Recognition and Clinical Importance of Thorium Dioxide (Thorotrast®) Deposits in Human Unstained Bone Marrow Smears

Direct microscopic estimation of the amount of hemosiderin in an unstained bone marrow smear is of great value in the clinical evaluation of iron deficiency and iron excess.' Usually, the highly refractile golden or amber crystals of hemosiderin are easily identified and need be differentiated only from particulate artifacts in the unstained marrow preparation. In the course of evaluating several thousand unstained marrow smears for hemosiderin content, smears from three patients have been encountered which contained extensive crystalline appearing aggregates of thorium dioxide (Thorotrast®). The physical appearance of these deposits was similar but not identical to that of hemosiderin. A history of previous injection of Thorotrast for diagnostic purposes was obtained only in these three patients. An appreciation of the differences in the appearance of Thorotrast and hemosiderin in the marrow may be useful in the clinical recognition of excessive tissue stores of these substances.