This report presents the first real-time measurement of absorbed radiation dose during radioimmunotherapy in mice. Dose rate and total dose at the center of the tumor were measured after administration of <tex-math>${}^{90}{\rm Y}\text{-labeled}$</tex-math> antibodies using a miniature metal oxide semiconductor field-effect transistor radiation dosimeter probe which was inserted into the center of the tumor volume. Continuous real-time measurements were made for as long as 23 h after injection of the radiolabeled antibodies. Comparison of the real-time dose-rate measurements with estimates based on the MIRD formalism indicates good agreement. The real-time measurements are further compared to measurements made in a second experiment in which groups of mice were sacrificed at individual times after injection of the same radiolabeled antibodies. The real-time measurements agree well with the measurements in excised tumors. The real-time measurements have greater time resolution and are much more efficient than traditional uptake measurements.

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