Extramedullary hematopoiesis

Findings

Renal ultrasound revealed large, echogenic kidneys with moderate hydronephrosis (Figure 1). Splenomegaly was also noted during the study. Non-contrast-enhanced CT showed extensive involvement of the abdomen with multiple soft-tissue density lesions. Masses were also seen surrounding, and inseparable from, the kidneys (Figure 2).

To further delineate the nature of these lesions, MRI was performed. T1-weighted MRI of the abdomen revealed low-signal lesions in the perirenal and pararenal locations, and heterogeneously hyperintense lesions in the pelvicalyceal regions (Figure 3). The pelvicalyceal lesions extended inferiorly along the proximal ureters in a symmetric fashion. T2-weighted MRI showed these masses to be homogeneously low-signal in all locations (Figure 4). Mild heterogeneous gadolinium enhancement was seen only in the pelvicalyceal components (Figure 5). Signal abnormalities were also noted in the liver and spleen on other images (not shown). Based on the imaging findings, extramedullary hematopoiesis (EMH), leukemia, lymphoma, and disseminated malignancy were the leading differential diagnostic considerations.

Pathologic Findings

CT- guided core biopsy and fine-needle aspiration of the right perirenal mass were performed. Hematoxylin and eosin stain showed numerous megakaryocytes, myeloid and erythroid precursors, and extracellular blood products (hemosiderin). No neoplastic cells were present. The final diagnosis was EMH (Figure 6).

Discussion

Extramedullary hematopoiesis is a well-recognized process in which the body attempts to maintain erythrogenesis in response to an alteration in the normal production of red blood cells.¹ It is observed in hemoglobinopathies, myeloproliferative disorders, neoplasms involving the bone marrow, and other conditions. Patients with hemoglobinopathies are more likely to have EMH in paraosseous locations, whereas patients with myeloproliferative disorders are more likely to have extraosseous masses.² More commonly observed areas of EMH include the paraspinal regions of the thorax, liver, and spleen, but it has been reported in other locations, including the adrenal gland, bowel, dura mater, and breast.^2-4 Only a few cases of perirenal and pelvicaliceal EMH have been reported in the literature.⁵

Extramedullary hematopoiesis in the abdomen most commonly involves the liver and spleen. Several different theories for the phenomenon have been proposed.^5,6 Recently, it has been postulated that hematogenous spread of multipotential stem cells occurs with eventual infiltration of various tissues and organs.⁵

Involvement of the kidneys with EMH is rare. It is more common in the parapelvic/pelvicalyceal areas (which are active during in utero erythrogenesis) than in the perirenal and pararenal locations. Perirenal and pararenal EMH can surround the kidneys without distorting their shape.⁷ In distinct contrast, our ultrasound showed significant hydronephrosis in large, echogenic kidneys. Other sonographic findings may be confused with renal cell carcinoma, renal lymphoma, or other diffuse pathologies such as polycystic kidney disease.⁸ Previous reports indicate that an enhancement pattern may be present in renal EMH .^7,9

The signal characteristics of these lesions on MRI suggested the presence of blood/iron products. As paramagnetic substances, iron in ferrous and ferric states (deoxyhemoglobin, methemoglobin, and hemosiderin) can shorten T1 and T2. Higher concentrations of hemosiderin, however, can cause a disproportionately greater effect on T2. As a result, appearance of blood products can vary from slightly increased signal to strikingly low signal on T1-weighted imaging.¹⁰ Decreasing T2 relaxation rates will result in low signal on T2-weighted imaging.

CONCLUSION

An important consequence of renal involvement of extramedullary hematopoiesis is renal failure from either obstruction or parenchymal infiltration. Recognition of the various manifestations of EMH is important for a prompt diagnosis and proper treatment. No imaging characteristic is pathognomonic, and a biopsy is often necessary to establish the definitive diagnosis. The patient received bilateral ureteral stents and is currently receiving low-dose radiation.

1. Resnick D. Hemoglobinopathies and other anemias. In: Resnick D. Diagnosis of Bone and Joint Disorders.Philadelphia, PA: W.B. Saunders, 2002:2171.
2. Dunnick NR. The Radiological Society of North America 85th Scientific Assembly and Annual Meeting: Image interpretation session: 1999. RadioGraphics.2000;20:257-278.
3. King BF, Kopecky KK, Baker MK, Clark SA. Extramedullary hematopoiesis in the adrenal glands: CT characteristics. J Comput Assist Tomogr. 1987;11:342-343.
4. Palmer GM, Shortsleeve MJ. Gastric polyps due to extramedullary hematopoiesis.AJR Am J Roentgenol.1998;171:531.
5. Rapezzi D, Racchi O, Ferraris AM. Perirenal extramedullary hematopoiesis in agnogenic myeloid metaplasia: MR imaging findings. AJR Am J Roentgenol.1997;168:1388-1389.
6. Dameshek W. Some speculations on the myeloproliferative syndromes. Blood.1951;6:372-375.
7. Georgiades CS, Neyman EG, Francis IR, et al. Typical and atypical presentations of extramedullary hematopoiesis. AJR Am J Roentgenol.2002;179:1239-1243.
8. Shawker TH, Hill M, Hill S, Garra B. Ultrasound appearance of extramedullary hematopoiesis. J Ultrasound Med.1987;6:283-290.
9. Kwak HS, Lee JM. CTfindings of extramedullary hematopoiesis in the thorax, liver and kidneys, in a patient with idiopathic myelofibrosis. J Korean Med Sci.2000;15:460-462.
10. Lee JW, Kim SH, Yoon CJ. Hemosiderin deposition on the renal cortex by mechanical hemolysis due to malfunctioning prosthetic cardiac valve: Report of MR findings in two cases. J Comput Assist Tomogr. 1999; 23:445-447.