Compton imaging for medical applications

Compton imaging exploits inelastic scattering, known as Compton scattering, using a Compton camera consisting of a scatterer detector in the front layer and an absorber detector in the back layer. This method was developed for astronomy, and in recent years, research and development for environmental and medical applications has been actively conducted. Compton imaging can discriminate gamma rays over a wide energy range from several hundred keV to several MeV. Therefore, it is expected to be applied to the simultaneous imaging of multiple nuclides in nuclear medicine and prompt gamma ray imaging for range verification in particle therapy. In addition, multiple gamma coincidence imaging is expected to be realized, which allows the source position to be determined from a single coincidence event using nuclides that emit multiple gamma rays simultaneously, such as nuclides that emit a single gamma ray simultaneously with positron decay. This review introduces various efforts toward the practical application of Compton imaging in the medical field, including in vivo studies, and discusses its prospects.

Metastasectomy versus radiation of secondary sites in stage IV breast cancer: Analysis from a national cancer registry

PURPOSE

Locoregional therapy at primary or secondary sites in breast cancer may be associated with improved survival as compared to systemic therapy alone. We explored the sociodemographic and clinicopathologic factors associated with the use of radiation versus surgical resection of metastatic sites (metastasectomy) in patients with de novo stage IV breast cancer, followed by the associated overall survival.

METHODS

We sampled the National Cancer Database for patients with de novo stage IV breast cancer, (2010-2017) and described cohort's characteristics using univariate analyses. We identified 5 subgroups based on malignant site involvement: 1. Bone only, 2. Brain only, 3. Liver only, 4. Lung only, and 5. Metastasis involving >1 site. Kaplan-Meier modeling with log-rank testing and multivariate Cox Regression analysis were used to explore differences in overall survival between those that received radiation at secondary sites and those that underwent metastasectomy.

RESULTS

N = 22,749patients were included in this analysis. Radiation (81.2%) was used more commonly than metastasectomy (28.8%). Metastasectomy was associated with better median overall survival across all 5 cohorts (p < .001), with the survival benefit being the most pronounced with lung only (OS: 56.9 months; HR 0.8, 95% CI 0.7-0.9, p = .032), or liver only (OS: 41.6 months; HR: 0.9; 95% CI: 0.7-1.1, p < .001) metastasis.

CONCLUSION

Metastasectomy in patients with de novo stage IV breast cancer may be associated with improved overall survival as compared to radiation of secondary lesions, particularly in those with only liver or lung involvement. Prospective randomized controlled trials investigating surgical resection of metastatic sites in patients with breast cancer are warranted.

Intercomparison of Monte Carlo calculated dose enhancement ratios for gold nanoparticles irradiated by X-rays: Assessing the uncertainty and correct methodology for extended beams

Results of a Monte Carlo code intercomparison exercise for simulations of the dose enhancement from a gold nanoparticle (GNP) irradiated by X-rays have been recently reported. To highlight potential differences between codes, the dose enhancement ratios (DERs) were shown for the narrow-beam geometry used in the simulations, which leads to values significantly higher than unity over distances in the order of several tens of micrometers from the GNP surface. As it has come to our attention that the figures in our paper have given rise to misinterpretation as showing 'the' DERs of GNPs under diagnostic X-ray irradiation, this article presents estimates of the DERs that would have been obtained with realistic radiation field extensions and presence of secondary particle equilibrium (SPE). These DER values are much smaller than those for a narrow-beam irradiation shown in our paper, and significant dose enhancement is only found within a few hundred nanometers around the GNP. The approach used to obtain these estimates required the development of a methodology to identify and, where possible, correct results from simulations whose implementation deviated from the initial exercise definition. Based on this methodology, literature on Monte Carlo simulated DERs has been critically assessed.

Hypofractionated adjuvant surgical cavity radiotherapy following resection of limited brain metastasis

BACKGROUND

Following surgical resection of oligometastatic disease to the brain there is a high rate of local relapse which is reduced by the addition of focal radiation therapy, often delivered as single fraction stereotactic radiosurgery (SRS) to the surgical cavity. This study audited the outcomes of an alternative approach using hypofractionated radiation therapy (HFRT) to the surgical resection cavity.

METHODS AND MATERIALS

Seventy-nine patients who received surgical resection and focal radiation therapy to the surgical cavity using HFRT with intensity modulated radiation therapy with or without stereotactic radiotherapy were identified. Doses were delivered in five fractions every second day for 10 days. Follow-up involved MRI surveillance with three-monthly MRI scans post resection. The major endpoints were local control at the surgical cavity site, and presence of radiation necrosis at the treated site.

RESULTS

Seventy-nine patients were included for the analysis with a median follow-up of 10.8 months. Of the cohort, 56% experienced intracranial progression, with all patients progressing distant to the resection cavity, and 7% progressing locally in addition. The one-year local control rate was 89.8%. The median progression-free survival was 10.0 months and median overall survival was 14.3 months. There was one CTCAE grade 3 toxicity of symptomatic radiation necrosis with no grade 4-5 toxicities seen.

CONCLUSIONS

The rate of local relapse following HFRT to the surgical cavity is low with minimal risk of radiation necrosis. HFRT can be considered as an alternative to SRS for focal radiotherapy after brain metastasis resection.

Synthesis, chemical and biochemical characterization of Lu2O3-iPSMA nanoparticles activated by neutron irradiation

Among the nanomaterials, rare sesquioxides (lanthanide oxides such as Lu₂O₃) are of interest due to their adequate thermal conductivity, excellent chemical stability, and high light output. The prostate-specific membrane antigen (PSMA) is an integral multifunctional protein overexpressed in various types of cancer cells. The radiolabeled PSMA inhibitor peptides (iPSMA) have demonstrated their usefulness as specific probes in the treatment and detection of a wide variety of neoplasms, mainly due to their high in vivo recognition by the PSMA protein. The objective of this research was to synthesize Lu₂O₃-iPSMA nanoparticles (NPs) and characterize their physicochemical properties before and after neutron activation, as well as to assess their biodistribution profile and in vitro potential to target cells overexpressing PSMA. The Lu₂O₃ NPs were synthesized by the precipitation-calcination method and conjugated to the iPSMA peptide using DOTA (1,4,7,10-tetraazocyclodecane-N,N',N″,N‴-tetraacetic acid) as a linking agent. Results of the physicochemical characterization by FT-IR and UV-Vis spectroscopies, SEM, TEM, DLS, HRTEM, SAED, DSC-TGA, and X-ray diffraction indicated the formation of Lu₂O₃-iPSMA NPs (diameter of 29.98 ± 9.07 nm), which were not affected in their physicochemical properties after neutron activation. ¹⁷⁷Lu₂O₃-iPSMA NPs showed high affinity (K_d = 5.7 ± 1.9 nM) for the PSMA protein, evaluated by the saturation assay on HepG2 hepatocellular carcinoma cells (PSMA-positive). The biodistribution profile of the nanosystem in healthy mice showed the main uptake in the liver. After irradiation, radioactive Lu₂O₃-iPSMA NPs exhibited radioluminescent properties, making the in vivo acquisition of their biodistribution, via optical imaging, possible. The results obtained from this research validate the execution of additional preclinical studies with the objective of evaluating the potential of the ¹⁷⁷Lu₂O₃-iPSMA NPs for the targeted radiotherapy and in vivo imaging of tumors overexpressing the PSMA protein.

Radiobiological and dosimetric assessment of DNA-intercalated 99mTc-complexes bearing acridine orange derivatives

BACKGROUND

Recently, a new family of 99mTc(I)-tricarbonyl complexes bearing an acridine orange (AO) DNA targeting unit and different linkers between the Auger emitter (99mTc) and the AO moiety was evaluated for Auger therapy. Among them, 99mTc-C3 places the corresponding radionuclide at a shortest distance to DNA and produces important double strand breaks (DSB) yields in plasmid DNA providing the first evidence that 99mTc can efficiently induce DNA damage when well positioned to the double helix. Here in, we have extended the studies to human prostate cancer PC3 cells using the 99mTc-C3 and 99mTc-C5 complexes, aiming to assess how the distance to DNA influences the radiation-induced biological effects in this tumoral cell line, namely, in which concerns early and late damage effects.

RESULTS

Our results highlight the limited biological effectiveness of Auger electrons, as short path length radiation, with increasing distances to DNA. The evaluation of the radiation-induced biological effects was complemented with a comparative microdosimetric study based on intracellular dose values. The comparative study, between MIRD and Monte Carlo (MC) methods used to assess the cellular doses, revealed that efforts should be made in order to standardize the bioeffects modeling for DNA-incorporated Auger electron emitters.

CONCLUSIONS

99mTc might not be the ideal radionuclide for Auger therapy but can be useful to validate the design of new classes of Auger-electron emitting radioconjugates. In this context, our results highlight the crucial importance of the distance of Auger electron emitters to the target DNA and encourage the development of strategies for the fine tuning of the distance to DNA for other medical radionuclides (e.g., 111In or 161Tb) in order to enhance their radiotherapeutic effects within the Auger therapy of cancer.

Intercomparison of dose enhancement ratio and secondary electron spectra for gold nanoparticles irradiated by X-rays calculated using multiple Monte Carlo simulation codes

PURPOSE

Targeted radiation therapy has seen an increased interest in the past decade. In vitro and in vivo experiments showed enhanced radiation doses due to gold nanoparticles (GNPs) to tumors in mice and demonstrated a high potential for clinical application. However, finding a functionalized molecular formulation for actively targeting GNPs in tumor cells is challenging. Furthermore, the enhanced energy deposition by secondary electrons around GNPs, particularly by short-ranged Auger electrons is difficult to measure. Computational models, such as Monte Carlo (MC) radiation transport codes, have been used to estimate the physical quantities and effects of GNPs. However, as these codes differ from one to another, the reliability of physical and dosimetric quantities needs to be established at cellular and molecular levels, so that the subsequent biological effects can be assessed quantitatively.

METHODS

In this work, irradiation of single GNPs of 50 nm and 100 nm diameter by X-ray spectra generated by 50 and 100 peak kilovoltages was simulated for a defined geometry setup, by applying multiple MC codes in the EURADOS framework.

RESULTS

The mean dose enhancement ratio of the first 10 nm-thick water shell around a 100 nm GNP ranges from 400 for 100 kVp X-rays to 600 for 50 kVp X-rays with large uncertainty factors up to 2.3.

CONCLUSIONS

It is concluded that the absolute dose enhancement effects have large uncertainties and need an inter-code intercomparison for a high quality assurance; relative properties may be a better measure until more experimental data is available to constrain the models.

Astatine-211 imaging by a Compton camera for targeted radiotherapy

Astatine-211 is a promising radionuclide for targeted radiotherapy. It is required to image the distribution of targeted radiotherapeutic agents in a patient's body for optimization of treatment strategies. We proposed to image ²¹¹At with high-energy photons to overcome some problems in conventional planar or single-photon emission computed tomography imaging. We performed an imaging experiment of a point-like ²¹¹At source using a Compton camera, and demonstrated the capability of imaging ²¹¹At with the high-energy photons for the first time.

Assessment of PSMA targeting ligands bearing novel chelates with application to theranostics: Stability and complexation kinetics of 68Ga3+, 111In3+, 177Lu3+ and 225Ac3

INTRODUCTION

Recent successes in the treatment of metastatic castration-resistant prostate cancer (mCRPCa) by systemic endoradiotherapy has sparked renewed interest in developing small molecule ligands targeting prostate-specific membrane antigen (PSMA) and chelators capable of stable complexation of metal radionuclides for imaging and therapy. As the size and coordination number of metals for imaging, such as ⁶⁸Ga³⁺, and for targeted therapy, such as ¹⁷⁷Lu³⁺ and ²²⁵Ac³⁺, are substantially different, they may show a preference for macrocycles of different denticity. We have prepared three simple conjugates that target PSMA and form radiometal complexes through coordination by either octa-, deca-, or dodecadentate tetraazacyclododecane chelators. The complex formation and metal ion selectivity of these constructs were determined at two relevant temperatures, complex stability was examined in vitro, and tumor targeting was demonstrated in preclinical PCa models with a view towards identifying a candidate with potential value as a theranostic agent for the imaging and therapy of mCRPCa.

METHODS

Three bifunctional chelates with high denticity, including the octadentate chelate DOTA, the decadentate 3p-C-DEPA and a novel dodecadentate analogue of DEPA, were synthesized and conjugated to a glutamate-urea-lysine (EuK) pharmacophore (EuK-DOTA, EuK-107 and EuK-106, respectively) to enable targeting of PSMA. The metal ion selectivity for each construct was determined by incubation at 25 °C and 95 °C with the trivalent radiometals ⁶⁸Ga³⁺, ¹¹¹In³⁺, ¹⁷⁷Lu³⁺ and ²²⁵Ac³⁺. PSMA binding affinity was determined by competitive binding using LNCaP cells, while in vivo tumor targeting of the ⁶⁸Ga-labeled constructs was examined by positron emission tomography (PET) in LNCaP xenograft tumor-bearing mice.

RESULTS

PMSA affinities (IC₅₀ values) were 13.3 ± 0.9 nM for EuK-DOTA, 18.0 ± 3.7 nM for EuK-107 and 42.6 ± 6.6 nM for EuK-106. EuK-107 and EuK-DOTA proved to rapidly and near quantitatively complex ⁶⁸Ga³⁺, ¹¹¹In³⁺, ¹⁷⁷Lu³⁺ and ²²⁵Ac³⁺ at 95 °C, with EuK-107 also rapidly complexing ¹¹¹In³⁺ and ¹⁷⁷Lu³⁺ at 25 °C. The inability of EuK-106 to chelate ¹⁷⁷Lu³⁺ and ²²⁵Ac³⁺ suggests that size of the cavity of the macrocylic ring may be more critical than the number of donor groups for the chelation of larger radiometals. In vivo, ⁶⁸Ga-EuK-107 proved to have similar uptake to ⁶⁸Ga-DKFZ-PSMA-617, a theranostic ligand currently in clinical evaluation, in a PSMA positive xenograft tumor model.

CONCLUSIONS

The broad metal ion selectivity, good in vitro affinity for PSMA and good in vivo tumor targeting suggest that EuK-107, with the 3p-C-DEPA chelator, merits further evaluation as a theranostics construct in prostate cancer.

Proton therapy of iris melanoma with 50 CGE : Influence of target volume on clinical outcome

PURPOSE

The aim of this study was to evaluate local tumour control, incidence of radiation-induced glaucoma and associated interventions of sector-based and whole anterior segment proton beam therapy (PBT) for the treatment of iris melanoma.

PATIENTS AND METHODS

We retrospectively analysed the data of 77 patients with iris melanoma who underwent PBT applied as 50 CGE in four daily fractions. Of the patients, 47 received PBT with a circular-shaped collimator and 30 with a conformal sector-shaped target volume. Local control, eye preservation and secondary glaucoma were evaluated.

RESULTS

Median follow-up time was 54.9 months. Local tumour control was 100% in patients receiving whole anterior segment irradiation. Two patients developed pigment dispersion in the non-irradiated area after sector-based PBT and received whole anterior segment salvage PBT. The mean volume of ciliary body irradiated was 89.0% and 34.9% for whole anterior segment and lesion-based irradiation, respectively. At the end of follow-up, secondary glaucoma was found in 74.3% of the patients with whole anterior segment irradiation and in 19.2% with sector-based irradiation. Patients with sector-based PBT had a stable visual acuity of logMAR 0.1, while it declined from logMAR 0.1 to 0.4 after whole anterior segment irradiation.

CONCLUSION

We found a significant reduction in radiation-induced secondary glaucoma and glaucoma-associated surgical interventions and stable visual acuity after sector-based irradiation compared with whole anterior segment irradiation. Sector-based irradiation revealed a higher risk for local recurrence, but selected patients with well-circumscribed iris melanoma benefit from applying a lesion-based target volume when treated with sector-based PBT.

Radiopharmaceutical chemistry of targeted radiotherapeutics, part 4: Strategies for 211At labeling at high activities and radiation doses of 211At α-particles

INTRODUCTION

Alpha particles are radiation of high energy and short range, properties that can lead to radiolysis-mediated complications in labeling chemistry at the high radioactivity levels required for clinical application. In previous papers in this series, we have shown that radiation dose has a profound effect on the astatine species that are present in the labeling reaction and their suitability for the synthesis of N-succinimidyl 3-[²¹¹At]astatobenzoate. The purpose of this study was to evaluate the effects of adding N-chlorosuccinimide (NCS) to the methanol solution used for initial isolation of ²¹¹At after distillation, a process referred to as ²¹¹At stabilization, on ²¹¹At chemistry after exposure to high radiation doses.

METHODS

High performance liquid chromatography was used to evaluate the distribution of ²¹¹At species present in methanol in the 500-65,000Gy radiation dose range and the synthesis of SAB from N-succinimidyl 3-(tri-n-butylstannyl)benzoate in the 500-120,000Gy radiation dose range using different ²¹¹At timeactivity combinations under conditions with/without ²¹¹At stabilization.

RESULTS

In the absence of NCS stabilization, a reduced form of astatine, At(2), increased with increasing radiation dose, accounting for about half the total activity by about 15,000Gy, while with stabilization, At(2) accounted for <10% of ²¹¹At activity even at doses >60,000Gy. SAB yields without stabilization rapidly declined with increasing dose, falling to ~20% at about 5000Gy while with stabilization, yields >80% were obtained with ²¹¹At solutions stored for more than 23h and receiving radiation doses >100,000Gy.

CONCLUSIONS

Adding NCS to the methanol solution used for initial isolation of ²¹¹At is a promising strategy for countering the deleterious effects of radiolysis on ²¹¹At chemistry.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

This strategy could facilitate the ability to perform ²¹¹At labeling at sites remote from its production and at the high activity levels required for clinical applications.

An evaluation in vitro of PARP-1 inhibitors, rucaparib and olaparib, as radiosensitisers for the treatment of neuroblastoma

BACKGROUND

The radiopharmaceutical (131)I-meta-iodobenzylguanidine ((131)I-MIBG) is an effective treatment for neuroblastoma. However, maximal therapeutic benefit from (131)I-MIBG is likely to be obtained by its combination with chemotherapy. We previously reported enhanced antitumour efficacy of (131)I-MIBG by inhibition of the poly(ADP-ribose) polymerase-1 (PARP-1) DNA repair pathway using the phenanthridinone derivative PJ34. Recently developed alternative PARP-1 inhibitors have greater target specificity and are expected to be associated with reduced toxicity to normal tissue. Therefore, our purpose was to determine whether the more specific PARP-1 inhibitors rucaparib and olaparib enhanced the efficacy of X-radiation or (131)I-MIBG.

METHODS

Radiosensitisation of SK-N-BE(2c) neuroblastoma cells or noradrenaline transporter gene-transfected glioma cells (UVW/NAT) was investigated using clonogenic assay. Propidium iodide staining and flow cytometry was used to analyse cell cycle progression. DNA damage was quantified by the phosphorylation of H2AX (γH2AX).

RESULTS

By combining PARP-1 inhibition with radiation treatment, it was possible to reduce the X-radiation dose or (131)I-MIBG activity concentration required to achieve 50 % cell kill by approximately 50 %. Rucaparib and olaparib were equally effective inhibitors of PARP-1 activity. X-radiation-induced DNA damage was significantly increased 2 h after irradiation by combination with PARP-1 inhibitors (10-fold greater DNA damage compared to untreated controls; p < 0.01). Moreover, combination treatment (i) prevented the restitution of DNA, exemplified by the persistence of 3-fold greater DNA damage after 24 h, compared to untreated controls (p < 0.01) and (ii) induced greater G2/M arrest (p < 0.05) than either single agent alone.

CONCLUSION

Rucaparib and olaparib sensitise cancer cells to X-radiation or (131)I-MIBG treatment. It is likely that the mechanism of radiosensitisation entails the accumulation of unrepaired radiation-induced DNA damage. Our findings suggest that the administration of PARP-1 inhibitors and (131)I-MIBG to high risk neuroblastoma patients may be beneficial.

Synthesis and evaluation of a new bifunctional NETA chelate for molecular targeted radiotherapy using(90)Y or(177)Lu

INTRODUCTION

Therapeutic potential of β-emitting cytotoxic radionuclides (90)Y and (177)Lu has been demonstrated in numerous preclinical and clinical trials. A bifunctional chelate that can effectively complex with the radioisotopes is a critical component for molecular targeted radiotherapy (90)Y and (177)Lu. A new bifunctional chelate 5p-C-NETA with a relatively long alkyl spacer between the chelating backbone and the functional unit for conjugation to a tumor targeting moiety was synthesized. 5p-C-NETA was conjugated to a model targeting moiety, a cyclic Arg-Gly-Asp-D-Tyr-Lys (RGDyK) peptide binding integrin αvβ3 protein overexpressed on various cancers. 5p-C-NETA was conjugated to c(RGDyK) peptide and evaluated for potential use in molecular targeted radiotherapy of (90)Y and (177)Lu.

METHODS

5p-C-NETA conjugated with c(RGDyK) was evaluated in vitro for radiolabeling, serum stability, binding affinity, and the result of the in vitro studies of 5p-C-NETA-c(RGDyK) was compared to that of 3p-C-NETA-c(RGDyK). (177)Lu-5p-C-NETA-c(RGDyK) was further evaluated for in vivo biodistribution using gliobastoma bearing mice.

RESULT

The new chelate rapidly and tightly bound to a cytotoxic radioisotope for cancer therapy, (90)Y or (177)Lu with excellent radiolabeling efficiency and maximum specific activity under mild condition (>99%, RT, <1 min). (90)Y- and (177)Lu-radiolabeled complexes of the new chelator remained stable in human serum without any loss of the radiolanthanide for 14 days. Introduction of the tumor targeting RGD moiety to the new chelator made little impact on complexation kinetics and stability with (90)Y or (177)Lu. (177)Lu-radiolabeled 5p-C-NETA-c(RGDyK) conjugate was shown to target tumors in mice and produced a favorable in vivo stability profile.

CONCLUSION

The results of in vitro and in vivo evaluation suggest that 5p-C-NETA is an effective bifunctional chelate of (90)Y and (177)Lu that can be applied for generation of versatile molecular targeted radiopharmaceuticals.