Thirteen patients with chronic granulocytic leukemia were given 28 separate irradiations to the spleen for the treatment of their disease. The radiation was given as a single dose that ranged between 100 and 700 rads. The changes in the circulating white blood cell (WBC) count could be represented by the expression$N=B_{0}+S_{0}(e^{-\beta t})$ where N is the number of WBC per mm3 of blood at day t; S0 is the number of WBC, at time zero, originating from the spleen; and B0 is the number of WBC stemming from nonirradiated parts of the body, mostly bone marrow. The disappearance rate β, determined by an indirect method, indicated that S0 cells were leaving the blood with a half-time of 4.5 days. The number of S0 cells could be calculated a posteriori by the expression$S_{0}=(N_{0}-N)/(1-e^{-\beta t})$, where N0 is the number of WBC before spleen irradiation and N the number of WBC at t days after irradiation.

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