Alpha-emitting Radionuclides Research Articles

Wide-band X-ray and gamma-ray imaging for clinical application; visualization of pharmacokinetics in targeted alpha therapy

Targeted alpha therapy (TAT) is a nuclear medicine therapy which administers alpha-emitting drugs and causes localized and effective damage to tumors. To investigate whether the drugs accumulate in the tumor correctly, in-situ visualization of the distribution of the drugs is essential. Visualization of alpha-emitting drugs using X-rays and gamma rays emitted simultaneously with the alpha-rays is suggested in theranostics, a combination of therapy and diagnostics. Since various X-rays and gamma rays ranging from a few tens of keV to several MeV are emitted from the alpha-emitting radionuclides, wide-band X-ray and gamma-ray imagers are required for a more versatile visualization of alpha-emitting drugs. Therefore, we developed a hybrid Compton camera (HCC) to visualize broadband X-rays and gamma rays. In this study, we visualized representative alpha-emitting drugs, Ra-223 dichloride and At-211 NaAt, using HCC. Ra-223 dichloride in the human body was imaged using gamma rays around 300 keV, and in-vivo imaging of At-211 NaAt in a mouse was performed using 79-keV X-rays. This study demonstrates the potential of HCC as a versatile imaging device for alpha-emitting drugs.

A calibration methodology for the novel radioluminescence detector systems

Radioluminescence mapping using well-characterized and optimized optical systems is an effective method for localizing contamination with alpha-emitting radionuclides. To apply this novel approach to radiological emergency management, nuclear safeguards, nuclear decommissioning, and nuclear forensics, an established traceability chain is required. This work presents the development and implementation of a novel calibration methodology to provide valuable information about, and confidence in, the performance of radioluminescence detection systems. The proposed calibration methodology is based on two complementary approaches: (a) application of well-characterized activity standards to establish a traceable relationship between radioluminescence intensity and alpha activity, and (b) use of all optical radiation-based devices that, when calibrated against an alpha activity standard, simulate the radioluminescence induced in nitrogen (N2) and nitric oxide (NO) gases by alpha particles in specific spectral regions. A dedicated 210Po alpha activity standard with a narrow peak of less than 32keV FWHM at 5.3MeV has been developed and used to characterize a lens-based radioluminescence detection system in terms of its sensitivity to alpha-induced radioluminescence in different atmospheres (air, N2, N2 ＋ NO mixture) in the UV-A and UV-C (solar blind) spectral regions. The characterized lens system was then used as an intermediary transfer device to cross-calibrate two portable integrating sphere-based radiance standards designed to simulate radioluminescence in the UV-A and UV-C spectral regions. Both radiance standards can be used as transfer standards simulating up to 510MBq when used in the UV-A and up to 8.7GBq when used in the UV-C spectral regions. They substantially simplify routine quality control of radioluminescence detection systems by eliminating the need for open alpha sources, which are always associated with strict radiation safety precautions. Furthermore, since radiance standards can operate in a wide range of intensities, linearity and detection limits of radioluminescence detectors can be readily determined. The design, construction, radiometric characterization, and calibration of dedicated transfer standards, as well as the development of new calibration procedures for radiometric traceability of radioluminescence detection systems, will enable appropriate accident and post-accident radiation measurements that will lead to more effective countermeasures and better protection of people, wildlife, and the environment.

Thermoluminescent dosimeters (TLD-100) for absorbed dose measurements in alpha-emitting radionuclides

Early works that used thermoluminescent dosimeters (TLDs) to measure absorbed dose from alpha particles reported relatively high variation (10%) between TLDs, which is undesirable for modern dosimetry applications. This work outlines a method to increase precision for absorbed dose measured using TLDs with alpha-emitting radionuclides by applying an alpha-specific chip factor (CF) that individually characterizes the TLD sensitivity to alpha particles. Variation between TLDs was reduced from 21.8% to 6.7% for the standard TLD chips and 7.9% to 3.3% for the thin TLD chips. It has been demonstrated by this work that TLD-100 can be calibrated to precisely measure the absorbed dose to water from alpha-emitting radionuclides.

Selection of radionuclide(s) for targeted alpha therapy based on their nuclear decay properties.

Targeted alpha therapy (TAT) is a promising form of oncology treatment utilising alpha-emitting radionuclides that can specifically accumulate at disease sites. The high energy and high linear energy transfer associated with alpha emissions causes localised damage at target sites whilst minimising that to surrounding healthy tissue. The lack of appropriate radionuclides has inhibited research in TAT. The identification of appropriate radionuclides should be primarily a function of the radionuclide's nuclear decay properties, and not their biochemistry or economic factors since these last two factors can change; however, the nuclear decay properties are fixed to that nuclide. This study has defined and applied a criterion based on nuclear decay properties useful for TAT. This down-selection exercise concluded that the most appropriate radionuclides are: 149 Tb, 211 At/ 211 Po, 212 Pb/ 212 Bi/ 212 Po, 213 Bi/ 213 Po, 224 Ra, 225 Ra/ 225 Ac/ 221 Fr, 226 Ac/ 226 Th, 227 Th/ 223 Ra/ 219 Rn, 229 U, 230 U/ 226 Th, and 253 Fm, the majority of which have previously been considered for TAT. 229 U and 253 Fm have been newly identified and could become new radionuclides of interest for TAT, depending on their decay chain progeny.

The Mapping of Alpha-Emitting Radionuclides in the Environment Using an Unmanned Aircraft System

The protection of first responders from radioactive contamination with alpha emitters that may result from a radiological accident is of great complexity due to the short range of alpha particles in the air of a few centimeters. To overcome this issue, for the first time, a system mounted on a UAS for the near-real-time remote measurement of alpha particles has been developed, tested, and calibrated. The new system, based on an optical system adapted to be installed on a UAS in order to measure the UV-C fluorescence emitted by alpha particles in the air, has been tested and calibrated, carried out in the laboratory and in field experiments using UV-C LEDs and 241Am sources. In experimental flights, the probability of detecting a point source was determined to be approximately 60%. In the case of a surface extended source, a detection efficiency per unit surface activity of 10 counts per second per MBq cm−2 was calculated. A background count rate of UV-C of around 26 ± 28 s−1 for an integration time of 0.1 s was measured during flights, which led to a decision threshold surface activity of 5 MBq cm−2.

Remote measurement and calibration of alpha-emitting radionuclides through the utilization of characteristic ultraviolet fluorescence

During the decommissioning of nuclear facilities and spent fuel reprocessing, conventional alpha particle detectors are insufficient for accurately measuring the emissivity of alpha particles due to their high radiation intensity and complex nuclide types. The air fluorescence method is developed to detect the α particle emissivity through measuring the intensity of ultraviolet fluorescence generated by the interaction between α particles and nitrogen or other inert gas mediums. This method offers advantages such as long detection range and immunity to γ radiation interference. The measurement results are primarily influenced by factors including α particle energy, detection distance, and gas medium composition, with photon noise and detection distance determining the detection limit. The obtained results indicate that: In the condition of measured in darkness, measurement time for 1000s, and distance for 1 m, The minimum detection limit for α sources is 7.6 kBq with a relative standard uncertainty of 10%.

Using VARSKIN+v1.2 to estimate dose from direct skin contamination with radionuclides 223 Ra, 212 Pb and 225 Ac; considerations for Nuclear Medicine staff and associated Personal Protective Equipment (PPE).

To calculate depth-weighted doses for 223 Ra, 212 Pb and 225 Ac for the skin sites of trunk, arms/legs, face, wrist, back of hand, fingertip, back and side of fingers using VARSKIN+v1.2. Published depth distribution histograms of the basal cells were used with dose averaging in VARSKIN+v1.2. A density correction factor was applied for the 1 g/cc within VARSKIN. Results were compared to the regulatory 70 µm depth and to average depth values for the skin sites. 223 Ra has no alpha component at the regulatory 70 µm. This dose is exceeded by the depth-weighted dose rates for all sites (except the fingertip) with factors ×74 (back of finger) to x3600 (trunk). 212 Pb and 225 Ac have alpha contributions at 70 µm. . For 212 Pb, this dose value is greater by over ×2 than the depth-weighted dose rate for the wrist, back of hand, and finger sites, and underestimates dose rates for the other sites. For 225 Ac, the 70µm dose rate is exceeded by the depth-weighted dose rates for the trunk, face, arms/legs by factors of ×4-10. Using fixed depth values, the depth-weighted dose rates are larger for all sites except the fingertip. The skin dose is also calculated for biological half-lives of 1, 3 and 6 h. Using the depth-weighted dose rates and a 3 h biological half-life, the activity for 500 mSv is in the range 9-177 Bq for the trunk, face, arms/legs, wrist and hand for all three radionuclides. For alpha-emitting radionuclides a depth-weighted calculation gives more representative dose values. The very low activity values for 500 mSv skin dose to be exceeded have implications for appropriate staff PPE and training.

Characterization of a segmented printed circuit board (PCB) as a standard for absorbed dose to water from alpha-emitting radionuclides.

Our previous work introduced and evaluated a standard for surface absorbed dose rate per unit radioactivity to water from unsealed alpha-emitting radionuclides used in targeted radionuclide therapy (TRT). An overall uncertainty over 4.0% at k=1 was reported for the absorbed dose to air measurements, which was partially attributed to the rotational alignment uncertainty in the geometrical setup. A printed circuit board (PCB) with a segmented guard was constructed to align the extrapolation chamber (EC) and the source plates using a differential capacitance technique. The PCB EC aimed to enhance the repeatability of the ionization current measurements. The PCB EC was evaluated using a thin film 210Po source. The measured absorbed dose to air cavity was compared with the Monte Carlo (MC) calculations. Using the extrapolation method, the surface absorbed dose rate to water was calculated. The PCB EC was constructed with a 4.50mm diameter collector surrounded by four sectors and a guard electrode. The sectors were isolated for rotational alignment and later connected to the guard for ionization current measurements. A bridge circuit measured differential capacitance between opposing sectors, and a hexapod motion stage rotated the source substrate to minimize the differential capacitance. The EC was evaluated using a 210Po source with a 3.20mm diameter and 1.253 Ci radioactivity. MC simulations were performed to calculate the , , and correction factors. Ionization current measurements were performed for air gaps in the 0.3-0.525mm range and surface absorbed dose rate to water was calculated. Rotational offsets of up to 3.0° were found and the current repeatability was found to increase with the absorbed dose to air uncertainty calculated to be ∼2.0%. Using the capacitance method, the effective EC diameter was measured to be 4.53mm. The recombination, polarity, and electrometer corrections were reported to be within 1.00% across all measurement trials. The MC-calculated correction factors were calculated to be much larger than the recombination and polarity correction factors. The average , , and corrections were calculated to be 1.063, 0.9402, and 2.136, respectively. The MC-calculated absorbed dose to air was found to overestimate the absorbed dose by over 4.00% when compared with the measured absorbed dose to air. The surface absorbed dose rate to water was calculated to be Gy/s/Bq with an overall uncertainty of 4.07%. The constructed PCB EC was deemed suitable as an absorbed dose standard. A repeatable rotational alignment was achieved using the differential capacitance technique. The metal electrodes on the PCB made a difference of<1.00% on the backscatter correction when compared to the EC comprised of polystyrene-equivalent collector. A 20% difference in the surface absorbed dose rate to water was found between the two ECs, which is attributed to the cavity diameter differences leading to different magnitudes of dose fall-off along the lateral direction.

Alpha-Emitting Radionuclides: Current Status and Future Perspectives.

The use of radionuclides for targeted endoradiotherapy is a rapidly growing field in oncology. In particular, the focus on the biological effects of different radiation qualities is an important factor in understanding and implementing new therapies. Together with the combined approach of imaging and therapy, therapeutic nuclear medicine has recently made great progress. A particular area of research is the use of alpha-emitting radionuclides, which have unique physical properties associated with outstanding advantages, e.g., for single tumor cell targeting. Here, recent results and open questions regarding the production of alpha-emitting isotopes as well as their chemical combination with carrier molecules and clinical experience from compassionate use reports and clinical trials are discussed.

Fluorine-18 incorporation and radiometal coordination in macropa ligands for PET imaging and targeted alpha therapy.

The development of theranostic agents for radiopharmaceuticals based on therapeutic alpha emitters marks an important clinical need. We describe a strategy for the development of theranostic agents of this type via the functionalization of the ligand with the diagnostic radionuclide fluorine-18. An analogue of macropa, an 18-membered macrocyclic chelator with high affinity for alpha therapeutic radiometals, was synthesized and its complexation properties with metal ions were determined. The new macropa-F ligand was used for quantitative radiometal complexation with lead-203 and bismuth-207, as surrogates for their alpha-emitting radioisotopes. As a diagnostic partner, a radiofluorinated macropa ligand was used for quantitative bismuth(III) and lead(II) complexation. All fluorine-18 and radiometal complexes are highly stable in human serum over several days. This study presents a new proof-of-principle approach for developing theranostic agents based on alpha-emitting radionuclides and fluorine-18.

Radioluminescence mapping of 241Am-doped environmental samples and nuclear materials

In the framework of the project RemoteALPHA, several radioluminescence scanning systems were developed to establish the metrological basis for the optical detection of alpha-emitting radionuclides. This work demonstrates an application of the ultraviolet imaging system for the characterization of low-activity alpha sources: contaminated environmental samples, and nuclear materials featuring depleted uranium. The measurements were performed at a reference distance of 2 m from the source. The weakest samples were measured in a chamber with an N2 + NO gas atmosphere. Using the scanning method, the total surface activity was calculated for analyzed samples, with the lowest reported value of just 51 Bq.

Advancing environmental monitoring: A comparative analysis of sequential radiochemical methods for precise alpha-particle spectrometry analysis of 241Am in soil and marine sediment samples

Environmental monitoring of alpha-emitting radionuclides is crucial for assessing the long-term impacts of nuclear activities and understanding ecosystem dynamics. Thus, long-term marine environmental monitoring needs a precise analytical method for determining isotopes in sediment and soil samples. Because 241Am is the first decay product of 241Pu, its activity in the environment will grow during the next 50 years. This study aims to compare and evaluate different sequential radiochemical methods for 241Am analysis, focusing on their precision and accuracy, to establish a more rapid, refined and reliable approach for environmental monitoring. Four radiochemical methods for 241Am determination in soil and marine sediment samples were compared using alpha-particle spectrometry. This sensitive technique plays a key role in quantifying low-level radioactive contaminants in sediment and soil samples. The procedure was optimised using reference materials prepared by IAEA: sediment (IAEA-300, IAEA-384 and IAEA-385) and soil (IAEA-327 and IAEA-375), and digestion, radiochemical separation, and purification on anion exchange resin (AG 1X8) and chromatographic resins (UTEVA, TRU) were used. The effectiveness of the Ca-oxalate precipitation method for iron removal was also evaluated. We used microcoprecipitation on neodymium fluoride (NdF3) to prepare measurement sources. As a significant outcome, we suggested the potential exclusion of co-precipitation (method D) in the determination of the 241Am isotope in sediment and soil samples. This recommendation aligns with our goal of developing a precise sequential radiochemical method for alpha-particle spectrometry. Our approach offers a valuable and optimised tool for monitoring the environmental impact of nuclear activities and accidents, particularly in marine ecosystems.

A GATE simulation study for dosimetry in cancer cell and micrometastasis from the 225Ac decay chain

BackgroundRadiopharmaceutical therapy (RPT) with alpha-emitting radionuclides has shown great promise in treating metastatic cancers. The successive emission of four alpha particles in the 225Ac decay chain leads to highly targeted and effective cancer cell death. Quantifying cellular dosimetry for 225Ac RPT is essential for predicting cell survival and therapeutic success. However, the leading assumption that all 225Ac progeny remain localized at their target sites likely overestimates the absorbed dose to cancer cells. To address limitations in existing semi-analytic approaches, this work evaluates S-values for 225Ac’s progeny radionuclides with GATE Monte Carlo simulations.MethodsThe cellular geometries considered were an individual cell (10 µm diameter with a nucleus of 8 µm diameter) and a cluster of cells (micrometastasis) with radionuclides localized in four subcellular regions: cell membrane, cytoplasm, nucleus, or whole cell. The absorbed dose to the cell nucleus was scored, and self- and cross-dose S-values were derived. We also evaluated the total absorbed dose with various degrees of radiopharmaceutical internalization and retention of the progeny radionuclides 221Fr (t1/2 = 4.80 m) and 213Bi (t1/2 = 45.6 m).ResultsFor the cumulative 225Ac decay chain, our self- and cross-dose nuclear S-values were both in good agreement with S-values published by MIRDcell, with per cent differences ranging from − 2.7 to − 8.7% for the various radionuclide source locations. Source location had greater effects on self-dose S-values than the intercellular cross-dose S-values. Cumulative 225Ac decay chain self-dose S-values increased from 0.167 to 0.364 GyBq−1 s−1 with radionuclide internalization from the cell surface into the cell. When progeny migration from the target site was modelled, the cumulative self-dose S-values to the cell nucleus decreased by up to 71% and 21% for 221Fr and 213Bi retention, respectively.ConclusionsOur GATE Monte Carlo simulations resulted in cellular S-values in agreement with existing MIRD S-values for the alpha-emitting radionuclides in the 225Ac decay chain. To obtain accurate absorbed dose estimates in 225Ac studies, accurate understanding of daughter migration is critical for optimized injected activities. Future work will investigate other novel preclinical alpha-emitting radionuclides to evaluate therapeutic potency and explore realistic cellular geometries corresponding to targeted cancer cell lines.

Recent Innovations and Nano-Delivery of Actinium-225: A Narrative Review.

The actinium-225 (225Ac) radioisotope exhibits highly attractive nuclear properties for application in radionuclide therapy. However, the 225Ac radionuclide presents multiple daughter nuclides in its decay chain, which can escape the targeted site, circulate in plasma, and cause toxicity in areas such as kidneys and renal tissues. Several ameliorative strategies have been devised to circumvent this issue, including nano-delivery. Alpha-emitting radionuclides and nanotechnology applications in nuclear medicine have culminated in major advancements that offer promising therapeutic possibilities for treating several cancers. Accordingly, the importance of nanomaterials in retaining the 225Ac daughters from recoiling into unintended organs has been established. This review expounds on the advancements of targeted radionuclide therapy (TRT) as an alternative anticancer treatment. It discusses the recent developments in the preclinical and clinical investigations on 225Ac as a prospective anticancer agent. Moreover, the rationale for using nanomaterials in improving the therapeutic efficacy of α-particles in targeted alpha therapy (TAT) with an emphasis on 225Ac is discussed. Quality control measures in the preparation of 225Ac-conjugates are also highlighted.

An Investigation on the Dose Distribution of Typical Alpha-Emitting Radionuclides in Nuclear Medicine Therapy

An Investigation on the Dose Distribution of Typical Alpha-Emitting Radionuclides in Nuclear Medicine Therapy

Динамика формирования поглощённых доз в крови лабораторных животных от альфа-излучающих радионуклидов с последовательными распадами

Since the beginning of the XXI century, active research has begun on the development of fundamentally new radiopharmaceutical drugs based on alpha-emitting maternal radionuclides for radionuclide therapy of micro-tumors and metastases. The main difficulty in assessing the absorbed doses of internal irradiation of such drugs is to take into account the radiation loads from all daughter radionuclides of sequential decay with alpha and beta transformations. The aim of the work is to develop a methodology for calculating the accumulated and total absorbed doses in the blood of laboratory animals from alpha-emitting radionuclides with successive decays within the framework of the compartment modeling method. To test the developed methodology and calculate the dosimetric characteristics, the free isotope Ac-225, injected into the blood as part of its chloride Ac-225Cl3, and the osteotropic preparation Ac-225-KHEDP (monokalium salt of hydroxyethylidendiphosphonic acid) labeled with it were selected. Quantitative data on the bio-distribution of Ac-225-KHEDP and Ac-225Cl3 in the body of intact mice were used to identify the necessary parameters. Calculations of the dosimetric characteristics of two Ac-225-labeled drugs in the blood of intact mice showed that the values of accumulated absorbed doses monotonically increase from the moment of drug administration, reaching their limit values equal to the total absorbed doses by about 300 hours. Alpha-emitting parent and daughter radionuclides make the main contribution to the values of total absorbed doses. The contribution of daughter beta-emitting radionuclides is significantly less, therefore, they can be ignored when preliminary estimates of the total absorbed doses are made. When using Ac-225-KHEDP, all radiation loads on the blood are approximately 2.5 times less compared to Ac-225Cl3, which is explained by the higher blood clearance value for Ac-225-KHEDP. The results obtained indicate the prospects for further studies of 225Ac-KHEDP and the possibility of its clinical application for the treatment of skeletal metastases.

Engineering CAR-T cells for radiohapten capture in imaging and radioimmunotherapy applications.

Rationale: The in vivo dynamics of CAR-T cells remain incompletely understood. Novel methods are urgently needed to longitudinally monitor transferred cells non-invasively for biodistribution, functionality, proliferation, and persistence in vivo and for improving their cytotoxic potency in case of treatment failure. Methods: Here we engineered CD19 CAR-T cells ("Thor"-cells) to express a membrane-bound scFv, huC825, that binds DOTA-haptens with picomolar affinity suitable for labeling with imaging or therapeutic radionuclides. We assess its versatile utility for serial tracking studies with PET and delivery of α-radionuclides to enhance anti-tumor killing efficacy in sub-optimal adoptive cell transfer in vivo using Thor-cells in lymphoma models. Results: We show that this reporter gene/probe platform enables repeated, sensitive, and specific assessment of the infused Thor-cells in the whole-body using PET/CT imaging with exceptionally high contrast. The uptake on PET correlates with the Thor-cells on a cellular and functional level. Furthermore, we report the ability of Thor-cells to accumulate cytotoxic alpha-emitting radionuclides preferentially at tumor sites, thus increasing therapeutic potency. Conclusion: Thor-cells are a new theranostic agent that may provide crucial information for better and safer clinical protocols of adoptive T cell therapies, as well as accelerated development strategies.

Mathematical Model for Evaluation of Tumor Response in Targeted Radionuclide Therapy with 211At Using Implanted Mouse Tumor.

Recent introduction of alpha-emitting radionuclides in targeted radionuclide therapy has stimulated the development of new radiopharmaceuticals. Preclinical evaluation using an animal experiment with an implanted tumor model is frequently used to examine the efficiency of the treatment method and to predict the treatment response before clinical trials. Here, we propose a mathematical model for evaluation of the tumor response in an implanted tumor model and apply it to the data obtained from the previous experiment of 211At treatment in a thyroid cancer mouse model. The proposed model is based on the set of differential equations, describing the kinetics of radiopharmaceuticals, the tumor growth, and the treatment response. First, the tumor growth rate was estimated from the control data without injection of 211At. The kinetic behavior of the injected radionuclide was used to estimate the radiation dose profile to the target tumor, which can suppress the tumor growth in a dose-dependent manner. An additional two factors, including the time delay for the reduction of tumor volume and the impaired recovery of tumor regrowth after the treatment, were needed to simulate the temporal changes of tumor size after treatment. Finally, the parameters obtained from the simulated tumor growth curve were able to predict the tumor response in other experimental settings. The model can provide valuable information for planning the administration dose of radiopharmaceuticals in clinical trials, especially to determine the starting dose at which efficacy can be expected with a sufficient safety margin.

Novel optical technologies for emergency preparedness and response: Mapping contaminations with alpha-emitting radionuclides

Radiological emergencies involving an accidental or deliberate dispersion of alpha-emitting radionuclides in the environment can cause significant damage to humans and societies in general. Currently, there is a metrology gap in managing such emergencies due to the lack of detectors that can measure alpha particles at distances greater than their range in air: most conventional alpha detectors are only effective when placed just a few centimeters above the contaminated area. This paper presents the development and testing of lens based optical detection systems that utilize alpha particle-induced ultraviolet (UV) luminescence of air, known as alpha radioluminescence. Telescopes based on fused silica and Poly(methyl 2-methylpropenoate) (PMMA) Fresnel lenses were investigated for their usability in facilitating emergency management related to alpha-emitting radionuclides. Careful matching of the diameter and focal length of the receiving optics, the response of the photocathode, and the passband of the filter allows detection sensitivities as high as 34s−1MBq−1 at 2m source-to-detector distance and background count rate of about 3s−1 in the UV-C spectral region, and suppression of daylight background count rate down to 16s−1. By flushing the source with nitrogen (N2) containing trace amounts of nitric oxide (NO), a groundbreaking sensitivity of 1.3×105s−1MBq−1 has been achieved, allowing detection limits as low as 100Bq with room lighting on, and 70Bq in a dark environment. In the UV-A spectral region, a detection limit of 4kBq could be achieved in a dark environment. These optical detection systems are aimed to facilitate a rapid, coordinated, and effective response in emergency situations involving the release of alpha-emitting radionuclides by mounting them on a tripod or an unmanned aerial device (UAV).

Intercomparison of S-Factor values calculated in Zubal voxelized phantom for eleven radionuclides commonly used in targeted prostate cancer therapy.

In this study we aimed at comparing various radionuclides ordinarily used in targeted prostate cancer therapy, thereby evaluating S-Factor parameter in the prostate organs as well as in its surrounding healthy tissues, namely the urinary bladder and rectum. InterDosi code version 1.1 was used to estimate S-Factor values in Zubal voxelized phantom for 11 radionuclides, namely 225Ac, 21At, 67Cu, 125I, 131I, 212Pb, 177Lu, 223Ra, 161Tb, 227Th and 90Y. The prostate organ was considered the source of different ionizing radiation emitted by the radionuclides cited above. The results showed that among all studied alpha-emitting radionuclides, 225 Ac, 223 Ra and 227 Th provide equidistantly the highest self-irradiation S-Factors whereas, 211At provides the lowest cross-irradiation S-Factors. On the other hand, considering only beta-emitting radionuclides, it is shown that 177Lu and 90Y induce respectively lowest and highest cross-absorption S-Factors on the surrounding healthy organs. We conclude that 177Lu and 211At are more adequate for prostate radionuclide therapy because they can relatively prevent surrounding organs from radiation toxicity and at the same time provide sufficient dose to treat the prostate tumor.