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ISSN (Print): 1874-4710
ISSN (Online): 1874-4729
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Sean L. Kitson Department of Biocatalysis and Isotope Chemistry Almac Sciences Almac House Craigavon Northern Ireland
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The present thematic “hot topic” issue in Current Radiopharmaceuticals is dedicated to the applications of molecular imaging
as major player in the diagnostic scenario of pancreatic neuroendocrine tumours (PanNETs). The therapeutic aspects using
radiopharmaceuticals have also been considered.
In the last years, we assisted to a progressive evolution of the available radionuclide imaging techniques dedicated to Pan-
NETs investigation, starting from the conventional Octreoscan© and moving forward to more sophisticated techniques such as
The possibility to provide an accurate characterization both of primary and metastatic disease is of utmost importance in the
clinical work-up of PanNETs and molecular imaging has revealed to be a cornerstone for Physicians to support the therapeutic
The present issue starts with an accurate description of physiopathological premises underlying the mechanisms of uptake
of several radiotracers that can be used, with main reference to those characterizing the presence of Somatostatin Receptors
(SSTRs), expressed by different subtypes of PanNETs. In this paper, Cuccurullo et al. begins with the explanation of the ancillary
role of 18F- Fluorodeoxyglucose (18F-FDG) . In NETs, 18F-FDG may act mainly as a negative prognostic indicator,
having capability to reveal dedifferentiated lesions in the restaging of individual patients with a critical clinical evolution. The
reason for its infrequent indication is consequently connected with the favorable biological behavior of the majority of Pan-
NETs, which are typically slow growing and express SSTRs according to their degree of differentiation. Therefore, using somatostatin
analogs (SSA) labeled with gamma or positron emitters, SSTRs can be used not only as a target for both diagnosis and
therapy, but also for a prognostic evaluation. Currently, the somatostatin theranostic model, based on diagnosis with radiotracers
and a Peptide Receptor Radionuclide Therapy (PRRT) performed with a similar molecule radiolabeled with a β- (or α,
hopefully in the future) emitter, represents one of the most successful options for the targeted therapy. Although radio-labeled
agonists usually provide efficient results, somatostatin antagonists (SS-ANTs) have been recently also proposed, being usable
for imaging and therapy. The Cuccurullo’s paper also highlights the reasons why the theranostic model based on SSTRs doesn’t
work in insulinoma, which scarcely express SSTR2 and SSTR5, wich are. the most important targets for the routinely used radiolabeled
somatostatin analogues. Therefore, in patients with a suspicious of insulinoma, different radiotracers, such as 18F
FluoroDOPA (18F-DOPA) or tracers targeting glucagon-like peptide-1 receptor, have to be preferred. However, it has to be
pointed out that, although F-DOPA showed interesting results in patients with PanNETs, its role is currently considered secondary
compared to somatostatin analogues, especially because of the lack of a theranostic model.
Starting from the classification and physiopathology of PanNETs, the paper by Briganti et al. deeply analyse the currently
available gamma-emitters for SSTRs evaluation . A detailed comparison of the diagnostic performance of the available tracers
and of hybrid machines compared to the traditional stand-alone tools is also included. In this paper, it is reported that 111Inpentetreotide
(Octreoscan©), successfully applied in well-differentiated (G1-G2) tumors, has lost its primary position because of
the better diagnostic accuracy of 68Ga DOTA-Peptides PET/CT, maintaining an ancillary role in PanNETs. Briganti et al. also
discuss preliminary data obtained with 99mTc-EDDA/HYNIC-TOC (Tektrotyd©), labeled with 99mTc, having more favorable
energy characteristics compared to 111Indium, radiolabeling Octreoscan©. To better identify a possible clinical role for Tektrotyd
©, in terms of cost/effectiveness, its pharmacokinetics and pharmacodynamics have been compared with respect to those of
Octreoscan© and PET DOTA-peptides.
The paper by Carollo et al. is focused on radiochemical issues, providing deep insights of the molecular imaging techniques
that can be applied in this setting, including a discussion on the comparison between the different radiotracers; the therapeutic
implications that might rise from a precise definition of disease extension and metabolic characterization are also evaluated .
This paper shows an overview of the radiopharmaceuticals that have been used so far in the imaging of PanNETs with insights
on the potential of new radiopharmaceuticals currently under clinical evaluation.
The implications for therapy of SSTRs imaging are fundamental for PanNETs patients work-up . The expression of
SSTRs is a pre-requisite for the success of PRRT, which has been tackled by Alsadik et al. in the current issue . The authors used a comprehensive literature search strategy of all studies published in English that can be found on SCOPUS and PubMed.
The results of PRRT, using 177Lutetium or 90Yttrium- DOTA- conjugated peptides in p-NETs, individuated either as a standalone
entity or as subgroup within the wider category of Gastro-entero-pancreatic neuroendocrine tumours (GEP NETs), have
been evaluated. This meta-analysis confirms that PRRT is a well-tolerated and effective treatment option for non-operable
and/or metastatic PanNETs. A strong support on a wider diffusion could be achieved from larger randomized controlled trials,
comparing PRRT with other treatment modalities.
The radiological aspects regarding the theranostic of PanNETs should not be underestimated neither be allocated on a second
level in comparison to molecular imaging. For this reason, this issue also includes an outline on the available conventional
radiological techniques that are currently used in PanNETs work-up . In the presence of a primary role of nuclear medicine,
either for a whole body analysis or for a molecular characterization, an extensive conventional radiological imaging is however
requested, remaining the foundation for the initial diagnosis and staging of these tumors.
In terms of technological advances, a significant improvement has been obtained with the advent of new hybrid machines
that could potentially improve the management of these patients . Hybrid PET/MRI systems are currently available in few
Centres world-wide, providing excellent results derived from the combination of PET with MRI, which is better for soft tissue
characterization and for reducing radiation exposure compared to CT . Furthermore, Diffusion-Weighted Imaging (DWI) is
an invaluable tool to depict small liver lesions undetectable by PET or CT; a further improvement allowed by MR in detecting
small hepatic metastases may be achieved combining 3-Tesla and DWI together with the administration of liver specific contrast
media. Hence, the best candidates to be imaged with SSTR PET/MRI are patients selected for hepatic de-bulking or having
liver predominant disease. in the perspective by Mapelli and Picchio, the capability of PET/MRI to better evaluate tumor
response to treatment difficult to be analyzed using traditional RECIST criteria is also discussed. The better evaluation of anatomic
changes and enhancement characteristics allowed by MRI may significantly implement SSTR PET, defining a more reliable
procedure to be used in place of CT and MRI alone or of PET/CT. In this context, SSTR-RADS Version 1.0 response criteria
have been recently introduced as a promising alternative and valuable tool for response assessment in NETs . To better
define the clinical role of PET/MRI in patients with p-NETs, it has to be pointed out that at present, this tool doesn’t show,
using a standard approach, the same diagnostic accuracy in detecting small lung lesions with respect to PET/CT. Therefore, in
presence of a negative or dubious pattern at pulmonary level, a chest CT scan would be recommended additionally to PET/MRI
[9, 10]. Moreover, MRI has a lower sensitivity in identifying hypersclerotic bone lesions compared to other imaging modalities
and SUV quantification with attenuation correction is still suboptimal . At present, these limitations, although scarcely significant
for a clinical utilization in PanNETs, have to be taken in account, looking forward to technological improvements, allowing
to obtain similar accuracies as for PET/CT
Based on the contents reported in the present issue, it can be concluded that in patients with PanNETs, a protean disease often
presenting with liver metastases, many topics have to be considered to understand the role of molecular imaging in the different
At first diagnosis, it is mandatory to detect primary tumour, more frequently identified using radiological techniques, and to
define whole body diffusion and molecular characterization. In the latter field, nuclear medicine techniques are determinant.
Being the major role actually played by PET/CT with 68Ga-DOTA peptides, the use of Octreoscan© is considered to be a second
choice, while the clinical position of Tektrotyd© is still to be fully defined. In patients with insulinoma, waiting for radiopharmaceuticals
having a better diagnostic accuracy, 18F-DOPA has to be preferred compared to radiolabeled somatostatin analogues.
Interesting perspectives seem to be related to a wider evaluation of innovative radiotracers, including GLP-1 Receptor
Peptides, as exendin, GRP receptor ligands, as bombesin agonists and antagonists, and gastrin/cholecystokinin analogs, as
minigastrin. Nevertheless, further studies, also better evaluating radiochemistry and pharmacokinetics, are needed before their
possible clinical application. Similarly, new approaches capable to early predict the efficacy of anti-angiogenesis treatments
could be defined through a research based on radiolabeled new drugs, such as bevacizumab. Conversely, it is currently too early
to predict the clinical impact of a technological revolution associated with fully digital PET scanners or with the use of alternative
radionuclides, with main reference to Cu-64.
This means that in the near future, the primacy of somatotastatin analogues labelled with 68Ga will remain unquestionable
in the general evaluation of PanNETs, also because of the theranostic model associated with PRRT. Regarding innovative radiopharmaceuticals,
neither radiolabeled somatostatin antagonists nor 11C-5-hydroxy-tryptophan (HTP), negatively affected by
the labelling with the short life radionuclide C-11, may actually cover a clinical role. Conversely, 18F-FDG may find a clinical
utility in restaging patients in whom a dedifferentiation is suspected, in order to better define therapeutic strategies.
Future developments will be based on a wider diffusion of more performing technologies, such as PET/MRI or digital PET,
in the clinical application of new radiopharmaceuticals and a more comprehensive multilevel integration of biologic information
pertaining to a specific tumor and single patient. This approach, possibly encompassing genomic considerations, is currently
evolving as a new entity denoted ‘precision medicine’ . In this context, together with nonspecific biomarkers, such as
chromogranin-A (CgA) and 5-hydroxyindoleacetic acid (5-HIAA), the evaluation of proliferative activity (Ki67) should be
considered. In the future, the clinical application of specific NET transcripts in whole blood could potentially conduct to an
earlier diagnosis, also providing information useful for prognostic stratification and a better definition of therapeutic strategies
. A further improvement is also required for an improved monitoring of tumor response. In PanNETs, radiological procedures
based on RECIST are unsatisfactory while newer radiological criteria are not yet consolidated; therefore, validated techniques utilizing molecular imaging, supported by the morpho-structural information allowed by CT or MRI should be implemented.
In this direction, more reliable results could be obtained through the development of new criteria, eventually including
the adoption of functional MRI, of more precise hematic biomarkers, of quantitative methods not only based on SUV.
In conclusion, a multidisciplinary approach is mandatory in clinical practice in roder to reach the best consensus on treatment
approach for each patient. In this context, together with the major role of nuclear medicine physicians and radiologists, of
oncologists, endocrinologists and surgeons, the contribution of experts in epidemiology, laboratory medicine and genomics is
advocated. Therefore, in agreement with L. Bodei, we can conclude that “a fusion product of molecular and genomic information
with tumor imaging is likely to be the quintessence of future NET diagnosis and define the progress from darkness to light”
In this special CRP issue, the production, radiochemical aspects, chelation and latest pre-clinical and clinical results are presented
for the application of 225Ac and 213Bi-based radiopharmaceuticals for Targeted Alpha Therapy (TAT).
The first block of papers discusses the production, radiochemical and chelation aspects for the production of radiopharmaceuticals
based on 225Ac and 213Bi.
In the review by A. Robertson et al. , an overview of the current supply of 225Ac/213Bi is discussed as well as alternative
production strategies to meet the current demand. Radiochemical aspects for isolating 225Ac from 229Th or from irradiated target
materials are discussed in the second part of this review. Finally, chelation aspects and challenges associated with the delivery
of alpha-emitting radionuclides to the targeting sites are described. All of these aspects are also discussed from the experiences,
capabilities and perspectives of TRIUMF.
The review by J. Engle  specifically aims to provide a broad overview of the current supply and possible additional
strategies for the production of 225Ac and 213Bi. These include discussion of the current status of 229Th, as well as alternative
production by irradiation of 226Ra with neutrons, protons and electrons and thorium with high energy protons.
In the review by J. Harvey , specific efforts pursued by the NorthStar company are reported and include the recovery of
229Th from fuel pellets, irradiation of 226-Radium with electrons and 232-Throium with high energy protons and testing of radiochemical
separation based on solid phase chromatography resins.
The next block of reviews and research articles report the results of pre-clinical and clinical studies using 225Ac and 213Bi.
In the research article by Z. Jiang et al. , in vivo animal studies using two different sources of 225Ac were evaluated. One
source was derived from the standard decay of 229Th and the second source was 225Ac extracted from proton irradiated thorium
target (containing 227Ac). Biodistribution studies of 225Ac in free form and chelated with DOTA were examined.
A research article by N. Pfankuchen et al.  provides results from the work on radiolabeling and pre-clinical evaluation of
225Ac attached to the zoledronic acid bone-seeking agent.
In the review by J. Jurcic , clinical results of using the 213Bi- and 225Ac-radiolabelled monoclonal antibody (Lintuzumab)
for therapy of acute myeloid leukemia (AML) are discussed.
In the review article by A. Morgenstern et al.  the current status of the field of TAT is given, including an overview of
the availability of the 225Ac and 225Ac/213Bi generator, as well as a broad overview of clinical trials using 225Ac and 213Bi.
The last review article discuss the importance of dosimetric and radiobiological considerations for the application of alpha
emitters by G. Sgouros et al. .
This special issue of Current Radiopharmaceuticals provides an up-to-date and useful comprehensive overview describing
the production, radiochemistry and use of both 213Bi and 225Ac and their important emerging clinical roles for therapeutic treatment
of a variety of specific cancers and chronic diseases.