Cancer is one of the main causes of human death worldwide (1). Along with chemotherapy and surgery, radiotherapy (RT), also termed as radiation therapy, is a well-established method of treating non-metastatic cancers (2–4). In current practice, more than half of the cancer patients receive RT as primary mode of cancer therapy or adjuvant mode of treatment along with chemotherapy, immunotherapy, or surgery (5). In RT, high doses of ionizing radiation are delivered to ablate cancer cells and suppress recurrence and progression of cancer cells. RT can be broadly categorized into three types: external beam RT (EBRT), systemic RT, and internal RT (6, 7). In EBRT, high-energy photon or electron or ion beams are employed to deliver radiation to the tumor volume by placing radiation source outside the patient’s body (2). Systemic radiation therapies such as targeted RT deliver radioisotopes labeled with carrier molecules with high affinity towards receptors overexpressed by the cancer cells, e.g., monoclonal antibodies (mAb), through ingestion, infusion using catheter, or intravenous injection. In internal RT, also known as brachytherapy (BT), minimal invasive methods are used to place the radiation sources either inside or in close proximity to the tumor volume. BT allows delivery of high doses of radiation precisely to the tumor volume, while minimizing radiation exposure to the healthy tissues and organs at risk. Due to the precise and targeted dose delivery characteristics of BT, it can be employed to effectively treat solid tumors with minimum side effects and short treatment time at low cost.

Clinical trials and preclinical studies using BT have reported promising outcomes. However, the logistical and operational difficulties associated with BT seed placement have impeded its successful application. For instance, in patients with prostate tumor, the transrectal ultrasound (TRUS)-based implantation approach is used to implant radioactive seeds within the tumor (8, 9). The seed implantation causes trauma and edema in the prostate gland. This may consequently result in inaccurate or off-target placement of the seeds. The placement of radioactive seeds outside the tumor volume may result in undesired radiation exposure to the organs at risk, e.g., urinary bladder and rectum. Further, inaccurate seed placement may produce non-uniform dose distribution and may consequently result in mild to severe clinical side effects. Additionally, post-implantation migration of seeds to the lungs has also been reported and may require seed removal (8, 10).

Recently, several preclinical studies on localized delivery of radioactive nanoparticles (radio-NPs) into the tumor, similar to BT, have been reported in the literature, and this technique is termed as nanobrachytherapy (11–13). In nanobrachytherapy, radio-NPs are injected intratumorally as an alternative to the implantation of radioactive seeds. One recent example of this mode of treatment is the work by Salvanou et al. (14), who reported the use of gold nanoparticles (AuNPs) radiolabeled with ²²⁵Ac (alpha emitter) as an unconventional BT procedure, involving intratumoral injection of these radiolabeled AuNPs. Such nanoparticle-based systems i) conserve the characteristics of BT, i.e., precise and targeted dose delivery; ii) can be administered through injection; and iii) have the ability to provide patient-specific treatment, as radiation dose can be divided into several fractions. Additionally, these radiopharmaceuticals do not need seed removal; hence, they can be handled easily and can be extremely useful. The nanometer size of these radiopharmaceuticals allows local diffusion from the site of injection and may result in homogeneous dose distribution within the tumor volume. Lastly, these nanomaterials (particularly high Z nanoparticles) can be used as multifunctional carriers to deliver radioisotopes to provide imaging and RT capabilities. Such radioactive high-Z nanoparticles may also enhance radiation dose through self-sensitization and may require less radioactivity in comparison with conventional BT.

In this article, we review the recent advancements in the synthesis and use of radio-NPs as nanobrachytherapeutic agents. The subsequent section presents a review and discussion on different techniques involved in radiosynthesis of nanoparticles. The particles emitted by radionuclides, present in the obtained radionuclide–nanoparticle complex, must deposit their energy locally and spare the surrounding normal tissues. Hence, in the succeeding section, the essential characteristics of radionuclide–nanoparticle complexes, which are vital to qualifying them as nanobrachytherapeutic agents, are discussed. After intratumoral injection, these radio-NPs diffuse 1–2 mm within the extracellular medium, from the site of injection (15), and are internalized by the tumor cells. Thus, different mechanisms involved in the internalization of radio-NPs by tumor cells are reviewed in the next section. Thereafter, we summarize and review the most recently published preclinical studies on nanobrachytherapy. Additionally, for any RT-based treatment, dosimetry and treatment planning are the two crucial steps to ensure and quantify its accuracy and efficacy. Hence, the subsequent section reviews the recent dosimetric studies on use of radio-NPs as nanobrachytherapeutic agents. Lastly, current challenges and future research directions on the role of radio-NPs in BT treatments are discussed.

Although several advances in cancer treatment have been made throughout the years, it is paramount to develop more precise diagnostic and therapeutic regimens essential to achieve better diagnostic and therapeutic outcomes. Tumor presents a multifactorial etiology, which makes it an extremely complex and heterogeneous disease, attributed to an almost unique expression of biomarkers from patient to patient. To circumvent this complexity, the development of so-called precision and personalized medicine is pivotal towards the battle against cancer (16). One of the major strategies is through the combination of nuclear medicine modalities and nanotechnology to offer unique opportunities to develop an effective single chemical entity with diagnostic and therapeutic capabilities for clinical applications in theranostic nanoradiopharmaceuticals. This is achieved by designing architectural radiolabeled nanoconstructs based on the amalgamation of four major components for the intended in vivo pharmacokinetics (17):

Among the different types of nanoparticles, AuNPs and iron oxide nanoparticles (IONPs) have gained more prominence due to their superior biocompatibility, low toxicity, ease in surface versatile functionalization and radiolabeling with a plethora of imaging, and therapeutic radionuclides towards the development of nanoradiopharmaceuticals for imaging and therapy of cancer. Translational medicine that makes use of nanoradio-pharmaceutical agents demonstrates excellent pharmacokinetics in terms of radiochemical production, purity and stability (nanoradioformulation integrity), biodistribution, dosimetry, low off-target localization, and favorable renal clearance profiles, which represent a versatile theranostic tool in cancer management, ranging from nuclear medicine imaging and image-guided surgery to alpha/beta-particle targeted therapy, and most recently targeted nanobrachytherapy (18–21). The use of targeted nanobrachytherapy through radiolabeled nanoparticles affords intra- or peritumoral administration, thus allowing less invasiveness and homogenizing the radiation dose deposition in the tumor as compared with conventional BT (22).

The radionuclides to be used for internal RT must deliver high doses of radiation locally and spare the surrounding normal tissues (5, 13). Hence, radionuclides emitting radiation with higher linear energy transfer (LET) are generally preferred. LET is the amount of energy transferred, by the emitted particles, to the medium traversed per unit distance. These radionuclides are categorized into three groups based on the emitted particle type (48). It includes α, β, and Auger particle-emitting radionuclides, as reported in Table 1 .

It is important to evaluate the suitability of these radionuclides for nanobrachytherapy applications. The must-have features for radionuclides can be classified into two main groups: i) physical and ii) biochemical characteristics. The physical characteristics to be considered are a) physical half-life; b) emitted particle type—α, β, and Auger electrons or photons; c) energy of the emitted particles; d) daughter product(s) and their stability; e) radionuclide purity and length of purification step; f) penetration depth of the emitted particles in the biological tissues; g) LET of the emitted particle; and h) size of the tumor to be treated (5, 13). Additionally, the biochemical characteristics to be evaluated are a) approach used to target tumor cells/tissues; b) retention of radio-NPs within the tumor; c) in vivo stability of the radionuclide–nanoparticle complex; and d) toxicity caused by the complex (53–55).

The physical half-life of the radionuclide should match with the in vivo pharmacokinetics of the radionuclide–nanoparticle complex (55). The life span (T) of the radionuclide can be estimated from the physical half-life (Tp) of the radionuclide and half-clearance time, also known as biological half-life (T_b ), of the radionuclide–nanoparticle complex using the relation 1/T = 1/T_p + 1/T_b (56). The physical half-life of the radionuclide can be known from the published radionuclide data; and to estimate the biological half-life of the radionuclide–nanoparticle complex, knowledge on the spatial and temporal distribution of the complex within the tumor and body is required. T_b depends on the mode of delivery, uptake, and metabolism of the radionuclide–nanoparticle complex by the tumor cells and its excretion from the patient’s body (48, 53, 54).

The radionuclides with physical half-life of between 6 h and 7 days are preferred for therapeutic purposes. An extremely short physical half-life hampers the flexibility in administration of the radiotherapeutic agent and is impractical for clinical use. On the contrary, the use of long-lived radionuclides may result in retention of radiation dose in the patient for a longer period of time. Furthermore, patients may be required to be isolated and admitted in the hospital, in order to minimize the risk of radiation exposure to the general public. Additionally, the biological half-life of the radionuclide–nanoparticle complex is dependent on the properties of the nanocarrier used. The nanocarriers with long biological half-life should be used with radionuclides having short physical half-life (13, 48). The radio-NPs must be efficiently retained within the tumor volume so that higher doses of radiation can be delivered to the tumor tissues. The use of nanocarriers with short biological half-life may result in excretion of radio-NPs with high activity and may need extensive management of radioactive waste. Hence, for efficient delivery of radiation dose, the radionuclide–nanoparticle complex with optimal physical and biological half-life must be selected (48, 54, 55).

α-Particle emitters such as ²²⁵Ac and ²¹¹At emit positively charged helium nuclei, having high higher LET and short penetration depths in biological tissues (5). For instance, ²²⁵Ac emits alpha particles in an energy range of 5–9 MeV and has LET value between 80 and 100 keV/μm and spatial penetration range between 40 and 100 μm. Hence, it has a probability of depositing most of the radiation within the tumor volume and can ablate tumor cells efficiently. Thus, α-emitting radionuclides are suitable in treating small or residual microscopic-size tumors. The main limitation of α-emitting radionuclides is that they have multiple daughter products with variable half-lives. Hence, migration of these nanocarriers labeled with α emitters can lead to significant damage to normal tissues (13, 14, 51, 57).

β-Emitting radionuclides are the most widely used radionuclides for internal RT purposes. The emitted electrons have lower LET and longer range (several millimeters) in comparison with the α emitters (58, 59). For example, ⁹⁰Y emits electrons with LET of 0.2 keV/μm and mean range of 3,960 μm. Hence, it may result in less cytotoxicity in comparison with the α-emitting radionuclides and radiation damage caused by these long-range β-particles, far from its origin, which is termed as “crossfire effect.” Thus, due to the long penetration depth of the emitted electrons (≈0.05–12 mm), β emitters are regarded as the most suitable for the treatment of large or bulky tumors (7). β-Emitting radionuclides ¹⁹⁸Au, ¹⁹⁹Au, ¹³¹I, and ¹⁷⁷Lu have been investigated as potential nanobrachytherapeutic agents (17, 37, 49, 50). ¹⁹⁸Au was used in the initial works of radioactive collidal gold (60). It is because ¹⁹⁸Au can be easily integrated with AuNPs. Some β-emitting radionuclides also decay with γ-radiation. For nanocarriers composed of high-Z materials, AuNP in particular, gamma radiation on interaction with the material of the nanoparticles may result in the enhancement of radiation dose deposition by the mechanism of radiosensitization (13, 15). Photoelectric effect plays a vital role in radiosensitization, and for Au, it is the strongest for gamma radiation of energy below or equal to 200 keV. ¹⁹⁸Au, ¹³¹I, and ¹⁷⁷Lu emit gamma radiation with energy >200 keV. Hence, the photoelectric effect for gold is the strongest for photons with energy lower than 200 keV. The gamma radiation emitted by these radionuclides does not provide maximum radiosensitization effect (5). However, ¹⁹⁹Au emits gamma radiations with maximum energy ≈158 keV. Thus, dose enhancement via radiosensitization effect can be expected. In this regard, gold was used as a nanomaterial in preclinical studies, using AuNPs radiolabeled with β-emitting radionuclides, due to its biocompatibility. Additionally, this gamma emission associated with β-emitting radionuclides can be advantageous in visualizing the spatial and temporal distribution of radio-NPs within the patient with the help of gamma scintigraphy techniques. Lastly, it should be considered that the long range of emitted electrons may result in non-specific cytotoxicity by depositing radiation dose to the surrounding normal cells/tissues (48, 54, 55).

Radionuclides emitting Auger electrons are considered to be beneficial in the treatment of small tumors or a cluster of tumor cells. This is attributed to higher cytotoxicity caused by these low-energy electrons (less than 500 eV or a few keV) with short range in the biological tissues (a few nanometers) (5, 54, 55). ¹⁰³Pd, ¹¹¹In, and ¹²⁵I have been used as nanobrachytherapeutic agents in preclinical studies involving tumor-bearing xenograft models (38, 45, 52). These radionuclides decay by internal conversion (IC) and electron capture (EC) mode and emit Auger electrons. The energy of the emitted Auger electrons range from ≈500 eV to a few keV with a spatial penetration depth of 2–500 nm. For effective ablation of tumor cells, these radionuclides must be internalized as close as possible to the cell nucleus. These radionuclides, ¹⁰³Pd, ¹¹¹In, and ¹²⁵I, also emit gamma radiation. ¹²⁵I and ¹⁰³Pd emit low-energy (30 keV) photons and have been used for low-dose-rate BT applications since the 1970s. The emitted photons deposit up to 98% of their energy within ≈5–8 cm of soft tissue and can be used to treat large and bulky tumors. ¹¹¹In also emits photons with energy greater than 200 keV and is not suitable for internal RT purposes or radiation dose enhancement through radiosensitization (13). In case of preclinical studies using xenograft models, the energy deposited to the tumor models is mainly due to these emitted Auger electrons and photoelectrons generated due to the interaction of low-energy photons and gold (7, 61).

Hence, the choice of the radionuclide also depends on the size of the tumor to be treated. It is because bulky tumors, micrometastases, and a small cluster of tumor cells require particles of specific energy for effective ablation of cancer cells. Further, the mode of radiosynthesis of nanoparticles and the length of the purification step (of radionuclides) must be selected according to the half-life of the radionuclide (54, 55). In terms of the spatial penetration depth and energy of the emitted particles, Auger and β-emitting radionuclides are most suitable for the treatment of solid tumors such as brain, breast, and prostate tumors by using nanobrachytherapy procedures (5, 13, 61).

Considering biochemical properties, a clinically acceptable radionuclide–nanoparticle complex must selectively concentrate within the tumor and have a prolonged retention. Also, it should have minimum or no uptake in the normal tissues or organs. Furthermore, the ratio of retention of a nanobrachytherapy agent should be high in the tumor volume in comparison with the normal tissues (10), so that high radiation doses can be delivered to the tumor volume and minimum or no radiation dose is delivered to the normal tissues or organs. Additionally, the radionuclide–nanoparticle complex should be stable enough at the time of injection and should have prolonged retention in vivo before it is excreted or metabolized (5, 13). Other biochemical features to be taken care of are low toxicity, appropriate pH, and optimal biological half-life. Furthermore, the radionuclide and nanoparticle (to which a radionuclide is attached) must have a high complexation yield and must form a stable complex in the biological environment (48, 53).

The four main mechanisms of nanomaterial internalization by cells are micropinocytosis, caveolae-mediated endocytosis, clathrin-mediated endocytosis, and a mechanism independent of caveolae or clathrin ( Figure 3 ) (62). The differential profile of AuNP internalization by different cell types depends on a large extent to the differences in their biophysical mechanisms, especially the cell membrane characteristics. Regarding the nanoparticle characteristics, the uptake is influenced significantly by the surface chemistry and the morphology of different nanomaterials. Additionally, one should be aware of the size of nanoparticle clusters that might be formed by aggregated particles in contact with cells, and the consequence of this aggregation in the internalization efficiency, as well as the location of nanoparticles and nanoaggregates in terms of organelles and intracellular vesicles (63, 64).

In 2008, Douglas and colleagues investigated the internalization and cytotoxicity of alginate–chitosan nanoparticles in 293T, COS7, and CHO cells. It was demonstrated that trypsinization can prevent alginate–chitosan nanoparticle internalization depending on the cell type. After trypsinization in 293T and COS7 cells, 75–85% of the binding efficiency to plasma membrane was lost, indicating that the interaction of those nanoparticles with the cells was mediated by chitosan and trypsin-sensitive proteins, but the same was not observed in CHO cells (65).

In the same study, it was observed that the vectors were not localized in lysosomes once they enter the cells, and the endocytic mechanism is different among the studied cell lines. For instance, clathrin-dependent endocytosis is important in 293T and COS7 cell lines, while caveolin-dependent internalization is significant for COS7 and CHO cells. Macropinocytosis was not relevant for any of the cell lines, but another mechanism dependent on actin microfilaments plays an important role for the internalization in 293T cells. This study supported the assumption that many factors are important for cell internalization and for the fate of nanomaterials in the cells, i.e., cell physiology, complex size, composition, and endocytosis mechanism. These parameters must be fully indicated in order to increase the success rate in the designed treatment (65).

The mechanism of internalization of 200-nm-diameter nanoparticles seems to be a combination of energy-dependent phagocytosis and clathrin-mediated endocytosis. But in all cases, the endocytoses were proven to be energy-dependent, while for smaller particles, an actin-dependent mechanism seems to play an important role. Caveolae-mediated endocytosis is the most important mechanism for 150- and 200-nm nanoparticles, but it is worthy to mention that all internalization pathways contribute to the internalization of 150-nm nanoparticles, and this might explain the higher efficiency of endocytosis for those particles (66). Positively charged nanoparticles were observed to be significantly more internalized than negatively charged ones (84% against 5%, respectively). It is clear that negatively charged particles must rely on surface functionalization so that receptor-mediated endocytosis can compensate for the lower internalization rates.

Bannunah and collaborators published a thorough study comparing negatively and positively charged particles, of different sizes, in terms of their epithelial and cell uptake efficiency, as well as their toxicity to CaCo-2 (human intestinal adenocarcinoma) and Calu-3 (human airway epithelial) cells. According to their study, positively charged nanoparticles cause higher levels of cytotoxicity as compared with negatively charged ones, and it might be due to the oxidative stress, mitochondrial damage, and cellular overall toxicity observed for those kinds of particles. Negatively charged particles are known to be less cytotoxic to epithelial cells, and this might be explained by the fact that those cells present a net negative charge in their extracellular portion of plasma membrane, enabling a better interaction with positively charged nanoparticles. The results obtained for other cell types are sometimes conflicting; therefore, more studies might be necessary in order to understand the mechanisms for each tissue (67).

When inhibitors of dynamin-dependent and clathrin-dependent endocytoses are used, it seems that both negatively and positively charged nanoparticles are not significantly internalized via a dynamin-dependent mechanism, but the inhibition of clathrin-mediated transport likely caused an increase in the transport of negatively charged particles, though with no effect on their cell uptake. Regarding positively charged particles, clathrin inhibition reduced by 46% their cell uptake and by 38% their transcellular transport, whereas micropinocytosis inhibition reduced the internalization of the same particles by 42%, and the transcellular transport by 38%, similarly to micropinocytosis inhibition by methyl-β-cyclodextrin (67). No effect on negatively charged nanoparticles was observed after micropinocytosis inhibition.

The disruption of microtubules with nocodazole had no effect on the internalization of any of the nanoparticles, but the transport across the cells was significantly impaired. Genistein, a tyrosine-kinase inhibitor, impaired both the internalization (50%) and the intracellular transport (48%) of negatively charged nanoparticles, leading to the assumption that caveolae-dependent endocytosis plays an important role for those nanoparticles (67).

The protein corona is another key factor to be considered when developing any nanomaterial with biomedical applications. The protein corona is formed whenever a nanomaterial is introduced into a complex protein aqueous system and consists in the rapid adsorption of the most abundant proteins onto the surface of the nanomaterial, followed by the exchange of at least part of these proteins for others with higher affinity for the nanomaterial. The result is a nanoparticle with a completely different surface coating than that predicted in the design phase, sometimes with tendency for aggregation or with higher stability, with different internalization rates (enhanced or impaired), and different pharmacokinetics (68, 69).

The composition of the protein corona is not universal, as it depends on the nanomaterial and on the previous coating. It was demonstrated, for instance, that citrate-coated IONPs were not stable while in contact with fetal calf serum proteins but were efficiently internalized by lymphoblastoid cells, while poly(acrylic acid)-coated IONPs were quite stable, although were poorly taken up, which can be a barrier to be faced by the nanomaterials inside the blood (68).

Another barrier that the nanoparticles must overcome is the reticuloendothelial system (RES), responsible for a rapid clearance of the nanomaterials once they enter in the bloodstream, decreasing their pharmacological action. Strategies to avoid the clearance by RES include surface modification with molecules that prevent opsonization and increase the half-life in blood, such as PEG. However, as described previously, many variables must be added, such as surface charge. Harush-Frenkel and collaborators verified once more the preferential internalization of positively charged nanoparticles (twice the endocytosis of their negatively charged counterparts) in HeLa cells, and after 45 min, the cells tend to decrease the uptake rate, characterizing a saturation phase (70).

Another factor that contributes to the decreased circulating time of nanoparticles in blood is the mononuclear phagocytic system, in which the macrophages quickly scavenge nanoparticles that are agglomerated or covered with the protein corona, preventing their arrival at the target site. Zhang and coworkers used the advantage of folic acid as a functionalizing agent in PEGylated superparamagnetic magnetite nanoparticles (circa 10 nm in diameter) in the internalization efficiency by mouse macrophages (RAW 264.7 cells) and human breast tumor (BT-20 cells). PEG was responsible in partially inhibiting the formation of the protein corona in order to decrease the recognition of nanoparticles by macrophages, whereas folic acid was added to the surface of nanoparticles to specifically target cancer cells overexpressing folate receptors in order to increase their uptake (71).

The alpha-, beta-, and Auger-emitting radionuclides have been investigated for nanobrachytherapy applications. A few preclinical studies on nanobrachytherapy applications using alpha, beta, and Auger emitters have been published in literature. The most recent ones are briefly discussed in this section, and a brief summary of these studies is also presented in Table 2 .

Cancer treatment is a multipronged approach wherein the combination of treatment regimens such as surgery, chemotherapy, RT, and more recently immunotherapy is adopted to achieve better therapeutic index. For example, immunotherapy alone has occasional responses, and benefits are limited to a minority of patients in limited disease sites due to immune evasion properties of tumor cells (79). A rare phenomenon called “abscopal effect” is observed with the local radiation treatments where tumors outside the treatment fields have been observed to shrink as a result of immune response provoked by RT (80–82). The abscopal effect is rare with radiation alone but profoundly observed in patients undergoing immune checkpoint blockade therapy (83–86). Similarly, immunotherapy can enhance the efficacy of RT via activation of the innate and adaptive immune system (79, 87). With the development of tumor-specific antibodies, immune checkpoint inhibitor antibodies, and chimeric antigen receptor T-cell therapies, immunotherapy has revolutionized the treatment of metastatic disease including melanoma, non-small cell lung cancer, and renal cell carcinoma (88, 89). The unique synergistic relationship between radiation and immunotherapy provides the benefit of controlling systemic disease with local delivery of treatment. Many clinical trials are ongoing, testing the outcomes (safety, tumor response, immune response, and toxicities) of the combination of immune checkpoint inhibitor therapeutics and RT for prostate cancer (castrate resistant), soft tissue sarcoma, BC, glioma, pancreatic cancer, and melanoma (79, 88, 89). The RT techniques employed are mostly EBRT, including highly conformal intensity modulated RT and stereotactic body RT (SBRT) combined with high-precision image guidance (IGRT). In many studies, immunotherapy was administered intratumorally to envisage lesser immune-related adverse events, better local immune response, and control of systemic metastatic disease (90–96).

In one of the studies, Moreau et al. investigated the use of multifunctional smart RT biomaterial (SRB) loaded with immunoadjuvants to study the abscopal effect of local RT (93). Lewis lung carcinoma (LLC) tumors were generated on both the right and left flanks of the mice, and one tumor was implanted with an SRB device loaded with CD40 antibody in PLGA matrix followed by RT. The SRB device releases immunotherapeutic drug intratumorally and provides image guidance for EBRT using small animal radiation research platform (SARRP). The treatment response was observed in both irradiated and un-irradiated tumors owing to the radiation-mediated systemic antitumor immune response.

There was one attempt to administer both radiation and an immune stimulator directly to the tumor. Sodium alginate formulation containing catalase was labeled with ¹³¹I and injected intratumorally. This creates in situ gelation to confine ¹³¹I within the tumor and alleviates tumor hypoxia (97). They also showed that when ¹³¹I was added with CpG oligonucleotide (immunostimulator) administered intratumorally and combined with systemic checkpoint blockade therapy (CTLA-4 antibodies), it leads to local tumor eradication as well as increased systemic immune response to inhibit distal metastasis and tumor recurrence.

Radiation in the form of nanoparticles is being investigated for intratumoral administration to reduce the side effects to normal tissues, and similarly, immunotherapeutic drugs are being administered intratumorally to avoid immune-related adverse events. The therapeutic combination of nanobrachytherapy and intratumoral immunotherapy has great potential to achieve higher therapeutic index in a synergistic manner. They may deliver larger doses of therapeutics to the tumor, reduce normal tissue toxicities of systemic delivery, eradicate distal malignant cells owing to the enhanced abscopal effect, and enhance the efficacy of immunotherapy as well as RT for multiple disease sites and larger patient base.

Radio-NPs are emerging as promising radiotherapeutic agents for cancer treatment and are being probed as a replacement to seed-based BT. Prior to using radio-NPs for RT applications, accurate dosimetric simulations are needed in order to determine the dose distribution within the tumor volume and the surrounding normal tissues. Monte Carlo simulation techniques can be used efficiently to determine the energy or dose distribution within the region of interest. As per our knowledge, only three studies addressing the problem of dosimetry for nanobrachytherapy using radio-NPs have been published so far. The main highlights of these three studies are briefly discussed below.

Laprise-Pelletier et al. (98) used both experimental and theoretical approaches to estimate the dose distribution maps in the tumor tissues. Initially, radio-NPs ¹⁰³Pd : Pd@Au were synthesized (15) and administered intratumorally to the mice bearing prostate cancer tumors. At different time points (2 h, 24 h, and 8 days), tumors were harvested and analyzed through optical and transmission electron microscopy (TEM). A comprehensive biodistribution study confirmed that more than 80% of radio-NPs were retained in the tumor volume and that a small percentage of NPs migrated to the liver and spleen. The intracellular distribution of ¹⁰³Pd : Pd@AuNPs was quantified through optical and TEM images. Maps and profiles of energy deposition at microscopic and macroscopic levels were estimated using these data. At the macroscopic level, the dose distribution, in terms of isodose curves, obtained for ¹⁰³Pd : Pd@AuNPs (also termed as “cloud” of radio-NPs) was compared with the dose distribution obtained for the conventional millimeter-sized, low-dose BT: ¹⁰³Pd seed. A sharper dose fall in the isodose curves estimated for a single injection of radio-NPs was reported in comparison with the conventional BT seed. This sharper dose fall was attributed to the attenuation of photons by the gold atoms present in the “cloud” of NPs. The authors stressed that this feature can be useful in effectively sparing organs at risk and delivering high doses of radiation to the tumor tissues, as NPs deposit very high doses of radiation in their immediate vicinity. The TEM images of the xenograft tumor cells were used to simulate the energy deposited by ¹⁰³Pd : Pd@AuNPs at the microscopic level. The Monte Carlo simulation was conducted in three steps: i) the TEM images representative of microdistribution of NPs were selected and digitized. These digitized images were virtually placed in the middle of 50-μm³ cubical phantom. ii) Nanoconstructs (r = 25 nm) with Pd core (r = 5 nm) coated with a thick layer of gold (r = 20 nm) were simulated. These nanoconstructs were positioned in the resampled TEM images, placed in the cubical phantom. iii) Emitted photons and electrons produced in the interactions were simulated, and energy deposition maps were computed. From the computed dose distribution, it was found that the highest dose deposition occurred in the immediate vicinity of the NPs. That is, the electrons escaping from the NPs lost most of their energy in a very short range, and almost no NPs were found in the nuclei. The same microdosimetric approach was used to quantify the radiosensitization effect induced by gold. In this case, two simulations were performed, considering i) ¹⁰³Pd : Pd core coated with gold and ii) ¹⁰³Pd : Pd not coated with gold. The estimated energy deposition map was reported as a ratio of energy-deposited values (energy deposited by radio-NPs coated with gold/energy deposited by radio-NPs not coated with gold). The ratio of energy-deposited value quantified the dose enhancement effect due to the presence of gold. Enhancement in dose by factor of 25, in the immediate vicinity of the NPs coated with gold, was observed, whereas for regions 2 μm further from the radio-NPs, no radiosensitization effect was observed. Hence, the radiosensitization effect was found to be extremely localized around the gold-coated radio-NPs. Based on the microdosimetric results, the authors concluded that reactive oxygen species (ROS) can be the main factor responsible for cell killing and observed strong tumor control effect (15).

Al-Yasiri et al. (99) used MCNP6.1.1 Monte Carlo (MC) code to construct a simple geometrical replica of a human prostate, containing a tumor inside it, and estimated the dose distribution due to the gold radio-NPs (¹⁹⁸AuNPs or ¹⁹⁹AuNPs) homogeneously distributed within the tumor. This simple model consisted of spheres representing the following: tumor (radius (r) = 0.4 cm) located within the prostate (r = 2 cm), prostate, bladder (r = 3.5 cm), and rectum (r = 1.5 cm). Dose distribution was estimated for tumor and other organs at risk (healthy prostate, bladder, and rectum), assuming that 10 mCi of ¹⁹⁸AuNPs/¹⁹⁹AuNPs was homogeneously distributed within the tumor volume. The authors reported that for both ¹⁹⁸AuNPs and ¹⁹⁹AuNPs, the maximum dose was deposited at the center of the tumor and decreased rapidly towards the tumor prostate interface and other surrounding organs. Owing to the high-energy beta emissions from ¹⁹⁸Au, high dose rates were reported for ¹⁹⁸Au at a) center of the tumor, 12 Gy/h; b) prostate tumor periphery, 1.46 Gy/h; c) prostate periphery, 0.1 Gy/h; d) center of the bladder, 0.013 Gy; and e) center of the rectum, 0.026 Gy/h. On the other hand, due to low-energy beta emissions for ¹⁹⁹Au, for the same locations, the dose rates were 1.6, 0.53, 0.26, 0.0013, and 0.004 Gy/h. Based on these findings, the authors concluded that ¹⁹⁸AuNPs are suitable for the treatment of solid tumors and that ¹⁹⁹Au can be used for imaging purposes.

In one of our recent in silico dosimetry study (61), we replicated the cell survival curves for three preclinical studies (10, 15, 100), published in literature, on the use of radioactive nanocarriers as nanobrachytherapeutic agents using a mathematical model (101) and EGSnrc (102) MC code. The mathematical model used took into account the doubling rate of tumor cells, complete repair of sublethal damage, uptake rate, and washout rate of nanocarriers to and from tumor cell monoexponential function of time. Furthermore, this study anticipated several possibilities and evaluated the dosimetric characteristics and therapeutic efficacy of nanoparticles radiolabeled with ¹⁰³Pd (Auger emitter), ¹⁵³Sm (medium energy beta emitter), and ¹⁹⁸Au (high-energy beta emitter). Initially, the dosimetric characteristics of ¹⁰³Pd, ¹⁵³Sm, and ¹⁹⁸Au were evaluated using single cell dosimetry (7). It was found that at the cellular level, ¹⁵³Sm deposited maximum dose, followed by ¹⁰³Pd and ¹⁹⁸Au. The least energy deposition for ¹⁹⁸Au was attributed to the emitted highly energetic beta particles. These beta particles exit the cell volume (radius = 5 μm) without depositing enough energy. Second, the estimated cell survival curves were found to be in good agreement with the experimental results published in literature. Lastly, we evaluated the impact of i) tumor size, ii) tumor type, and iii) amount of injected activity on the cell survival curves. We found that ¹⁵³Sm and ¹⁹⁸Au effectively ablated tumor cells for all three cases with minimum injected activity (≤20 MBq), whereas for ¹⁰³Pd, higher radioactivity was required to achieve a similar effect. Hence, we concluded that for radioresistant, large size (≈1 cm³) and rapidly growing tumors, ¹⁵³Sm and ¹⁹⁸Au can be conclusively used as nanobrachytherapeutic agents, whereas ¹⁰³Pd is only suitable for small-size (≈0.3 cm³) tumors that have injected activity ≥60 MBq.

The application of interstitial BT is impeded by several posttreatment adverse effects or symptoms and the associated operational and logistical complications. The emerging nano-platforms can be used to efficiently deliver radiopharmaceuticals to the tumor. In comparison with free radioisotopes or radioisotopes functionalized with single tumor-specific biomolecules, nanoparticles can be loaded with higher doses of radioactivity, and multiple radioisotopes can be accommodated within a single nanoparticle. Moreover, these nanocarriers can also provide several additional functions, for instance: i) photothermal effect, ii) load chemotherapeutic drug, iii) radiosensitization in case of high-Z nanoparticles, and iv) real-time tumor imaging. Hence, they can be helpful in improving the efficacy or optimizing the therapeutic planning of internal RT or systemic therapy. Radio-NPs, injected intratumorally, can directly deliver radiation dose to the tumor like BT; and this technique is termed as nanobrachytherapy.

In this article, we review the recent progress in the radiosynthesis of the nanoparticles and their use for nanobrachytherapy applications. Recent progress on the i) radiosynthesis methods, ii) selection of a radionuclide for nanobrachytherapy application, iii) modes of internalization of nanocarriers, and iv) the most recent preclinical and dosimetric studies on BT are discussed.

The intratumoral (i.t.) injection of radio-NPs for nanobrachytherapy applications is associated with several challenges and shortcomings. The two main obstacles that have impeded the clinical translation of radio-NPs are i) leakage of fraction of injected radio-NPs from the tumor and ii) inhomogeneous distribution of radioactivity within the tumor post i.t. injection.

Intratumoral retention of radio-NPs is crucial for the therapeutic effectiveness of nanobrachytherapy application and must be maximized. It also reduces the risk of irradiating normal tissues or healthy organs (especially the liver and spleen). Both inhomogeneous intratumoral radioactivity distribution and leakage of radio-NPs from the tumor post injection are caused by irregular tumor vasculature, variable blood and lymph flow, and pressure gradients. Since tumors are unique, the radio-NP leakage ratio and distribution of NPs within the tumor may vary from patient to patient. This will probably make treatment planning and dosimetric computations complicated and challenging. The tumor retention of radio-NPs can be improved by i) functionalizing surface of radio-NPs with tumor-specific biomolecules or ii) co-injecting biocompatible polymers that sequester NPs within the tumor, along with radio-NPs. The delivery systems, injected intratumorally, that can homogeneously distribute radioactivity throughout the tumor volume with minimal leakage have not been developed yet.

Consequently, for clinical translation of intratumorally injected radio-NPs for nanobrachytherapy applications, the injected radio-NPs should i) have high intratumoral retention and ii) homogeneously distribute radioactivity throughout the tumor volume.

Hence, more comprehensive biodistribution studies are required to understand and control the excretion routes of radio-NPs. Furthermore, intratumoral distribution and diffusion of NPs depend on i) tumor architecture and its density; ii) interstitial fluid pressure; iii) tumor vasculature, blood flow, and lymph flow; and iv) specifics of extracellular matrix of the tumor. These factors must be investigated in a wide range of tumors in order to reduce the inhomogeneity in the intratumoral radioactivity distribution. Lastly, for accurate computation of dose distribution at the cellular and subcellular levels within a tumor injected with radio-NPs, the computational model should consider i) in vivo microscopic distribution of radio-NPs, ii) complex cell geometry, and iii) distribution of radio-NPs near and within the nucleus. The computational model should also include all possible physics processes that are susceptible to participate in radiation dose distribution at the microscopic scale.

BS: initial idea, conceptualization, data collection, and manuscript preparation. LF: manuscript preparation. VT: manuscript preparation. SS: initial idea, basic framework, review of the manuscript, and supervision. TF: supervision and manuscript review. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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