The relationship between marrow distribution as imaged with 99Tc(m) sulfur colloid and histologic estimate of bone marrow cellularity was examined in 101 patients. The patients were divided into three groups according to marrow cellularity: normocellular, hypercellular or hypocellular marrow. The marrow distribution of these patients was graded as extended, not extended or not visualized. A general association between marrow cellularity and marrow distribution was found. Seventy seven per cent of patients with normocellular marrow had limitation of marrow within normal sites and a similar number of patients with reactive hypercellular marrow had marrow extension. On analysis of the patients that were exceptions to this generalization the following observations were made. In patients in whom the marrow is hypercellular and infiltrated with abnormal cells or tissue the marrow distribution may not be visualized with radiocolloid. Stimulation of hematopoiesis may result in marrow hypercellularity without marrow extension beyond the usual sites. Stimulation of one or more hematopoietic cell lines may result in marrow extension without marrow hypercellularity, suggesting an impairment of hematopoietic cell response. Peripheral marrow extension as observed with radiocolloid, therefore, is not simply a reflection of marrow hypercellularity but probably represents a response of the bone marrow stroma to situations in which one or more hematopoietic cell lines is stimulated.
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