Practical determination of patient-specific marrow dose using radioactivity concentration in blood and body

Accurate determination of red marrow radiation is important because myeloto xicity is often dose limiting in radioimmunotherapy. The S-value methodolog y assumes a fixed red marrow mass as defined by the standard Medical intern al Radiation Dose (MIRD) mathematic phantom. Substantial error can be intro duced in marrow radiation estimates because red marrow mass varies from pat ient to patient. In this work we describe a patient-specific marrow dosimet ry methodology that does not require an explicit estimate of marrow mass. M ethods: Photon radiation to marrow from ail sources can be considered as th e total body to marrow. Based on photon radiation from body and electron ra diation from blood, a patient-specie marrow dose can be determined by count ing blood and total body radioactivity and measuring body weight. Results: The deviation in marrow dose calculation using total body to represent all photon radiation was 3.9% in 66 patients administered I-131-labeled antibod ies and was 9.1% in 18 patients administered Cu-67-labeled antibodies. The differences between this patient-specific approach and estimates based on s tandard anatomy were considerable, ranging from -35% to 88%. The difference s were greater when patients' weights differed substantially from the MIRD reference man phantom. Conclusion: For radiopharmaceuticals that do not bin d marrow, patient-specific marrow dosimetry can be independent of the actua l marrow mass of a patient. Patient-specific marrow dosimetry can be determ ined using radioactivity concentration in blood and body.