**3. Actinium-225**

225Ac (T1/2 = 10 days), like 223Ra, is α-emitter radiopharmaceutical that decays to stable 209Bi through six radionuclides [24]. The 225Ac radioelement is a targeted alpha treatment that improves survival in individuals with metastatic castrationresistant prostate cancer. 225Ac emits a particle with E<sup>α</sup> = 6 MeV energy when it decays, yielding net four particles and three-particle disintegrations, the majority of which are high energy and useful gamma emissions, including 213Bi (T1/2 = 45.6 m; E<sup>α</sup> = 6 MeV Emax(β) = 444 keV and Eγ=440 keV), where this line has been used in imaging drug distribution [18]. Other daughters include 221Fr (T1/2 = 4.8 min; E<sup>α</sup> = 5 Mev and 218 keV γ energy line emission), 217At (T1/2 = 32.3 ms; E<sup>α</sup> = 7 MeV), 213Po

**Figure 2.** *The 225Ac radiopharmaceuticals position in radioactive 233U decay series.*

*Radium-223 and Actinium-225 α-Emitter Radiopharmaceuticals in Treatment… DOI: http://dx.doi.org/10.5772/intechopen.99756*

#### **Figure 3.**

*Schematic diagram depicting the concept of targeted radionuclide therapy employing alpha (α) particles in a tumor-targeting construct. The high linear energy transfer (LET) and short-range of particles make them highly desirable for use in cancer therapy.*

(T1/2 = 4.2 μs; E<sup>α</sup> = 8 MeV), 209Tl (T1/2 = 2.2 m; Emax(β) = 659 keV) (stable). Given 225Ac with a 10 d half-life, the high emission of alpha particles, and the favorable, fast 209Bi stable decay chain, this radionuclide is recognized to have a promising potential for cancer usage [2]. **Figure 2** illustrates the decay pattern for 225Ac. Also, **Table 1** shows 225Ac characteristics of radiopharmaceuticals. 225Ac is a potential candidate among the α particle-producing ions with characteristics suited for usage in targeted α therapy (TAT) (**Figure 3**).

The radiological half-life of 225Ac is 9.92 days, allowing it to be sent to clinics distant from the location of manufacture. Furthermore, this lengthy half-life is compatible with the use of macromolecular targeting vectors, such as antibodies or nanoparticles, which have long in-vivo circulation durations. As it decays to stable 209Bi, 225Ac produces eight short-lived progenies, producing a total of four highenergy a particle that kill cancer cells (**Figure 2**). Notably, in both in vitro and in vivo settings, 225Ac is far more potent than its daughter nuclide, 213Bi.

#### **4. Dose calculations**

The estimations of the doses were carried out with the application "Internal Computer Dose Assessment" (IDAC-Dose2.1). The dose coefficients of patients having radiopharmaceutical exams in nuclear medicine were calculated using this program by ICRP. In addition, dose estimates using the same ICRP computer architecture for the internal dose evaluation are the basis of the IDAC-Dose2.1 program.

For a time-independent system, the mean absorbed dose (D) to a target region (rT) is calculated using the equation below [25]:

$$(r\_T, T\_D) = \sum\_{r\_S} \tilde{A}(r\_S, T\_D).S(r\_T \leftarrow r\_S) \tag{1}$$

where *A r* <sup>~</sup>ð Þ *<sup>S</sup>*, *TD* is the cumulated activity (Bq) in source region *rS* over the integration period *TD*, and *S r*ð Þ *<sup>T</sup> rS* is the mean absorbed dose (Gy/Bq) in the


*\*Radiation weighting factor for α-radiation is 5, unit Gy/Bq as proposed by the ICRP [26].*

*\*\*Radiation weighting factor of 20 for α-radiation with unit Sv/Bq.*

*ICRP = International Commission on Radiological Protection.*

#### **Table 2.**

*Intravenous 223Ra and 225Ac radiopharmaceutical doses and the doses absorbed (Gy/Bq) determined by computer-assessing computer-dose 2.1 in human body organs.*

target tissue per nuclear transformation in the source region and defined by this Equation [26]:

$$\mathcal{S}(\mathbf{r}\_{\rm T} \leftarrow \mathbf{r}\_{\rm S}) = \sum\_{i} \Delta\_{i}.\mathbf{q}(\mathbf{r}\_{\rm T} \leftarrow \mathbf{r}\_{\rm S}, \mathbf{E}\_{i})\tag{2}$$

**Figure 5.** *The absorbed dose distribution of 225Ac radiopharmaceutical in some body organs.*

where φð Þ rT rS, Ei is the specific absorbed fractions (SAFs) value, and; Δ<sup>i</sup> ¼ *EiYi* (where *Ei* is the yield and *Yi* is the mean energy or part of the energy distribution for β-decay) of the *i* � *th* the nuclear transition of the radionuclide in joules [27].

The absorbed dose of radiopharmaceuticals 223Ra and 225Ac was estimated with IDAC-Dose2.1 and given in **Table 2**. The findings were computed after one hour of intravenous doses. In the prostate organ, the absorbed doses of 223Ra and 225Ac radiopharmaceuticals were determined to be 9.47 � <sup>10</sup>�<sup>9</sup> Gy/Bq and 1.91E-9Gy/Bq, respectively. This number represents 1% of the total body dosage. The greatest and least absorbed doses of 223Ra were observed in the Thymus (9.53 � <sup>10</sup>�<sup>8</sup> Gy/Bq) and Eye lenses (1.30 � <sup>10</sup>�<sup>10</sup> Gy/Bq) organs, respectively, according to biokinetics distribution. In addition, the 225Ac distribution in bodily organs reveals that the Spleen (1.47 � <sup>10</sup>�<sup>8</sup> Gy/Bq) has the greatest concentration absorbed dosage and the Eye lenses have the lowest.

**Figures 4** and **5** indicates doses taken in some human body organs by 223Ra and 225Ac radiopharmaceuticals, respectively. The histogram shows that 50 percent of the absorbed dosage is accumulated in six organs in both radiopharmaceuticals. These six organs are thymus, Spleen, lunge, kidney, colon wall, small intestine wall, Spleen, lung, kidney, colon wall, small intestine wall, and liver.
