Bone marrow dosimetry for radioimmunotherapy: theoretical considerations.

In most implementations of radioimmunotherapy, the red marrow is the dose-limiting organ. Estimates of the radioactivity concentration in this organ are usually based on blood sampling or bone marrow biopsy. This work presents a simple equation which may be used to calculate the red marrow-to-blood activity concentration ratio given the hematocrit and the red marrow extracellular fluid fraction of a patient. Also presented are a series of equations which may be used to correct the activity concentration in bone marrow biopsies for bone, yellow marrow and blood contamination. Assuming a hematocrit of 0.47 and a red marrow extracellular fluid fraction of 0.19, the predicted red marrow-to-blood activity concentration ratio is 0.36. This value is consistent with experimental studies. Depending upon the characteristics of the bone marrow biopsy sample, the red marrow activity concentration may be underestimated by a factor of 1.5-5. The correction factor for an "average" biopsy is 2.73. If the cortical bone cap is removed before the sample is weighed and counted, the correction factor is 1.92.