MRD by the referring cytopathologist (May-Grünwald-Giemsa stained smear). However, examination of histopathological sections including HE, IgA, IgG, IgM, Igλ, Igκ, CD79a, BLA36, CD3, Melan A, cytokeratin, lysozyme and fascin stained sections revealed a diagnosis of a histiocytic tumour. This case was excluded from our case series (Mellor et al 2008).