Extramedullary haemopoiesis usually occurs in association with haematological disorders—in particular, myelofibrosis—and normally occurs in the reticulo-endothelial system, involving the liver, spleen, and lymph nodes. The kidneys and other organs are less often affected. In addition, single lineage haemopoiesis may occur, although it does not usually form mass-like lesions. This report describes a focus of erythropoiesis occurring in a renal cell carcinoma.

A 55 year old woman underwent a right radical nephrectomy and the specimen measured 10 × 6 × 6 cm. A 2.5 × 2.0 × 2.0 cm circumscribed nodule was present. Microscopic examination showed a clear cell renal carcinoma, nuclear grade 2, with central cystic degeneration. A single, extremely small focus of erthyropoiesis was present within a central small capillary, consisting of approximately 20 nucleated red blood cells (fig 1). A preoperative haemoglobin concentration was normal, at 132 g/litre (normal range, 115–185).

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Figure 1

 (A) Low power appearance of renal cell carcinoma with focus of erythropoiesis. (B) Intracapillary erythropoiesis.

Extramedullary haemopoiesis has been reported in the kidneys, usually associated with idiopathic myelofibrosis.^1,2 A renal cell carcinoma associated with a perirenal liposarcoma and extramedullary haemopoiesis has been documented.³ A superficial, spindle cell lipoma from the neck with extramedullary erythropoiesis has also been reported.⁴ Extramedullary haemopoiesis also occurs in hepatic angiomyolipoma (but not in renal angiomyolipoma) and in other hepatic tumours, an occurrence thought to be related to the hepatic sinusoidal endothelium.⁵ Foci of haemopoiesis or erythropoiesis have been described adjacent to recent, acute myocardial infarcts, thought to be a manifestation of altered cytokine production.⁶ Isolated megakaryocytes are a normal occurrence in the capillaries of the lung.⁷ They have been cited to occur in sentinel lymph nodes,⁸ although in lymph nodes they are usually present as part of microscopic foci of erythropoiesis and granulopoiesis. Extramedullary haemopoiesis usually occurs in tissues with a milieu that supports the proliferation of primitive haemopoietic bone marrow elements. Filtration of clonogenic bone marrow cells within supportive tissues is one pathogenetic mechanism considered in the pathogenesis of extramedullary haemopoiesis, whereas the migratory nature of megakaryocytes may explain their presence in aberrant sites in the absence of extramedullary haemopoiesis. Although this case may represent a transitory erythropoietic focus, a rare erythropoietin induced occurrence of erythropoiesis within a renal cell carcinoma is perhaps a more plausible explanation. Although it has been reported that 74% of renal cell carcinomas show strong erythropoietin immunolocalisation,⁹ foci of associated erythropoiesis appear to be unusual.