Contribution to red marrow absorbed dose from total body activity: A correction to the MIRD method

The contribution to red marrow absorbed dose from beta -emitting radionucli des distributed uniformly in the total body can be overestimated using eith er MIRD 11 or MIRDOSE3. The S value assigned to the red marrow target regio n from activity distributed in the remainder of the body is of particular c oncern. The assumption that the specific absorbed fraction for total body i rradiating red marrow and other skeletal tissues is the inverse of the tota l-body mass can result in an inappropriate remainder-of-body contribution t o marrow dose. We evaluated differences in the calculation of marrow dose u sing MIRD 11 and MIRDOSE3 formulations and developed methods to correct the results from either to remove inappropriate contributions. When bone takes up significantly less activity than is predicted from an apportionment of remainder-tissue activity based on mass, the standard remainder-of-body cor rection may substantially overestimate the electron component of the S valu e from remainder tissues to red marrow using either MIRD 11 or MIRDOSE3. If bone takes up activity, this contribution is negligible using MIRD 11 S va lues but remains with MIRDOSE3 S values. This overestimate can be significa nt, particularly when the residence time of activity in the remainder of th e body is much higher than in the red marrow and a different correction is needed. As the ratio of the remainder of body to marrow residence time is l owered, the overestimate becomes less significant. Conclusion: In this arti cle, we show the magnitude of this overestimate (which is most important fo r nuclides with large "nonpenetrating" emission components and for pharmace uticals that have a large ratio of remainder of body to marrow residence ti mes), show the appropriate corrections to be made in each case, and propose a new method for calculating marrow dose contributions that will avoid thi s complication in future applications. Because all models give approximate doses for real patients, with uncertainties within those involved in these corrections, we do not suggest that changes be made to existing marrow dose estimates. We suggest only that future calculations be as accurate as poss ible.