Initial Safety and Tumor Control Results From a "First-in-Human" Multicenter Prospective Trial Evaluating a Novel Alpha-Emitting Radionuclide for the Treatment of Locally Advanced Recurrent Squamous Cell Carcinomas of the Skin and Head and Neck

Clinical Best Practices for Radiation Safety During an Alpha DaRT Treatment

ABSTRACT

Alpha DaRT is a new alpha radiation treatment for treating solid tumors and is currently being evaluated through clinical trials worldwide. Being a novel radiation treatment, it is important to discuss the safety considerations and procedures that are needed to ensure safe use of this unique approach. The objective of this article is to provide a set of recommendations-radiation safety best practices that were developed based on operational and clinical experience.

Efficacy and safety of diffusing alpha-emitter radiation therapy (DaRT) for head and neck cancer recurrence after radiotherapy

BACKGROUND

To evaluate the efficacy and safety of diffusing alpha-emitter radiation therapy (DaRT) for recurrent head and neck cancer (rHNC) after radiotherapy.

METHODS

This study was a multicenter prospective clinical trial. Eligibility criteria included all patients with biopsy-proven rHNC and history of radiotherapy. The efficacy of DaRT was evaluated in terms of tumor shrinkage after 10 weeks of DaRT seed implantation. To assess safety of DaRT, radioactivity levels in blood and urine were measured, and incidence and grade of adverse events (AEs) were evaluated.

RESULTS

Between 2019 and 2021, DaRT was performed in 11 patients and completed in 10 patients with 11 tumors. The tumor sites included the tongue (n = 3), buccal mucosa (2), lips (2), floor of the mouth (1), soft palate (1), nose (1), and subcutaneous layer (1). Nine tumors were confirmed to be squamous cell carcinoma, and the remaining two tumors were basal cell carcinoma and neuroblastoma. Complete response (CR) and partial response (PR) were observed in three and six patients, respectively. The response rate was 81.8%. The maximum average radioactivity levels in blood and urine were 42.5 Bq/cm3 and 8.4 Bq/cm3, respectively, on the second day after implantation. Forty AEs were observed in all 11 patients, including 22 Grade 1 AEs, 16 Grade 2, and 2 Grade 3 (hypertension and seed remnants).

CONCLUSION

The initial response of rHNC after radiotherapy to DaRT was favorable, and the incidence and grade of AEs were acceptable, as compared to existing therapies.

Stereotactic implantation of diffusing alpha-emitters radiation therapy sources in the swine brain: a potential new focal therapy for brain tumors

PURPOSE

Diffusing alpha-emitters Radiation Therapy ("Alpha DaRT") is a new cancer treatment modality that employs radium-224-loaded metal sources implanted in solid tumors to disperse alpha-emitting atoms within a therapeutic "kill-zone" of a few millimeters around each source. Preclinical studies have demonstrated tumor growth delay in various cancer types, including glioblastoma multiforme, and the method is used in clinical trials for patients with skin and head and neck cancer. This study aims to assess the safety and feasibility of implementing Alpha DaRT for brain tumor treatment in a large animal model.

METHODS

Alpha-DaRT sources were delivered via image-guided stereotactic implantation into both hemispheres of eight swine. 1-3 layers of radial deployment of 7 sources were delivered through a single penetration point into each hemisphere. A 90-day follow-up period included clinical evaluation, brain MRI, head CT, blood, CSF, urine, and feces sampling, and an analysis of source location over time. Brain tissue pathology was performed on termination.

RESULTS

Alpha-DaRT sources were reproducibly and efficiently delivered to the brain cortex and subcortex. No unexpected abnormalities were detected in blood or CSF samples. MRI and CT scans revealed no evidence of major bleeding or infection. Measurements of 212Pb in blood and CSF exhibited the expected exponential decay from day 7 to day 14 post-source implantation. Minimal spatial and temporal movements of the sources were noted. Histopathological analysis demonstrated locally confined findings in brain parenchyma in a very close proximity to the sources.

CONCLUSION

Alpha-DaRT sources can be safely delivered into a large animal brain using image-guided stereotactic implantation. These findings support further exploration of Alpha DaRT as a potential treatment modality for brain tumors.

Modeling absorbed alpha particle dose from diffusing alpha-emitters radiation therapy in changing tissue volumes

BACKGROUND

Diffusing alpha-emitters Radiation Therapy ("Alpha DaRT") is a promising new radiation therapy modality for treating bulky tumors. 224Ra-carrying sources are inserted intratumorally, producing a therapeutic alpha-dose region with a total size of a few millimeter via the diffusive motion of 224Ra's alpha-emitting daughters. Clinical studies of Alpha DaRT have reported 100% positive response (30%-100% shrinkage within several weeks), with post-insertion swelling in close to half of the cases. While dosimetry recommendations informed by the effects of edema are standard in some radiation therapy modalities, the effect of edema and tumor shrinkage on the absorbed dose delivered by Alpha DaRT is still unknown.

PURPOSE

The aim of this work is to develop a simple model for Alpha-DaRT dose deposition in a time-dependent tissue volume in order to study the effect of geometrical changes in source location due to edema and tumor shrinkage on the delivered alpha particle dose.

METHODS

We perform FEM-based dose deposition modeling for a single Alpha-DaRT source inside shrinking and swelling tissues. Gradual volume change models were used for shrinkage and swelling, and an additional immediate volume gain model was also used for "worst case" swelling. Volume change rates were estimated from source location data from serial scans acquired at time of insertion and removal for seven patients treated using Alpha DaRT. We calculate absorbed dose profiles under both the high- and low-diffusion regimes described by the Diffusion-Leakage model.

RESULTS

Changes in tissue volume can lead to over- or underestimation of the calculated absorbed dose. In the low-diffusion regime, gradual tissue shrinkage can result in an increase of 100% and gradual swelling can result in a 35% decrease in absorbed dose compared to a calculation in static tissue. Although immediate post-insertion swelling can reduce the absorbed dose by close to 65% for very closely spaced sources, in all cases analyzed the final absorbed dose continues to exceed the 10 Gy target. These effects are less severe in the high-diffusion regime.

CONCLUSIONS

These results indicate that tissue swelling and shrinkage can have a non-negligible effect on the tumor absorbed dose. Further study of tissue dynamics during Alpha-DaRT treatment will be necessary for improvements in dosimetry practice.

Exploring biochemical considerations for diffusive alpha radiation therapy (DaRT) models

Diffusing alpha-emitting Radiation Therapy (DaRT) is a cancer treatment currently undergoing clinical trials. DaRT utilizes localized 224-Radium (224Ra) seeds to deliver high linear energy transfer (LET) alpha radiation. Its main advantage over other alpha radiation treatments is that the diffusion of 224Ra decay chain products allows for a more spatially distributed dose. In silico models are used to simulate the physical dynamics of DaRT and the diffusion of DaRT progeny radionuclides into cancer tissue. These models mostly rely on physical principles, often neglecting biochemical interactions with the tumor microenvironment (TME), which affect DaRT dosimetry in human cancer tissue. Here, we address this gap by reviewing how the daughter isotope 212-Lead (212Pb) interacts with chemically heterogeneous TMEs during DaRT treatments. 212Pb is given special attention due to its high physiological activity and long half-life compared to other DaRT radionuclides. By investigating Pb-binding molecules in the TME and their molecular dynamics, we aim to highlight key biochemical processes to be considered by computational models. We identify several species with prevalent roles in cancer tissue as possible binding partners with 212Pb. These species include Glutathione (GSH), Metallothioneins (MTs), Calmodulin (CaM), and Human Serum Albumin (HSA). GSH, MTs, CaM, and HSA were selected based on their known ability to bind to Pb and their concentration in cancer tissue and were examined for their variability in diverse TMEs. Ultimately, this article seeks to guide future research by providing a basic framework of molecular species important for the accurate simulation of DaRT within the TME.

Melanoma Cells from Different Patients Differ in Their Sensitivity to Alpha Radiation-Mediated Killing, Sensitivity Which Correlates with Cell Nuclei Area and Double Strand Breaks

Background/Objective: In this study, for the first time, we examined and compared the sensitivity of four patient-derived cutaneous melanoma cell lines to alpha radiation in vitro and analyzed it in view of cell nucleus area and the formation of double-strand breaks (DSB). Melanoma cells sensitivity to alpha radiation was compared to photon radiation effects. Furthermore, we compared the sensitivity of the melanoma cells to squamous cell carcinoma. Methods: Human melanoma cell lines YDFR.C, DP.C, M12.C, and M16.C, and the squamous cell carcinoma cell line, CAL 27, were irradiated in vitro using Americium-241 as alpha-particle source. Cells were irradiated with doses of 0 to 2.8 gray (Gy). Cell viability, DNA DSB, and nuclear size were measured. Results: 1. Alpha radiation caused death and proliferation arrest of all four melanoma cell lines, but inter-tumor heterogeneity was observed. 2. The most sensitive cell line (DP.C) had a significantly larger nucleus area (408 µm2) and the highest mean number of DSB per cell (9.61) compared to more resistant cells. 3. The most resistant cell, M16.C, had a much lower nucleus area (236.99 µm2) and DSB per cell (6.9). 4. Alpha radiation was more lethal than photon radiation for all melanoma cells. 5. The SCC cell, CAL 27, was more sensitive to alpha radiation than all melanoma cells but had a similar number of DSB (6.67) and nucleus size (175.49 µm2) as the more resistant cells. 6. The cytotoxic effect of alpha radiation was not affected by proliferation arrest after serum starvation. 7. Killing of cells by alpha radiation was marginally elevated by ATR or topoisomerase 1 inhibition. Conclusions: This study demonstrates that various human melanoma cells can be killed by alpha radiation but exhibit variance in sensitivity to alpha radiation. Alpha radiation applied using the Intra-tumoral Diffusing alpha-emitters Radiation Therapy (Alpha DaRT) methodology may serve as an efficient treatment for human melanoma.

Feasibility and safety of endoscopic ultrasound-guided diffusing alpha emitter radiation therapy for advanced pancreatic cancer: Preliminary data

Background and study aims Pancreatic cancer is a devastating disease with limited locoregional treatment options. Diffusing alpha-emitter radiation therapy (Alpha DaRT), a novel cancer treatment using alpha-particle interstitial radiotherapy, may help address this challenge. The aim of this study was to evaluate the feasibility and safety of endoscopic ultrasound (EUS)-guided Alpha DaRT for advanced pancreatic cancer. Patients and methods Patients with inoperable locally advanced or metastatic pancreatic adenocarcinoma were treated with EUS-guided Alpha DaRT insertion. The Alpha DaRT sources were delivered into pancreatic tumors using a standard EUS needle with a novel proprietary applicator. Adverse events (AEs) were assessed based on the Common Terminology Criteria for Adverse Events version 5.0. Tumor response was evaluated by imaging 4 to 6 weeks post treatment. Results The first five patients were treated between March and September 2023. The procedure was technically successful in all cases, with Alpha DaRT sources inserted into the target tumor. Estimated gross tumor volume coverage ranged from 8% to 44%. Fourteen AEs were reported among three patients. Four were serious AEs, none of which was associated with the treatment, but rather, with disease progression or medical assistance in dying. Only two AEs (mild) were deemed possibly related to the study device. At the 35-day visit, two patients had progressive disease and three had stable disease, with one of the latter showing partial response 2 months post procedure. Conclusions Preliminary results from this first-in-human trial indicate that EUS-guided Alpha DaRT treatment for unresectable pancreatic cancer is feasible and safe, with no device-associated serious AEs. Further investigation of this promising novel modality is underway.

A finite volume approach to dosimetric calculations in diffusing alpha-emitters radiation therapy

Objective.In diffusing alpha-emitters radiation therapy ('Alpha DaRT'), the diffusion-leakage (DL) model is used to determine the spatial distributions of the emitters and the corresponding alpha dose, critical for a successful treatment. This work first introduces a finite volume (FV) approach to develop numerical schemes to simulate the DL model in one, two and three dimensions then presents how variations over realistic ranges of the DL model parameters related to desorption, diffusion and leakage processes affect the alpha dose distribution and the position of the clinically significant alpha particle10Gy isodose. This work also presents the effects of three modeling approximations: two source geometry approximations (solid cylinder instead of hollow, pixelized cross section instead of circular), and one dosimetric approximation (single-source dose superposition instead of multiple-sources direct dose calculation).Approach.The introduced FV approach was used to obtain spatial distributions of the emitters, from which the corresponding alpha dose distributions were calculated under the assumption of a local deposition of the alpha particles' energies. Variation ranges of the DL model parameters were based on previously published data. For each modeling approximation studied, the error and relative error on the alpha dose distribution were calculated and the displacement of the10Gy isodose was evaluated.Main results.Over realistic ranges, the desorption probabilities, diffusion lengths, and leakage probabilities affect the position of the alpha particle10Gy isodose by∼0.1mm,∼1.5mm and∼0.5mm, respectively. The three modeling approximations studied have a negligible effect on the alpha particle10Gy isodose position, with displacements⩽0.01mm.Significance.This work quantitatively evaluates the relative importance of different parameters and approximations in Alpha-DaRT alpha dose calculations based on their impact not only on the dose variation at a given distance from the source but also on the displacement of clinically significant isodoses.

First measurements of radon-220 diffusion in mice tumors, towards treatment planning in diffusing alpha-emitters radiation therapy

BACKGROUND

Diffusing alpha-emitters radiation therapy ("Alpha-DaRT") is a new method for treating solid tumors with alpha particles, relying on the release of the short-lived alpha-emitting daughter atoms of radium-224 from interstitial sources inserted into the tumor. Alpha-DaRT tumor dosimetry is governed by the spread of radium's progeny around the source, as described by an approximate framework called the "diffusion-leakage model". The most important model parameters are the diffusion lengths of radon-220 and lead-212, and their estimation is therefore essential for treatment planning.

PURPOSE

Previous works have provided initial estimates for the dominant diffusion length, by measuring the activity spread inside mice-borne tumors several days after the insertion of an Alpha-DaRT source. The measurements, taken when lead-212 was in secular equilibrium with radium-224, were interpreted as representing the lead-212 diffusion length. The aim of this work is to provide first experimental estimates for the diffusion length of radon-220, using a new methodology.

METHODS

The diffusion length of radon-220 was estimated from autoradiography measurements of histological sections taken from 24 mice-borne subcutaneous tumors of five different types. Unlike previous studies, the source dwell time inside the tumor was limited to 30 min, to prevent the buildup of lead-212. To investigate the contribution of potential non-diffusive processes, experiments were done in two sets: fourteen in vivo tumors, where during the treatment the tumors were still carried by the mice with active blood supply, and 10 ex-vivo tumors, where the tumors were excised before source insertion and kept in a medium at37 ∘ C $37^\circ {\text{C}}$ with the source inside.

RESULTS

The measured diffusion lengths of radon-220, extracted by fitting the recorded activity pattern up to 1.5 mm from the source, lie in the range0.25 - 0.6 mm ${0.25-0.6}\nobreakspace {\text{mm}}$ , with no significant difference between the average values measured in in-vivo and ex-vivo tumors:L R n i n - v i v o = 0.40 ± 0.08 mm $L_{Rn}^{in-vivo}=0.40{\pm }0.08\nobreakspace {\text{mm}}$ versusL R n e x - v i v o = 0.39 ± 0.07 mm $L_{Rn}^{ex-vivo}=0.39{\pm }0.07\nobreakspace {\text{mm}}$ . However, in-vivo tumors display an enhanced spread of activity 2-3 mm away from the source. This effect is not explained by the current model and is much less pronounced in ex-vivo tumors.

CONCLUSIONS

The average measured radon-220 diffusion lengths in both in-vivo and ex-vivo tumors are consistent with published data on the diffusion length of radon in water and lie close to the upper limit of the previously estimated range of0.2 - 0.4 mm $0.2-0.4\nobreakspace {\text{mm}}$ . The observation that close to the source there is no apparent difference between in-vivo and ex-vivo tumors, and the good agreement with the theoretical model in this region suggest that the spread of radon-220 is predominantly diffusive in this region. The departure from the model prediction in in-vivo tumors at large radial distances may hint at potential vascular contribution, which will be the subject of future works.

Extended Follow-Up Outcomes from Pooled Prospective Studies Evaluating Efficacy of Interstitial Alpha Radionuclide Treatment for Skin and Head and Neck Cancers

The initial favorable efficacy and safety profile for Alpha DaRT have been demonstrated (NCT04377360); however, the longer-term safety and durability of the treatment are unknown. This pooled analysis of four prospective trials evaluated the long-term safety and efficacy of Alpha DaRT for the treatment of head and neck or skin tumors. A total of 81 lesions in 71 patients were treated across six international institutions, with a median follow-up of 14.1 months (range: 2-51 months). Alpha DaRT sources were delivered via a percutaneous interstitial technique and placed to irradiate the tumor volume with the margin. The sources were removed two to three weeks following implantation. A complete response was observed in 89% of treated lesions (n = 72) and a partial response in 10% (n = 8). The two-year actuarial local recurrence-free survival was 77% [95% CI 63-87]. Variables, including recurrent versus non-recurrent lesions, baseline tumor size, or histology, did not impact long-term outcomes. Twenty-seven percent of patients developed related acute grade 2 or higher toxicities, which resolved with conservative measures. No grade 2 or higher late toxicities were observed. These data support the favorable safety profile of Alpha DaRT, which is currently being explored in a pivotal US trial.

APR-246 as a radiosensitization strategy for mutant p53 cancers treated with alpha-particles-based radiotherapy

Radiation therapy (RT) remains a common treatment for cancer patients worldwide, despite the development of targeted biological compounds and immunotherapeutic drugs. The challenge in RT lies in delivering a lethal dose to the cancerous site while sparing the surrounding healthy tissues. Low linear energy transfer (low-LET) and high linear energy transfer (high-LET) radiations have distinct effects on cells. High-LET radiation, such as alpha particles, induces clustered DNA double-strand breaks (DSBs), potentially inducing cell death more effectively. However, due to limited range, alpha-particle therapies have been restricted. In human cancer, mutations in TP53 (encoding for the p53 tumor suppressor) are the most common genetic alteration. It was previously reported that cells carrying wild-type (WT) p53 exhibit accelerated senescence and significant rates of apoptosis in response to RT, whereas cells harboring mutant p53 (mutp53) do not. This study investigated the combination of the alpha-emitting atoms RT based on internal Radium-224 (224Ra) sources and systemic APR-246 (a p53 reactivating compound) to treat tumors with mutant p53. Cellular models of colorectal cancer (CRC) or pancreatic ductal adenocarcinoma (PDAC) harboring mutant p53, were exposed to alpha particles, and tumor xenografts with mutant p53 were treated using 224Ra source and APR-246. Effects on cell survival and tumor growth, were assessed. The spread of alpha emitters in tumors was also evaluated as well as the spatial distribution of apoptosis within the treated tumors. We show that mutant p53 cancer cells exhibit radio-sensitivity to alpha particles in vitro and to alpha-particles-based RT in vivo. APR-246 treatment enhanced sensitivity to alpha radiation, leading to reduced tumor growth and increased rates of tumor eradication. Combining alpha-particles-based RT with p53 restoration via APR-246 triggered cell death, resulting in improved therapeutic outcomes. Further preclinical and clinical studies are needed to provide a promising approach for improving treatment outcomes in patients with mutant p53 tumors.

DNA damage response in a 2D-culture model by diffusing alpha-emitters radiation therapy (Alpha-DaRT)

Diffusing alpha-emitters radiation therapy (Alpha-DaRT) is a unique method, in which interstitial sources carrying 224Ra release a chain of short-lived daughter atoms from their surface. Although DNA damage response (DDR) is crucial to inducing cell death after irradiation, how the DDR occurs during Alpha-DaRT treatment has not yet been explored. In this study, we temporo-spatially characterized DDR such as kinetics of DNA double-strand breaks (DSBs) and cell cycle, in two-dimensional (2D) culture conditions qualitatively mimicking Alpha-DaRT treatments, by employing HeLa cells expressing the Fucci cell cycle-visualizing system. The distribution of the alpha-particle pits detected by a plastic nuclear track detector, CR-39, strongly correlated with γH2AX staining, a marker of DSBs, around the 224Ra source, but the area of G2 arrested cells was more widely spread 24 h from the start of the exposure. Thereafter, close time-lapse observation revealed varying cell cycle kinetics, depending on the distance from the source. A medium containing daughter nuclides prepared from 224Ra sources allowed us to estimate the radiation dose after 24 h of exposure, and determine surviving fractions. The present experimental model revealed for the first time temporo-spatial information of DDR occurring around the source in its early stages.

A diffusion-leakage model coupled with dose point kernels (DPK) for dosimetry of diffusing alpha-emitters radiation therapy (DaRT)

BACKGROUND

Diffusing alpha-emitters radiation therapy (DaRT) is a novel brachytherapy technique that leverages the diffusive flow of 224Ra progeny within the tumor volume over the course of the treatment. Cell killing is achieved by the emitted alpha particles that have a short range in tissue and high linear energy transfer. The current proposed absorbed dose calculation method for DaRT is based on a diffusion-leakage (DL) model that neglects absorbed dose from beta particles.

PURPOSE

This work aimed to couple the DL model with dose point kernels (DPKs) to account for dose from beta particles as well as to consider the non-local deposition of energy.

METHODS

The DaRT seed was modeled using COMSOL multiphysics and the DL model was implemented to extract the spatial information of the diffusing daughters. Using Monte-Carlo (MC) methods, DPKs were generated for 212Pb, 212Bi, and their progenies since they were considered to be the dominant beta emitters in the 224Ra radioactive decay chain. A convolution operation was performed between the integrated number densities of the diffusing daughters and DPKs to calculate the total absorbed dose over a 30-day treatment period. Both high-diffusion and low-diffusion cases were considered.

RESULTS

The calculated DPKs showed non-negligible energy deposition over several millimeters from the source location. An absorbed dose >10 Gy was deposited within a 1.8 mm radial distance for the low diffusion case and a 2.2 mm radial distance for the high diffusion case. When the DPK method was compared with the local energy deposition method that solely considered dose from alpha particles, differences above 1 Gy were found within 1.3 and 1.8 mm radial distances from the surface of the source for the low diffusion and high diffusion cases, respectively.

CONCLUSIONS

The proposed method enhances the accuracy of the dose calculation method used for the DaRT technique.

Dose and DNA damage modelling of diffusing alpha-emitters radiation therapy using Geant4

PURPOSE

Diffusing alpha-emitters radiation therapy (DaRT) is a brachytherapy technique using α-particles to treat solid tumours. The high linear energy transfer (LET) and short range of α-particles make them good candidates for the targeted treatment of cancer. Treatment planning of DaRT requires a good understanding of the dose from α-particles and the other particles released in the 224Ra decay chain.

METHODS

The Geant4 Monte Carlo toolkit has been used to simulate a DaRT seed to better understand the dose contribution from all particles and simulate the DNA damage due to this treatment.

RESULTS

Close to the seed α-particles deliver the majority of dose, however at radial distances greater than 4 mm, the contribution of β-particles is greater. The RBE has been estimated as a function of number of double strand breaks (DSBs) and complex DSBs. A maximum seed spacing of 5.5 mm and 6.5 mm was found to deliver at least 20 Gy RBE weighted dose between the seeds for RBEDSB and RBEcDSB respectively.

CONCLUSIONS

The DNA damage changes with radial distance from the seed and has been found to become less complex with distance, which is potentially easier for the cell to repair. Close to the seed α-particles contribute the majority of dose, however the contribution from other particles cannot be neglected and may influence the choice of seed spacing.

The low-LET radiation contribution to the tumor dose in diffusing alpha-emitters radiation therapy

BACKGROUND

Diffusing alpha-emitters Radiation Therapy ("Alpha DaRT") is a new technique that enables the use of alpha particles for the treatment of solid tumors. Alpha DaRT employs interstitial sources carrying a few μ $\mu$ Ci of224 $^{224}$ Ra below their surface, designed to release a chain of short-lived atoms (progeny of224 $^{224}$ Ra) which emit alpha particles, along with beta, Auger, and conversion electrons, x- and gamma rays. These atoms diffuse around the source and create-primarily through their alpha decays-a lethal high-dose region measuring a few millimeters in diameter.

PURPOSE

While previous studies focused on the dose from the alpha emissions alone, this work addresses the electron and photon dose contributed by the diffusing atoms and by the atoms remaining on the source surface, for both a single Alpha DaRT source and multi-source lattices. This allows to evaluate the low-LET contribution to the tumor dose and tumor cell survival, and demonstrate the sparing of surrounding healthy tissue.

METHODS

The low-LET dose is calculated using the EGSnrc and FLUKA Monte Carlo (MC) codes. We compare the results of a simple line-source approximation with no diffusion to those of a full simulation, which implements a realistic source geometry and the spread of diffusing atoms. We consider two opposite scenarios: one with low diffusion and high212 $^{212}$ Pb leakage, and the other with high diffusion and low leakage. The low-LET dose in source lattices is calculated by superposition of single-source contributions. Its effect on cell survival is estimated with the linear quadratic model in the limit of low dose rate.

RESULTS

For sources carrying 3  μ $\umu$ Ci/cm224 $^{224}$ Ra arranged in a hexagonal lattice with 4 mm spacing, the minimal low-LET dose between sources is∼ 18 - 30 $\sim 18-30$  Gy for the two test cases and is dominated by the beta contribution. The low-LET dose drops below 5 Gy∼ 3 $\sim 3$  mm away from the outermost source in the lattice with an effective maximal dose rate of< 0.04 $<0.04$  Gy/h. The accuracy of the line-source/no-diffusion approximation is∼ 15 % $\sim 15\%$ for the total low-LET dose over clinically relevant distances (2-4 mm). The low-LET dose reduces tumor cell survival by a factor of∼ 2 - 200 $\sim 2-200$ .

CONCLUSIONS

The low-LET dose in Alpha DaRT can be modeled by conventional MC techniques with appropriate leakage corrections to the source activity. For 3  μ $\umu$ Ci/cm224 $^{224}$ Ra sources, the contribution of the low-LET dose can reduce cell survival inside the tumor by up to two orders of magnitude. The low-LET dose to surrounding healthy tissue is negligible. Increasing source activities by a factor of 5 can bring the low-LET dose itself to therapeutic levels, in addition to the high-LET dose contributed by alpha particles, leading to a "self-boosted" Alpha DaRT configuration, and potentially allowing to increase the lattice spacing.

A finite element method for modeling diffusion of alpha-emitting particles in tissue

BACKGROUND

Diffusing alpha-emitters Radiation Therapy ("DaRT") is a promising new modality for the treatment of solid tumors. Interstitial sources containing 224 Ra are inserted into the tumor, producing alpha particles via the decay of 224 Ra and its daughters. The alpha particles are able to produce a "kill region" of several mm due to the diffusion of the alpha-emitting atoms. The Diffusion-Leakage (D-L) model has been proposed to describe the movement of the alpha-emitters used in DaRT in tumor tissue.

PURPOSE

To date, estimating the dose delivered under the D-L model has been accomplished with numerical solutions based on finite difference methods, namely DART1D and DART2D, as well as with asymptotic expressions for the long time limit. The aim of this work is to develop a flexible method of finite elements for solving the D-L model and to validate prior solutions of the D-L model.

METHODS

We develop a two-dimensional finite element solution to the D-L model implemented using the FEniCS software library. Our approach solves the variational formulation of the D-L equations on an unstructured mesh of triangular Lagrangian elements. We calculate the local dose in the mid- and axial planes of the source and validate our results against the one- and two-dimensional solutions obtained using the previously proposed numerical scheme, DART1D and DART2D. We use our model to estimate the change in dose in the source midplane as a function of the physical parameters used in the D-L model.

RESULTS

The local dose at the end of a 30 day treatment period estimated by our numerical method differs from DART1D and DART2D by less than 1% in the source midplane and less than 3% along the source axis over clinically relevant distances, with the largest discrepancies in high gradient areas where the Finite Element Method (FEM) mesh has a higher element density. We find that within current experimentally estimated ranges for D-L model parameters, the dose in the source midplane at a distance of 2 mm can vary by over a factor of 3.

CONCLUSIONS

The 2D finite element model reproduces the calculated dose obtained with DART1D and DART2D under the assumptions D-L model. The variation in predicted dose within current experimental ranges for model parameters suggests the necessity of further studies to better determine their statistical distributions. Finally, the FEM model can be used to calculate dose from DaRT in a variety of realistic 2D geometries beyond the D-L model.

Targeted Radium Alpha Therapy in the Era of Nanomedicine: In Vivo Results

Targeted alpha-particle therapy using radionuclides with alpha emission is a rapidly developing area in modern cancer treatment. To selectively deliver alpha-emitting isotopes to tumors, targeting vectors, including monoclonal antibodies, peptides, small molecule inhibitors, or other biomolecules, are attached to them, which ensures specific binding to tumor-related antigens and cell surface receptors. Although earlier studies have already demonstrated the anti-tumor potential of alpha-emitting radium (Ra) isotopes-Radium-223 and Radium-224 (223/224Ra)-in the treatment of skeletal metastases, their inability to complex with target-specific moieties hindered application beyond bone targeting. To exploit the therapeutic gains of Ra across a wider spectrum of cancers, nanoparticles have recently been embraced as carriers to ensure the linkage of 223/224Ra to target-affine vectors. Exemplified by prior findings, Ra was successfully bound to several nano/microparticles, including lanthanum phosphate, nanozeolites, barium sulfate, hydroxyapatite, calcium carbonate, gypsum, celestine, or liposomes. Despite the lengthened tumor retention and the related improvement in the radiotherapeutic effect of 223/224Ra coupled to nanoparticles, the in vivo assessment of the radiolabeled nanoprobes is a prerequisite prior to clinical usage. For this purpose, experimental xenotransplant models of different cancers provide a well-suited scenario. Herein, we summarize the latest achievements with 223/224Ra-doped nanoparticles and related advances in targeted alpha radiotherapy.

Radiometals in Imaging and Therapy: Highlighting Two Decades of Research

The present article highlights the important progress made in the last two decades in the fields of molecular imaging and radionuclide therapy. Advancements in radiometal-based positron emission tomography, single photon emission computerized tomography, and radionuclide therapy are illustrated in terms of their production routes and ease of radiolabeling. Applications in clinical diagnostic and radionuclide therapy are considered, including human studies under clinical trials; their current stages of clinical translations and findings are summarized. Because the metalloid astatine is used for imaging and radionuclide therapy, it is included in this review. In regard to radionuclide therapy, both beta-minus (β-) and alpha (α)-emitting radionuclides are discussed by highlighting their production routes, targeted radiopharmaceuticals, and current clinical translation stage.

In-silico calculations of DNA damage induced by α-particles in the 224Ra DaRT decay chain for a better understanding of the radiobiological effectiveness of this treatment

Diffusing alpha-emitters radiation Therapy (DaRT) is an interstitial brachytherapy technique using 224Ra seeds. For accurate treatment planning a good understanding of the early DNA damage due to α-particles is required. Geant4-DNA was used to calculate the initial DNA damage and radiobiological effectiveness due to α-particles with linear energy transfer (LET) values in the range 57.5-225.9 keV/μm from the 224Ra decay chain. The impact of DNA base pair density on DNA damage has been modelled, as this parameter varies between human cell lines. Results show that the quantity and complexity of DNA damage changes with LET as expected. Indirect damage, due to water radical reactions with the DNA, decreases and becomes less significant at higher LET values as shown in previous studies. As expected, the yield of complex double strand breaks (DSBs), which are harder for a cell to repair, increases approximately linearly with LET. The level of complexity of DSBs and radiobiological effectiveness have been found to increase with LET as expected. The quantity of DNA damage has been shown to increase for increased DNA density in the expected base pair density range of human cells. The change in damage yield as a function of base pair density is largest for higher LET α-particles, an increase of over 50% for individual strand breaks between 62.7 and 127.4 keV/μm. This change in yield shows that the DNA base pair density is an important parameter for modelling DNA damage particularly at higher LET where the DNA damage is greatest and most complex.

Simulation of particle release for Diffusing Alpha-Emitters Radiation Therapy

We used Monte Carlo simulations to study release of ²²⁴Ra daughter nuclei from the seed used for Diffusing Alpha-Emitters Radiation Therapy (DART). Calculated desorption probabilities for ²¹⁶Po (15%) and ²¹²Pb (12%) showed that they make a significant contribution to total release from the seed. We also showed that the dose to tissue from decays inside the 10 mm long seed exceeds 2.9 Gy for initial ²²⁴Ra activity of 3 μCi (111 kBq).

Diffusing Alpha-Emitters Radiation Therapy Promotes a Proimmunogenic Tumor Microenvironment and Synergizes With Programmed Cell Death Protein 1 Blockade

PURPOSE

Diffusing alpha-emitters Radiation Therapy (DaRT) releases alpha-emitting atoms into the tumor microenvironment. The treatment effectively ablates human and mice xenografts and shows 100% response rates in skin or head and neck squamous cell carcinoma patients. DaRT induces specific and systemic antitumor immune activation and synergizes with immune stimulation and modulation in mice. Here, the transcriptional profile activated by DaRT, and its potential to enhance responsiveness to immune checkpoint inhibition by programmed cell death protein 1 (PD-1) blockade were studied.

METHODS AND MATERIALS

Squamous cell carcinoma tumor- bearing BALB/C mice were treated with DaRT or inert seeds in combination with anti-PD-1 (aPD-1) or IgG control antibody. Sixteen days after seed insertion, tumors and spleens were subjected to immunophenotyping and immunohistochemical staining. Combination of DaRT and aPD-1 was tested for efficacy. Gene expression analysis was performed on mRNA extracted from tumors 7 days after DaRT or inert insertion using Nanostring PanCancer-IO-360 panel, and tumors and spleens were subjected to flow cytometry analysis.

RESULTS

DaRT in combination with aPD-1 delayed tumor development, induced CD3 and CD8 lymphocytes infiltration more efficiently than either monotherapy. The combined treatment reduced splenic polymorphonuclear myeloid derived suppressor cells more than aPD-1 therapy or control. Granzyme B release in the tumor was increased only in the combinational treatment and was correlated with T-lymphocyte infiltration. Gene expression and gene set enrichment analysis of mRNA levels 7 days after DaRT insertion indicated that DaRT upregulated apoptosis, p53 signaling, G1/S-related arrest, interferon signaling and myeloid related transcription, while downregulating DNA repair, cell proliferation, and notch-related transcription. Flow cytometry showed that DaRT increased dendritic cells activation and led to changes in MDSCs distribution.

CONCLUSIONS

DaRT promotes a "hot" tumor microenvironment and changes in immune suppression that lead to a potentiation of aPD-1 blockade induced effector T cell function and improved treatment efficacy. This study provides rationale for investigating DaRT and aPD-1 combination in patients with squamous cell carcinoma.

Alpha dose modeling in diffusing alpha-emitters radiation therapy-Part I: single-seed calculations in one and two dimensions

BACKGROUND

Diffusing alpha-emitters Radiation Therapy ("DaRT") is a new method, presently in clinical trials, which allows treating solid tumors by alpha particles. DaRT relies on interstitial seeds carrying μCi-level ²²⁴ Ra activity below their surface, which release a chain of short-lived alpha emitters that spread throughout the tumor volume primarily by diffusion. Alpha dose calculations in DaRT are based on describing the transport of alpha emitting atoms, requiring new modeling techniques.

PURPOSE

A previous study introduced a simplified framework, the "Diffusion-Leakage (DL) model", for DaRT alpha dose calculations, and employed it to a point source, as a basic building block of arbitrary configurations of line sources. The aim of this work, which is divided into two parts, is to extend the model to realistic seed geometries (in Part I), and to employ single-seed calculations to study the properties of DaRT seed lattices (Part II). Such calculations can serve as a pragmatic guide for treatment planning in future clinical trials.

METHODS

We derive a closed-form asymptotic solution for an infinitely long cylindrical source, and extend it to an approximate time-dependent expression that assumes a uniform temporal profile at all radial distances from the source. We then develop a finite-element one-dimensional numerical scheme for a complete time-dependent solution of this geometry and validate it against the closed-form expressions. Finally, we discuss a two-dimensional axisymmetric scheme for a complete time-dependent solution for a seed of finite diameter and length. Different solutions are compared over the relevant parameter space, providing guidelines on their usability and limitations.

RESULTS

We show that approximating the seed as a finite line source comprised of point-like segments significantly underestimates the correct alpha dose, as predicted by the full two-dimensional calculation. The time-dependent one-dimensional solution is shown to coincide to sub-percent-level with the two-dimensional calculation in the seed midplane, and maintains an accuracy of a few percent up to ∼2 mm from the seed edge.

CONCLUSIONS

For actual treatment plans, the full two-dimensional solution should be used to generate dose lookup tables, similarly to the TG-43 format employed in conventional brachytherapy. Given the accuracy of the one-dimensional solution up to ∼2 mm from the seed edge it can be used for efficient parametric studies of DaRT seed lattices.

Alpha dose modeling in diffusing alpha-emitters radiation therapy. Part II: Lattice studies

BACKGROUND

Diffusing alpha-emitters radiation therapy ("DaRT") is a new method, presently in clinical trials, which allows treating solid tumors by alpha particles. DaRT relies on interstitial seeds carrying μCi-level ²²⁴ Ra activity on their surface, which release a chain of short-lived alpha emitters that spread throughout the tumor volume primarily by diffusion. Alpha dose calculations in DaRT are based on describing the transport of alpha emitting atoms, requiring new modeling techniques.

PURPOSE

A previous study introduced a simplified framework, the "diffusion-leakage (DL) model," for DaRT alpha dose calculations, and employed it to a point source, as a basic building block of arbitrary configurations of line sources. The aim of this work, which is divided into two parts, is to extend the model to realistic seed geometries (in Part I), and to employ single-seed calculations to study the properties of DaRT seed lattices (Part II). Such calculations can serve as a pragmatic guide for treatment planning in future clinical trials.

METHODS

We employ the superposition of single-seed solutions, developed in Part I, to study the alpha dose in DaRT seed lattices and investigate the sensitivity of the required seed activity and spacing to changes in the DL model parameters and to seed placement errors.

RESULTS

We show that the rapid fall-off of the dose, which guarantees sparing healthy tissue already 2-3 mm away from the tumor, strongly favors a hexagonal, rather than square, seed placement pattern. Realistic variations in the seed manufacturing parameters (²²⁴ Ra activity and emission rate of its daughters) are shown to have a negligible effect on the required lattice spacing. On the other hand, tumor parameters (i.e., diffusion lengths and ²¹² Pb leakage probability), as well as seed placement errors, have a significant effect.

CONCLUSIONS

In most cases, hexagonal lattice spacing on the scale of ∼3.5-4.5 mm using seeds carrying a few μCi/cm ²²⁴ Ra will enable overcoming realistic uncertainties in measured tumor environment parameters, as well as seed placement errors, and result in therapeutically relevant alpha dose levels.

Learning from the past: a century of accuracy, aspirations, and aspersions in brachytherapy

The oldest form of radiation therapy, brachytherapy, has been investigated and reported in the scientific and medical literature for well over a century. Known by many names over the years, radium-based, empirical practices evolved over decades to contemporary practice. This includes treatment at various dose rates using multiple radionuclides or even electrically generated photon sources. Predictions or prognostications of what may happen in the future enjoy a history that spans centuries, e.g. those by Nostradamus in the 1500s. In this review article, publications from several eras of past practice between the early 1900s and the late 2010s where the authors address the "future of brachytherapy" are presented, and for many of these publications, one can use the benefit of the intervening years to comment on the accuracy or the inaccuracies inherent in those publications. Finally, recently published papers are reviewed to examine current expectations for the future practice of brachytherapy.

Radiolabeled nanomaterials for biomedical applications: radiopharmacy in the era of nanotechnology

BACKGROUND

Recent advances in nanotechnology have offered new hope for cancer detection, prevention, and treatment. Nanomedicine, a term for the application of nanotechnology in medical and health fields, uses nanoparticles for several applications such as imaging, diagnostic, targeted cancer therapy, drug and gene delivery, tissue engineering, and theranostics.

RESULTS

Here, we overview the current state-of-the-art of radiolabeled nanoparticles for molecular imaging and radionuclide therapy. Nanostructured radiopharmaceuticals of technetium-99m, copper-64, lutetium-177, and radium-223 are discussed within the scope of this review article.

CONCLUSION

Nanoradiopharmaceuticals may lead to better development of theranostics inspired by ingenious delivery and imaging systems. Cancer nano-theranostics have the potential to lead the way to more specific and individualized cancer treatment.

A New Approach for a Safe and Reproducible Seeds Positioning for Diffusing Alpha-Emitters Radiation Therapy of Squamous Cell Skin Cancer: A Feasibility Study

Simple Summary

The Diffusing Alpha-emitters Radiation Therapy (DaRT) is a novel brachytherapy technique employing 224-radium enriched seeds releasing short-lived alpha-emitting atoms into the tumour. DaRT overcomes the main obstacle in employing alpha radiation for cancer treatments in liquid and solid media caused by their short range. The aim of the study is to improve the DaRT technique with an external radio-opaque template that can help clinicians predict the correct number of sources to achieve tumour coverage. Furthermore, the template is used to aid clinicians in visualizing lesions and their eventual subcutaneous extension. Finally, it is also utilized on treatment day to ensure that the sources are properly inserted into the tumour.

Abstract

The purpose of this study is to discuss how to use an external radio-opaque template in the Diffusing Alpha-emitters Radiation Therapy (DaRT) technique’s pre-planning and treatment stages. This device would help to determine the proper number of sources for tumour coverage, accounting for subcutaneous invasion and augmenting DaRT safety. The procedure will be carried out in a first phase on a phantom and then applied to a clinical case. A typical DaRT procedure workflow comprises steps like tumour measurements and delineation, source number assessment, and therapy administration. As a first step, an adhesive fiberglass mesh (spaced by 2 mm) tape was applied on the skin of the patient and employed as frame of reference. A physician contoured the lesion and marked the entrance points for the needles with a radio opaque ink marker. According to the radio opaque marks and metabolic uptake the clinical target volume was defined, and with a commercial brachytherapy treatment planning system (TPS) it was possible to simulate and adjust the spatial seeds distribution. After the implant procedure a CT was again performed to check the agreement between simulations and seeds positions. With the procedure described above it was possible to simulate a DaRT procedure on a phantom in order to train physicians and subsequently apply the novel approach on patients, outlining the major issues involved in the technique. The present work innovates and supports DaRT technique for the treatment of cutaneous cancers, improving its efficacy and safety.

Acute and late complications and toxicities of skin brachytherapy

Skin cancer is the most prevalent malignancy, and its incidence is rising. Surgery is the primary treatment, however, morbidity of surgical resection in certain parts of the body warrant alternate treatment options such as radiation therapy. Complex topology of the skin can pose a challenge for treatment with photon and electron external beam radiotherapy techniques. Brachytherapy can be an attractive radiotherapy technique for these regions. Furthermore, brachytherapy results in lower dose to the underlying deep organs. The goal of this manuscript is to provide a brief overview of the role of brachytherapy for skin malignancies and to outline potential acute and long-term toxicities.

Radon-220 diffusion from 224Ra-labeled calcium carbonate microparticles: Some implications for radiotherapeutic use

Alpha-particle emitting radionuclides continue to be the subject of medical research because of their high energy and short range of action that facilitate effective cancer therapies. Radium-224 (²²⁴Ra) is one such candidate that has been considered for use in combating micrometastatic disease. In our prior studies, a suspension of ²²⁴Ra-labeled calcium carbonate (CaCO₃) microparticles was designed as a local therapy for disseminated cancers in the peritoneal cavity. The progenies of ²²⁴Ra, of which radon-220 (²²⁰Rn) is the first, together contribute three of the four alpha particles in the decay chain. The proximity of the progenies to the delivery site at the time of decay of the ²²⁴Ra-CaCO₃ microparticles can impact its therapeutic efficacy. In this study, we show that the diffusion of ²²⁰Rn was reduced in labeled CaCO₃ suspensions as compared with cationic ²²⁴Ra solutions, both in air and liquid volumes. Furthermore, free-floating lead-212 (²¹²Pb), which is generated from released ²²⁰Rn, had the potential to be re-adsorbed onto CaCO₃ microparticles. Under conditions mimicking an in vivo environment, more than 70% of the ²¹²Pb was adsorbed onto the CaCO₃ at microparticle concentrations above 1 mg/mL. Further, the diffusion of ²²⁰Rn seemed to occur whether the microparticles were labeled by the surface adsorption of ²²⁴Ra or if the ²²⁴Ra was incorporated into the bulk of the microparticles. The therapeutic benefit of differently labeled ²²⁴Ra-CaCO₃ microparticles after intraperitoneal administration was similar when examined in mice bearing intraperitoneal ovarian cancer xenografts. In conclusion, both the release of ²²⁰Rn and re-adsorption of ²¹²Pb are features that have implications for the radiotherapeutic use of ²²⁴Ra-labeled CaCO₃ microparticles. The release of ²²⁰Rn through diffusion may extend the effective range of alpha-particle dose deposition, and the re-adsorption of the longer lived ²¹²Pb onto the CaCO₃ microparticles may enhance the retention of this nuclide in the peritoneal cavity.

The Potentiation of Anti-Tumor Immunity by Tumor Abolition with Alpha Particles, Protons, or Carbon Ion Radiation and Its Enforcement by Combination with Immunoadjuvants or Inhibitors of Immune Suppressor Cells and Checkpoint Molecules

The delivery of radiation therapy (RT) for cancer with intent to cure has been optimized and standardized over the last 80 years. Both preclinical and clinical work emphasized the observation that radiation destroys the tumor and exposes its components to the immune response in a mode that facilitates the induction of anti-tumor immunity or reinforces such a response. External beam photon radiation is the most prevalent in situ abolition treatment, and its use exposed the “abscopal effect”. Particle radiotherapy (PRT), which has been in various stages of research and development for 70 years, is today available for the treatment of patients in the form of alpha particles, proton, or carbon ion radiotherapy. Charged particle radiotherapy is based on the acceleration of charged species, such as protons or carbon-12, which deposit their energy in the treated tumor and have a higher relative biological effectiveness compared with photon radiation. In this review, we will bring evidence that alpha particles, proton, or carbon ion radiation can destroy tumors and activate specific anti-tumor immune responses. Radiation may also directly affect the distribution and function of immune cells such as T cells, regulatory T cells, and mononuclear phagocytes. Tumor abolition by radiation can trigger an immune response against the tumor. However, abolition alone rarely induces effective anti-tumor immunity resulting in systemic tumor rejection. Immunotherapy can complement abolition to reinforce the anti-tumor immunity to better eradicate residual local and metastatic tumor cells. Various methods and agents such as immunoadjuvants, suppressor cell inhibitors, or checkpoint inhibitors were used to manipulate the immune response in combination with radiation. This review deals with the manifestations of particle-mediated radiotherapy and its correlation with immunotherapy of cancer.

Targeted Alpha Therapy: Progress in Radionuclide Production, Radiochemistry, and Applications

This review outlines the accomplishments and potential developments of targeted alpha (α) particle therapy (TAT). It discusses the therapeutic advantages of the short and highly ionizing path of α-particle emissions; the ability of TAT to complement and provide superior efficacy over existing forms of radiotherapy; the physical decay properties and radiochemistry of common α-emitters, including ²²⁵Ac, ²¹³Bi, ²²⁴Ra, ²¹²Pb, ²²⁷Th, ²²³Ra, ²¹¹At, and ¹⁴⁹Tb; the production techniques and proper handling of α-emitters in a radiopharmacy; recent preclinical developments; ongoing and completed clinical trials; and an outlook on the future of TAT.

RIG-1-Like Receptor Activation Synergizes With Intratumoral Alpha Radiation to Induce Pancreatic Tumor Rejection, Triple-Negative Breast Metastases Clearance, and Antitumor Immune Memory in Mice

Diffusing alpha-emitting radiation therapy (DaRT) employs intratumoral Ra-224-coated seeds that efficiently destroy solid tumors by slowly releasing alpha-emitting atoms inside the tumor. In immunogenic tumor models, DaRT was shown to activate systemic antitumor immunity. Agonists of the membrane-bound toll-like receptors (TLRs) enhanced these effects and led to tumor rejection. Here, we examined the combination of DaRT with agents that activate a different type of pattern recognition receptors, the cytoplasmatic RIG1-like receptors (RLRs). In response to cytoplasmatic viral dsRNA, RLRs activate an antiviral immune response that includes the elevation of antigen presentation. Thus, it was postulated that in low-immunogenic tumor models, RLR activation in tumor cells prior to the induction of their death by DaRT will be superior compared to TLR activation. Intratumoral cytoplasmatic delivery of the dsRNA mimic polyIC by polyethylenimine (PEI), was used to activate RLR, while polyIC without PEI was used to activate TLR. PolyIC(PEI) prior to DaRT synergistically retarded 4T1 triple-negative breast tumors and metastasis development more efficiently than polyIC and rejected panc02 pancreatic tumors in some of the treated mice. Splenocytes from treated mice, adoptively transferred to naive mice in combination with 4T1 tumor cells, delayed tumor development compared to naïve splenocytes. Low-dose cyclophosphamide, known to reduce T regulatory cell number, enhanced the effect of DaRT and polyIC(PEI) and led to high long-term survival rates under neoadjuvant settings, which confirmed metastasis clearance. The epigenetic drug decitabine, known to activate RLR in low doses, was given intraperitoneally prior to DaRT and caused tumor growth retardation, similar to local polyIC(PEI). The systemic and/or local administration of RLR activators was also tested in the squamous cell carcinoma (SCC) tumor model SQ2, in which a delay in tumor re-challenge development was demonstrated. We conclude that RIG-I-like activation prior to intratumoral alpha radiation may serve as a potent combination technique to reduce both tumor growth and the spread of distant metastases in low-immunogenic and metastatic tumor models.

Atomic Nanogenerators in Targeted Alpha Therapies: Curie's Legacy in Modern Cancer Management

Atomic in vivo nanogenerators such as actinium-225, thorium-227, and radium-223 are of increasing interest and importance in the treatment of patients with metastatic cancer diseases. This is due to their peculiar physical, chemical, and biological characteristics, leading to astonishing responses in otherwise resistant patients. Nevertheless, there are still a few obstacles and hurdles to be overcome that hamper the broader utilization in the clinical setting. Next to the limited supply and relatively high costs, the in vivo complex stability and the fate of the recoiling daughter radionuclides are substantial problems that need to be solved. In radiobiology, the mechanisms underlying treatment efficiency, possible resistance mechanisms, and late side effect occurrence are still far from being understood and need to be unraveled. In this review, the current knowledge on the scientific and clinical background of targeted alpha therapies is summarized. Furthermore, open issues and novel approaches with a focus on the future perspective are discussed. Once these are unraveled, targeted alpha therapies with atomic in vivo nanogenerators can be tailored to suit the needs of each patient when applying careful risk stratification and combination therapies. They have the potential to become one of the major treatment pillars in modern cancer management.