Influence of total-body mass on the scaling of S-factors for patient-specific, blood-based red-marrow dosimetry

To perform patient-specific, blood-based red-marrow dosimetry, dose conversion factors (the S factors in the MIRD formalism) have to be scaled by patients' organ masses. The dose to red marrow includes both self-dose and cross-irradiation contributions. Linear mass scaling for the self-irradiation term only is usually applied as a first approximation, whereas the cross-irradiation term is considered to be mass independent. Recently, the need of a mass scaling correction on both terms, not necessarily linear and dependent on the radionuclide, has been highlighted in the literature. S-factors taking into account different mass adjustments of organs are available in the OLINDA/EXM code. In this paper, a general algorithm able to fit the mass-dependent factors S-rm <- tb and S-rm <- rm is suggested and included in a more general equation for red-marrow dose calculation. Moreover, parameters to be considered specifically for therapeutic radionuclides such as I-131, Y-90 and Lu-177 are reported. The red-marrow doses calculated by the traditional and new algorithms are compared for 131I in ablation therapy (14 pts), Lu-177- (13 pts) and Y-90- (11 pts) peptide therapy for neuroendocrine tumours, and 90Y-Zevalin therapy for NHL (21 pts). The range of differences observed is as follows: -36% to -10% for I-131 ablation, -22% to 5% for Lu-177-DOTATATE, -9% to 11% for Y-90-DOTATOC and -8% to 6% for Y-90-Zevalin. All differences are mostly due to the activity in the remainder of the body contributing to cross-irradiation. This paper quantifies the influence of mass scaling adjustment on usually applied therapies and shows how to derive the appropriate parameters for other radionuclides and radiopharmaceuticals.