Dosimetry in Radiopharmaceutical Therapy

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FIGURE 1.
Graphical representation of dose deposition ranges delivered by different radionuclides having varying modes of decay. Top row shows relative geometric dose deposition delivered by point source of activity. Bottom row shows same data represented with 2-dimensional curve, illustrated as point spread function. Figure illustrates variable dose deposition properties and is not to scale.
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FIGURE 2.
General workflow for RPT dosimetry. Process begins with test administration (may be either pretherapy administration or first cycle of multidose therapy regimen). Serial quantitation measurements can then support calculation of absorbed doses, either in terms of tumor and organ mean doses (D) or dose distributions (D[x,y,z]). Dose estimation per unit of administered activity can then be used to tailor treatment. The term dose metric may refer to absorbed dose (for physical dosimetry) or biologically effective dose (BED), equieffective dose (EQD2_α/β), or effective uniform dose (EUD) (for bioeffect dosimetry).
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FIGURE 3.
Hybrid SPECT/planar imaging approach to imaging-based measurement of time–activity data (55).

TABLE 1.

Summary Characteristics of Commonly Used Radionuclides in RPT

Isotope	Primary emission	Half-life	LET	Maximum range in tissue (therapeutic radiation)
²¹¹At	α	7.21 h	High	80 μm
²¹²Pb	α	10.6 h	High	100 μm
²¹³Bi	α	45.6 min	High	100 μm
²²³Ra	α	11.4 d	High	70 μm
²²⁵Ac	α	10.0 d	High	85 μm
²²⁷Th	α	18.7 d	High	70 μm
⁶⁷Cu	β	61.8 h	Low	2.1 mm
⁹⁰Y	β	64.1 h	Low	11 mm
¹³¹I	β	8.02 d	Low	3.3 mm
¹⁵³Sm	β	46.5 h	Low	3.3 mm
¹⁷⁷Lu	β	6.65 d	Low	1.8 mm