Brachytherapy for patients with uveal melanoma: historical perspectives and future treatment directions

Personalized treatment approaches in intraocular cancer

Background

Intraocular malignant tumors represent a severe disease that threatens vision as well as life. To better extend the life of the patient, preserve visual function, and maintain ocular aesthetics, selecting the appropriate timing and methods of treatment becomes crucial.

Main text

With the continuous advancement of medical technology, the techniques and methods for treating intraocular malignant tumors are constantly evolving. While surgery was once considered the optimal method to prolong patient survival and prevent local recurrence, the discovery and application of various treatments such as radiotherapy, laser therapy, chemotherapy, cryotherapy, and monoclonal antibodies have led to a greater diversity of treatment options. This diversity offers more possibilities to develop personalized treatment plans, and thereby maximize patient benefit. This article reviews the various treatment methods for intraocular malignant tumors, including indications for treatment, outcomes, and potential complications.

Conclusions

Differentiating small intraocular malignant tumors from pigmented lesions is challenging, and ongoing monitoring with regular follow-up is required. Small to medium-sized tumors can be treated with radiotherapy combined with transpupillary thermotherapy. Depending on the tumor's distance from the optic disc, surgery with partial resection may be considered for distant tumors, while proximal tumors may require complete enucleation. Systemic chemotherapy has been widely applied to patients with retinal tumors, lymphomas, and intraocular metastatic cancers, but has limited efficacy in patients with choroidal melanoma. Antagonists of Vascular Endothelial Growth Factor (Anti-VEGF) drugs can improve patient vision and quality of life, while the efficacy of immunotherapy and molecular targeted therapy is still under research.

Anterior plaque brachytherapy placement for treatment of iris and iridociliary melanomas - Surgical procedure and institutional experience

Surgical placement of eye plaque brachytherapy (EPB) is the standard of care for the treatment of uveal melanomas, including iris/iridociliary melanomas. However, unique challenges exist in anterior EPB placement. Here, we describe a surgical technique for anterior EPB placement when placement requires plaque positioning onto the cornea. Blunt conjunctival peritomy exposes the sclera overlying the tumor. A "dummy" plaque is placed, with positioning confirmed by direct visualization. The amniotic membrane is draped across the cornea and anchored with the eyelet sutures, the plaque is placed overlying the membrane, the conjunctiva is closed over the plaque, and a temporary tarsorrhaphy is performed. One week later, the conjunctival incision is reopened for plaque/amniotic membrane removal. This technique was employed in the treatment of 12 iris/iridociliary melanomas at our institution, with no instances of corneal damage. In placing an anterior plaque, employing this technique allows appropriate cancer treatment while optimizing patient comfort and corneal integrity.

Cancer stem-like cells in uveal melanoma: novel insights and therapeutic implications

Uveal melanoma (UM) is the most common primary ocular tumor in the adult population. Even though these primary tumors are successfully treated in 90% of cases, almost 50% of patients ultimately develop metastasis, mainly in the liver, via hematological dissemination, with a median survival spanning from 6 to 12 months after diagnosis. In this context, chemotherapy regimens and molecular targeted therapies have demonstrated poor response rates and failed to improve survival. Among the multiple reasons for therapy failure, the presence of cancer stem-like cells (CSCs) represents the main cause of resistance to anticancer therapies. In the last few years, the existence of CSCs in UM has been demonstrated both in preclinical and clinical studies, and new molecular pathways and mechanisms have been described for this subpopulation of UM cells. Here, we will discuss the state of the art of CSC biology and their potential exploitation as therapeutic target in UM.

Iodine-125 brachytherapy for choroidal melanoma by using Ocuprosta seeds with indigenous non-collimated plaques: Our initial experience

PURPOSE

Brachytherapy is the gold-standard treatment for choroidal melanoma. This study evaluated iodine-125 brachytherapy by using Ocuprosta seeds with indigenous non-collimated plaques in Asian patients.

METHODS

Retrospective single-center study in a tertiary care hospital of 12 eyes with choroidal melanoma in 12 Asian patients who underwent brachytherapy with Ocuprosta seeds fixed on non-collimated plaques and had a follow-up of at least 32 months (mean: 42.4 ± 9.5 months; median: 40 months). Radiotherapy was planned after developing the digital 3D model of the tumor within the eye by using radiological images and clinical pictures. Ocuprosta iodine-125 seeds were used on indigenous non-collimated gold plaques to deliver the radiation for precalculated time. "Successful outcome" was taken as a decrease in the volume of the tumor, and "unsuccessful outcome" was defined as no change in the tumor volume or increase in the tumor volume at 24 months after brachytherapy.

RESULTS

The mean decrease in tumor volume was 21% (914.5 ± 912.2 mm3 to 495.7 ± 633.6 mm3) after brachytherapy, which correlated with the baseline volume of the tumor. Ten eyes (83.3%) showed a reduction in tumor volume, whereas two eyes showed an increase in the volume of the tumor after brachytherapy. One of the cases with a reduction in tumor size developed neovascular glaucoma. Enucleation was done in three eyes. A globe salvage rate of 75% and tumor regression rate of 83% were seen in the present study using Ocuprosta seeds.

CONCLUSIONS

Iodine-125 brachytherapy with uncollimated indigenous gold plaques is an effective treatment modality for choroidal melanomas in Asian patients.

Depression in Uveal Melanoma Survivorship: Examining Psychological Predictors of Adjustment in the First Year Following Diagnosis

BACKGROUND

A rare cancer, uveal melanoma (UM) affects 5 in 1 million adults annually. Research on predictors of mental health in UM survivors is scarce.

PURPOSE

In this prospective study, we tested models that postulate interactions between illness perceptions and coping processes in predicting depressive symptoms 1 year following UM diagnosis.

METHODS

Participants' approach- and avoidance-oriented coping processes and illness perceptions specific to control and chronicity were assessed. Participants (N = 107) completed assessments prior to diagnosis (T0), and 1 week (T1), 3 months (T2), and 12 months after UM diagnosis (T3).

RESULTS

At T1, a significant avoidance coping × chronicity perception interaction (b = 1.84, p = .03) indicated that the link between higher avoidance coping and greater T3 depressive symptoms was stronger for participants with prolonged chronicity perceptions (b = 17.13, p < .001). Chronicity perceptions at T2 interacted significantly with approach-oriented coping at all time points; the link between higher approach coping and lower T3 depressive symptoms was stronger for participants with prolonged chronicity perceptions at T2. Interactions between control perceptions and coping did not significantly predict T3 depressive symptoms.

CONCLUSIONS

Findings lend partial support to predictive models that consider the combined, interacting influence of chronicity perceptions and coping processes on depressive symptoms in survivors of eye cancer.

Tumor Shape-Specific Brachytherapy Implants by 3D-Printing, Precision Radioactivity Painting, and Biomedical Imaging

In brachytherapy (BT), or internal radiation therapy, cancer is treated by radioactive implants. For instance, episcleral plaques (EPs) for the treatment of uveal melanoma, are designed according to generic population approximations. However, more personalized implants can enhance treatment precision through better adjustment of dose profiles to the contours of cancerous tissues. An original approach integrating biomedical imaging, 3D printing, radioactivity painting, and biomedical imaging, is developed as a workflow for the development of tumor shape-specific BT implants. First, computer-aided design plans of EP are prepared according to guidelines prescribed by the Collaborative Ocular Melanoma Study protocol. Polyetheretherketone (PEEK), a high-performance polymer suitable for permanent implants, is used to 3D-print plaques and the geometrical accuracy of the printed design is evaluated by imaging. The possibility to modulate the dose distribution in a tridimensional manner is demonstrated by painting the inner surfaces of the EPs with radioactive 103Pd, followed by dose profile measurements. The possibility to modulate dose distributions generated by these 3D-printed plaques through radioactivity painting is therefore confirmed. Ex vivo surgical tests on human eyeballs are performed as an assessment of manipulation ease. Overall, this work provides a solution for the fabrication of tumor-specific radioactive implants requiring higher dose precision.

Dosimetric Investigation of Six Ru-106 Eye Plaques by EBT3 Radiochromic Films and Monte Carlo Simulation

Background

Ophthalmic brachytherapy using radioactive plaques is an effective technique for the treatment of uveal melanoma. Ru-106 eye plaques are considered as interesting issue due to their steep gradient dose. The pre-planning evaluation of dosimetric parameters is essential for the treatment planning system.

Objective

The current study aims at providing dose distributions of six Ru-106 eye plaques (CCA, CCB, CGD, CIB, COB and COD) using radiochromic EBT3 film, Geant4 Monte Carlo toolkit and the treatment planning software (Plaque Simulator).

Material and Methods

In this experimental study, an in-house phantom was employed for depth dose measurements with EBT3 films. Also, Geant4.10.5 scoring mesh was implemented to obtain the 2D dose distribution of the plaques. The results were compared with Plaque Simulator software and the manufacturer's (BEBIG) data. The gamma index criterion (3%/3 mm) was used to evaluate dose distributions obtained by the film measurements and Geant4 simulation.

Results

A good agreement was achieved between simulation and experimental results. Gamma index passing rate was 94.2%, 89.3%, 88.2%, 82.2%, 92.2% and 90.1% for CCA, CCB, CGD, CIB, COB and COD plaques, respectively. Absolute dose rate (mGy/min) obtained by EBT3 film at the depth of 2 mm was 79.4 mGy/min, 81.0 mGy/min, 78.6 mGy/min, 62.2 mGy/min, 75.2 mGy/min and 81.2 mGy/min for CCA, CCB, CGD, CIB, COB and COD plaques, respectively.

Conclusion

The measured dose distributions and lateral dose profiles may be utilized in the treatment planning system to cover clinical volumes such as the clinical target volume and the gross tumor volume.

New Treatment Horizons in Uveal and Cutaneous Melanoma

Melanoma is a complex and heterogeneous malignant tumor with distinct genetic characteristics and therapeutic challenges in both cutaneous melanoma (CM) and uveal melanoma (UM). This review explores the underlying molecular features and genetic alterations in these melanoma subtypes, highlighting the importance of employing specific model systems tailored to their unique profiles for the development of targeted therapies. Over the past decade, significant progress has been made in unraveling the molecular and genetic characteristics of CM and UM, leading to notable advancements in treatment options. Genetic mutations in the mitogen-activated protein kinase (MAPK) pathway drive CM, while UM is characterized by mutations in genes like GNAQ, GNA11, BAP1, EIF1AX, and SF3B1. Chromosomal aberrations, including monosomy 3 in UM and monosomy 10 in CM, play significant roles in tumorigenesis. Immune cell infiltration differs between CM and UM, impacting prognosis. Therapeutic advancements targeting these genetic alterations, including oncolytic viruses and immunotherapies, have shown promise in preclinical and clinical studies. Oncolytic viruses selectively infect malignant cells, inducing oncolysis and activating antitumor immune responses. Talimogene laherparepvec (T-VEC) is an FDA-approved oncolytic virus for CM treatment, and other oncolytic viruses, such as coxsackieviruses and HF-10, are being investigated. Furthermore, combining oncolytic viruses with immunotherapies, such as CAR-T cell therapy, holds great potential. Understanding the intrinsic molecular features of melanoma and their role in shaping novel therapeutic approaches provides insights into targeted interventions and paves the way for more effective treatments for CM and UM.

Radiotherapy on-chip: microfluidics for translational radiation oncology

The clinical importance of radiotherapy in the treatment of cancer patients justifies the development and use of research tools at the fundamental, pre-clinical, and ultimately clinical levels, to investigate their toxicities and synergies with systemic agents on relevant biological samples. Although microfluidics has prompted a paradigm shift in drug discovery in the past two decades, it appears to have yet to translate to radiotherapy research. However, the materials, dimensions, design versatility and multiplexing capabilities of microfluidic devices make them well-suited to a variety of studies involving radiation physics, radiobiology and radiotherapy. This review will present the state-of-the-art applications of microfluidics in these fields and specifically highlight the perspectives offered by radiotherapy on-a-chip in the field of translational radiobiology and precision medicine. This body of knowledge can serve both the microfluidics and radiotherapy communities by identifying potential collaboration avenues to improve patient care.

Treatment of Uveal Melanoma With Radioactive Iodine 125 Implant Compared With Proton Beam Radiotherapy

Objective

To review the current state of radiation therapy for uveal melanoma and compare particle radiation and brachytherapy.

Patients and Methods

The medical records of 156 patients treated for uveal melanoma between May 30, 2012, and March 16, 2020, were retrospectively reviewed. Treatments consisted of either radioactive iodine 125 implant (RAI) or fractionated proton radiation (proton beam therapy [PBT]). Baseline characteristics were compared using a Wilcoxon rank sum test or χ² test. Outcomes were compared using Cox proportional hazards regression models or logistic regression models.

Results

The median length of follow-up after treatment was 2.7 years (range, 0.5 to 9.0 years). Patients who underwent treatment with RAI were older (median age, 67 vs 59 years; P<.001) and had a lower tumor classification (American Joint Commission on Cancer; P=.001) compared with those who underwent PBT. There was no significant difference between RAI and PBT in the outcomes of liver metastases, death, enucleation, tearing, vision loss, retinal detachment, tumor thickness, conjunctivitis, optic neuropathy, iris neovascularization, or neovascular glaucoma (all P>.05). Patients who underwent RAI treatment had significantly higher risk of diplopia (P<.001), cataract progression (P<.001), and maculopathy (P=.03) compared with those who received PBT. Patients who underwent RAI were at higher risk of eyelash loss (P=.006) compared with the PBT group.

Conclusion

Treatment with PBT and RAI has similar efficacy; however, there are differences in the adverse outcomes associated with these 2 modalities.

Atypical choroidal nevus in a subject with a germline PALB2 pathogenic variant

Recent evidence suggests that PALB2 variants may increase risk for the development of uveal melanoma and uveal melanocytic neoplasms. Here we report a case of an atypical choroidal nevus in a patient with a personal history of cancer and pathogenic PALB2 germline variant. A 75-year-old white female presented with an elevated predominantly amelanotic choroidal lesion OS. On examination and ophthalmic imaging, the mass measured 8.8 mm × 6.5 mm × 1.5 mm. The mass showed predominantly medium to high reflectivity on diagnostic A-scan and acoustic hollowing on B-scan. OCT over the lesion showed no subretinal fluid. The patient has a personal history of breast cancer and gastric adenoma and a strong family history of cancer. The patient was found to have a pathogenic truncating variant in PALB2 (rs118203998 c.3549C > A, p.Y1183*). Together with our previous findings of pathogenic PALB2 variants in uveal melanoma patients, this new finding of an atypical choroidal nevus in a patient with a pathogenic PALB2 germline variant suggests that pathogenic PALB2 variants may be a risk factor for uveal melanocytic neoplasms. This finding warrants further assessment of the prevalence and progression of uveal melanocytic neoplasms in PALB2 pathogenic variant carriers.

Local tumor control and treatment related toxicity after plaque brachytherapy for uveal melanoma: A systematic review and a data pooled analysis

Uveal melanoma (UM) represents the most common primary intraocular tumor, and nowadays eye plaque brachytherapy (EPB) is the most frequently used visual acuity preservation treatment option for small to medium sized UMs. The excellent local tumor control (LTC) rate achieved by EPB may be associated with severe complications and adverse events. Several dosimetric and clinical risk factors for the development of EPB-related ocular morbidity can be identified. However, morbidity predictive models specifically developed for EPB are still scarce. PRISMA methodology was used for the present systematic review of articles indexed in PubMed in the last sixteen years on EPB treatment of UM which aims at determining the major factors affecting local tumor control and ocular morbidities. To our knowledge, for the first time in EPB field, local tumor control probability (TCP) and normal tissue complication probability (NTCP) modelling on pooled clinical outcomes were performed. The analyzed literature (103 studies including 21,263 UM patients) pointed out that Ru-106 EPB provided high local control outcomes while minimizing radiation induced complications. The use of treatment planning systems (TPS) was the most influencing factor for EPB outcomes such as metastasis occurrence, enucleation, and disease specific survival, irrespective of radioactive implant type. TCP and NTCP parameters were successfully extracted for 5-year LTC, cataract and optic neuropathy. In future studies, more consistent recordings of ocular morbidities along with accurate estimation of doses through routine use of TPS are needed to expand and improve the robustness of toxicity risk prediction in EPB.

Uveal melanoma diagnosis and current treatment options (Review)

Uveal melanoma is a rare condition accounting for only 5% of all primary melanoma cases. Still, it is the most frequently diagnosed primary intraocular malignant tumor in adults. Almost 90% of the tumors involve the choroid and only a small percentage affects the ciliary body or the iris. There is a consistent difference in incidence between different regions with individuals of northern European descent having a significantly higher risk as compared to Hispanics, Asians, and Blacks. Among the many risk factors, mutations in the G protein subunit alpha Q (GNAQ) or G protein subunit alpha 11 (GNA11) genes and different receptors are highly suggestive. While iris melanoma can easily be noticed by the patient itself or diagnosed at a routine slit-lamp evaluation, a consistent percentage of posterior uveal tumors are incidentally diagnosed at funduscopic evaluation as they can evolve silently for years, especially if located in the periphery. Uveal melanoma classifications rely on the tumor size (thickness and basal diameter) and also on intraocular and extraocular extension. The differential diagnosis with pseudomelanomas is carried out according to the tumor aspect and position. Iris melanoma has a better prognosis and a lower mortality rate as compared to choroidal melanoma that has a much higher rate of metastasis (50% of the patients) and a subsequent limited life expectancy from 6 to 12 months. While conservative therapeutic options for the primary tumor, relying on different surgical excision techniques and/or irradiation therapies, offer good local tumor control, the treatment options for metastatic disease, although numerous, are still inadequate in preventing a fatal outcome.

Radiation Retinopathy: Detection and Management Strategies

A gradual shift in trend from primary enucleation to globe salvaging radiation therapy for the management of ocular tumors has resulted in the rise of several post-treatment ocular complications including radiation retinopathy. Radiation retinopathy is a chronic, progressive, and occlusive vasculopathy that can manifest anytime between 1 month to 15 years after starting radiation therapy. The aim of treatment in most of these cases is to prevent further vision loss. Treatment options such as laser photocoagulation, anti-vascular endothelial growth factor and intraviral steroids have been described. However, despite several advances in diagnostic and therapeutic modalities, a significant proportion of eyes with radiation retinopathy eventually go blind. This review summarises some of the clinical features, investigative modalities, and recent therapeutic strategies used in the management of radiation retinopathy.

Fine-Needle Aspiration Biopsy of Intraocular Mass-Like Lesions

OBJECTIVES

The purpose of this study was to evaluate the diagnostic performance of fine-needle aspiration (FNA) biopsy for intraocular mass-like lesions and its contributing factors.

METHODS

Intraocular FNA cases were retrieved and reviewed along with histopathologic follow-ups, if available. The effects of rapid on-site evaluation (ROSE), repeated biopsy, and adjunct immunocytochemical studies on cytologic diagnoses were analyzed.

RESULTS

Of 72 FNA biopsies from 63 patients, nondiagnostic biopsy was seen in 17 cases (24%), whereas a definitive diagnosis was rendered in 39 cases (54%). The cytologic diagnoses correlated well with histopathologic follow-ups with a concordance rate of 61%. Almost all nondiagnostic biopsies (16/17, 94%) were seen in cases in which ROSE was not performed. Of the 7 patients in whom biopsy was repeated, a definitive diagnosis was rendered in 4 cases (57%). Immunocytochemistry was performed in the majority of cases with a malignant diagnosis, especially in metastatic tumors (75%).

CONCLUSIONS

Our data demonstrates that FNA is an effective tool for the diagnosis of intraocular tumors. ROSE, repeated biopsy, and adjunct immunocytochemistry can help reduce the nondiagnostic rate and/or enhance diagnosis of malignancy, further improving FNA diagnostic performance.

Current molecular and clinical insights into uveal melanoma (Review)

Uveal melanoma (UM) represents the most prominent primary eye cancer in adults. With an incidence of approximately 5 cases per million individuals annually in the United States, UM could be considered a relatively rare cancer. The 90-95% of UM cases arise from the choroid. Diagnosis is based mainly on a clinical examination and ancillary tests, with ocular ultrasonography being of greatest value. Differential diagnosis can prove challenging in the case of indeterminate choroidal lesions and, sometimes, monitoring for documented growth may be the proper approach. Fine needle aspiration biopsy tends to be performed with a prognostic purpose, often in combination with radiotherapy. Gene expression profiling has allowed for the grading of UMs into two classes, which feature different metastatic risks. Patients with UM require a specialized multidisciplinary management. Primary tumor treatment can be either enucleation or globe preserving. Usually, enucleation is reserved for larger tumors, while radiotherapy is preferred for small/medium melanomas. The prognosis is unfavorable due to the high mortality rate and high tendency to metastasize. Following the development of metastatic disease, the mortality rate increases to 80% within one year, due to both the absence of an effective treatment and the aggressiveness of the condition. Novel molecular studies have allowed for a better understanding of the genetic and epigenetic mechanisms involved in UM biological activity, which differs compared to skin melanomas. The most commonly mutated genes are GNAQ, GNA11 and BAP1. Research in this field could help to identify effective diagnostic and prognostic biomarkers, as well as novel therapeutic targets.

Ultrasound Biomicroscopy Documented Anterior Uveal Melanoma Regression after Ruthenium-106 Plaque Therapy

Introduction

Ultrasound biomicroscopy (UBM) is the only widely used method for the evaluation of anterior uveal melanoma (AUM).

Objective

Documentation of regression of AUM treated with ruthenium-106 (Ru-106) plaque types CCB and CCC using UBM.

Methods

This single institution-based retrospective case series involved 10 Caucasian patients with AUM followed after brachytherapy with UBM from January 2014 until February 2019. The largest prominence of the tumor perpendicular to the sclera or the cornea (including scleral/corneal thickness) (D) and the largest basal dimension (B) were measured in millimeters with UBM for all patients prior to the brachytherapy and at 4-month interval follow-up. Tumor regression was calculated as a percentage of decrease in the initial D and B values.

Results

The study involved 10 patients with a mean age of 64.4 years (yr) (range 46-80 yr). D ranged from 1.82 to 5.5 mm (median 2.99 mm) and B from 2.32 to 12.38 mm (median 4.18 mm). The apical radiation dose in all patients was 100 Gy. The median follow-up was 42.02 months. Regression for D was 21.11 ± 13.66%, 31.09 ± 14.66%, and 34.92 ± 19.86% at 1st, 2nd, and 3rd year of the follow-up, respectively, while for B it was 21.58 ± 16.05%, 28.98 ± 17.71%, and 32.06 ± 18.96%, respectively. Tumor recurrence was documented in 2/10 patients.

Conclusion

The major regression of AUM, treated with Ru-106 plaque types CCB and CCC, was documented in the first 2 years after brachytherapy in our study group. In the following years, only minimal regression was documented that warns of the need for close monitoring and active search for local recurrences.

Therapy of uveal melanoma A Review

The aim of intraocular melanoma therapy is to achieve local tumor control, reduce the risk of metastasis development, preserve the eyeball and possibly the visual function of the eye. The choice of therapeutic approach requires a comprehensive view and individual approach to each patient with uveal melanoma. Factors considered include local finding (location, tumor size and shape, tumor activity, central visual acuity, intraocular complications), age and the patients overall physical and psychological condition, as well as the patients wishes. The most widely used method of uveal melanoma treatment is radiotherapy. The effect of radiation is caused by the absorption of ionizing radiation energy, the effect of radiation on the cell is manifested by cell death (depletion), or by a cytogenetic information change (mutation). Brachytherapy uses scleral applicators with radionuclide - ruthenium (Ru-106) applicators dominate in Europe and iodine (I-125) applicators in the USA. In external radiotherapy, the source of ionizing radiation is outside the patients body. Both stereotactic radiosurgery and fractionated stereotactic radiotherapy are used. In the Czech Republic, treatment is carried out using Leksell gamma knife or CyberKnife, while proton therapy dominates in the world. The development of serious radiation complications (radiation retinopathy, neuropathy, neovascular glaucoma, toxic tumor syndrome, etc.) should be considered. Surgical therapy involves a variety of invasive procedures. Iridectomy is performed for iris melanoma. Anteriorly located choroidal melanomas and / or ciliary body melanomas can be resolved by transscleral resection (exoresection). For posterior choroidal melanomas, a combination of external tumor irradiation with pars plana vitrectomy is used. Enucleation is a method of choice in advanced tumors that cannot be effectively irradiated. Orbital exenteration is indicated in advanced tumors with extrabulbar spread or in relapsed tumor after previous enucleation.

Design and Challenges of Radiopharmaceuticals

This review describes general concepts with regard to radiopharmaceuticals for diagnostic or therapeutic applications that help to understand the specific challenges encountered during the design, (radio)synthesis, in vitro and in vivo evaluation and clinical translation of novel radiopharmaceuticals. The design of a radiopharmaceutical requires upfront decisions with regard to combining a suitable vector molecule with an appropriate radionuclide, considering the type and location of the molecular target, the desired application, and the time constraints imposed by the relatively short half-life of radionuclides. Well-designed in vitro and in vivo experiments allow nonclinical validation of radiotracers. Ultimately, in combination with a limited toxicology package, the radiotracer becomes a radiopharmaceutical for clinical evaluation, produced in compliance with regulatory requirements for medicines for intravenous (IV) injection.