Observational retrospective study of altered biodistribution of tositumomab and ¹³¹I-tositumomab

The tositumomab/¹³¹I-tositumomab radioimmunotherapy regimen is administered as a dosimetric dose followed by a therapeutic dose. The biodistribution of the dosimetric dose is assessed by quantitative calculations of whole-body residence time (TBRT) and visual examination of whole-body γ-camera images, to determine the administered radioactivity dose and whether a therapeutic dose can be administered. We investigated whether altered biodistribution of ¹³¹I-tositumomab could be identified using quantitative TBRT. Methods: BioClinica, Inc., provided γ-camera images to an independent reviewer to assess altered ¹³¹I-tositumomab biodistribution in patients reported to a registry. Results: Of 2,649 therapeutic doses, 5 (0.2%) were cancelled because of altered biodistribution as determined by γ-camera images and TBRT. Of these, 3 γ-camera images were assessed by the independent reviewer; one showed altered biodistribution (0.04%) and was in agreement with the TBRT on-site calculation. Conclusion: TBRT alone should be used to determine altered biodistribution and hence whether to administer the therapeutic dose.