Correlation of dose with toxicity and tumour response to 90Y- and 177Lu-PRRT provides the basis for optimization through individualized treatment planning

177 Lu-DOTATATE PRRT Safety and Organ-at-Risk Dosimetry in Patients With Gastroenteropancreatic Neuroendocrine Tumors : Data From the Prospective Phase 2 LUMEN Study

PURPOSE

The aim of this study was to assess the association among toxicity, dosimetry of organs-at-risk, and disease progression in patients with gastroenteropancreatic neuroendocrine tumors (GEP-NETs) treated with 177 Lu-DOTATATE.

PATIENTS AND METHODS

Thirty-seven patients with GEP-NETs underwent 177 Lu-DOTATATE peptide receptor radionuclide therapy (PRRT) in a single-arm, prospective, phase 2 study, where patients were followed up with blood tests, isotopic glomerular filtration rate (iGFR), and imaging examinations (CT/MRI and PET) every 6 months until disease progression. Adverse events (AEs) graded per CTCAEv4.03 and occurring during treatment were collected and followed up until resolution. Dosimetry, including biologically effective doses (BEDs) to kidneys, BED to bone marrow, and absorbed dose (AD) to spleen, was performed after each PRRT cycle. Statistical analyses explored associations among dosimetry, toxicity, and patient progression free-survival.

RESULTS

The most common AEs were anemia and lymphopenia (65%), followed by thrombocytopenia and fatigue (each 51%), alopecia (46%), and nausea (41%). The most common grade ≥3 AE was lymphopenia (43%). There was no grade ≥3 nephrotoxicity. The median iGFR % decrease was 11% ( P < 0.001), at a median follow-up of 23 months. iGFR %decrease and renal BED did not correlate (Spearman ρ = -0.09). Similarly, no significant association was found between bone marrow BED or spleen AD and the grades of hematological toxicities. We observed no association between progression free-survival and either the decline of renal function or the occurrence of hematological toxicities during PRRT.

CONCLUSIONS

This study confirms the safety profile of 177 Lu-DOTATATE PRRT in patients with GEP-NETs irrespective of the dosimetry of organs at risk. Kidney, bone marrow, and spleen dosimetry measures were not associated with renal or hematological toxicity.

Impact of the Reference Multiple-Time-Point Dosimetry Protocol on the Validity of Single-Time-Point Dosimetry for [177Lu]Lu-PSMA-I&T Therapy

Internal dosimetry supports safe and effective patient management during radionuclide therapy. Yet, it is associated with high clinical workload, costs, and patient burden, as patient scans at multiple time points (MTPs) must be acquired. Dosimetry based on imaging at a single time point (STP) has continuously gained popularity. However, MTP protocols, used as a reference to judge the validity of STP dosimetry, differ depending on local requirements and deviate from the unknown patient-specific ground truth pharmacokinetics. The aim of this study was to compare the error and optimum time point for different STP approaches using different reference MTP protocols. Methods: Whole-body SPECT/CT scans of 7 patients (7.4-8.9 GBq of [177Lu]Lu-PSMA-I&T) were scheduled at 24, 48, 72, and 168 h after injection. Sixty lesions, 14 kidneys, and 10 submandibular glands were delineated in the SPECT/CT data. Two curve models, that is, a mono- and a biexponential model, were fitted to the MTP data, in accordance with goodness-of-fit analysis (coefficients of variation, sum of squared errors). Three population-based STP approaches were compared: one method published by Hänscheid et al., one by Jackson et al., and one using population-based effective half-lives in the mono- or biexponential curve models. Percentage differences between STP and MTP dosimetry were evaluated. Results: Goodness-of-fit parameters show that a monoexponential function and a biexponential function with shared population-based parameters and physical tail are reasonable reference models. When comparing both reference models, we observed maximum differences of -44%, -19%, and -28% in the estimated absorbed doses for lesions, kidneys, and salivary glands, respectively. STP dosimetry with an average deviation of less than 10% from MTP dosimetry may be feasible; however, this deviation and the optimum imaging time point showed a dependence on the chosen reference protocol. Conclusion: STP dosimetry for [177Lu]Lu-PSMA therapy is promising to boost the integration of dosimetry into clinical routine. According to our patient cohort, 48 h after injection may be regarded as a compromise for STP dosimetry for lesions and at-risk organs. The results from this analysis show that a common gold standard for dosimetry is desirable to allow for reliable and comparable STP dosimetry.

A review of 177Lu dosimetry workflows: how to reduce the imaging workloads?

177 Lu radiopharmaceutical therapy is a standardized systemic treatment, with a typical dose of 7.4 GBq per injection, but its response varies from patient to patient. Dosimetry provides the opportunity to personalize treatment, but it requires multiple post-injection images to monitor the radiopharmaceutical's biodistribution over time. This imposes an additional imaging burden on centers with limited resources. This review explores methods to lessen this burden by optimizing acquisition types and minimizing the number and duration of imaging sessions. After summarizing the different steps of dosimetry and providing examples of dosimetric workflows for177 Lu -DOTATATE and177 Lu -PSMA, we examine dosimetric workflows based on a reduced number of acquisitions, or even just one. We provide a non-exhaustive description of simplified methods and their assumptions, as well as their limitations. Next, we detail the specificities of each normal tissue and tumors, before reviewing dose-response relationships in the literature. In conclusion, we will discuss the current limitations of dosimetric workflows and propose avenues for improvement.

Future Perspectives of Artificial Intelligence in Bone Marrow Dosimetry and Individualized Radioligand Therapy

Radioligand therapy is an emerging and effective treatment option for various types of malignancies, but may be intricately linked to hematological side effects such as anemia, lymphopenia or thrombocytopenia. The safety and efficacy of novel theranostic agents, targeting increasingly complex targets, can be well served by comprehensive dosimetry. However, optimization in patient management and patient selection based on risk-factors predicting adverse events and built upon reliable dose-response relations is still an open demand. In this context, artificial intelligence methods, especially machine learning and deep learning algorithms, may play a crucial role. This review provides an overview of upcoming opportunities for integrating artificial intelligence methods into the field of dosimetry in nuclear medicine by improving bone marrow and blood dosimetry accuracy, enabling early identification of potential hematological risk-factors, and allowing for adaptive treatment planning. It will further exemplify inspirational success stories from neighboring disciplines that may be translated to nuclear medicine practices, and will provide conceptual suggestions for future directions. In the future, we expect artificial intelligence-assisted (predictive) dosimetry combined with clinical parameters to pave the way towards truly personalized theranostics in radioligand therapy.

Absorbed Dose-Response Relationship in Patients with Gastroenteropancreatic Neuroendocrine Tumors Treated with [177Lu]Lu-DOTATATE: One Step Closer to Personalized Medicine

[177Lu]Lu-DOTATATE has been approved for progressive and inoperable gastroenteropancreatic neuroendocrine tumors (GEP-NETs) that overexpress somatostatin receptors. The absorbed doses by limiting organs and tumors can be quantified by serial postinfusion scintigraphy measurements of the γ-emissions from 177Lu. The objective of this work was to explore how postinfusion [177Lu]Lu-DOTATATE dosimetry could influence clinical management by predicting treatment efficacy (tumor shrinkage and survival) and toxicity. Methods: Patients with GEP-NETs treated with [177Lu]Lu-DOTATATE between 2016 and 2022 and who underwent dosimetry were included. Absorbed doses were calculated for healthy organs (liver, kidneys, bone marrow, and spleen) and tumors using PLANET Dose and the local energy deposition method based on serial posttreatment SPECT/CT. Up to 5 lesions per site were selected and measured on images collected at baseline and 3 mo after treatment end (measurement masked to the somatostatin receptor imaging uptake). For toxicity assessment, laboratory parameters were regularly monitored. Clinical data, including time to death or progression, were collected from the patients' health records. Correlations between absorbed doses by organs and toxicity and between absorbed doses by lesions and tumor volume variation were studied using regression models. Results: In total, 35 dosimetric studies were performed in patients with mostly grade 2 (77%) tumors and metastases in liver (89%), lymph nodes (77%), and bone (34%), and 146 lesions were analyzed: 1-9 lesions per patient, mostly liver metastases (65%) and lymph nodes (25%). The median total absorbed dose by tumors was 94.4 Gy. The absorbed doses by tumors significantly decreased between cycles. The absorbed dose by tumors was significantly associated with tumor volume variation (P < 0.001) 3 mo after treatment end, and it was a significant prognostic factor for survival. Toxicity analysis showed a correlation between the decrease of hematologic parameters such as lymphocytes or platelet concentrations and the absorbed doses by the spleen or bone marrow. The mean absorbed dose by the kidneys was not correlated with nephrotoxicity during the studied period. Conclusion: In patients treated with [177Lu]Lu-DOTATATE for GEP-NETs, tumor and healthy organ dosimetry can predict survival and toxicities, thus influencing clinical management.

Advances in Radionuclide Therapies for Patients with Neuro-endocrine Tumors

PURPOSE OF REVIEW

To provide insights into the role of peptide receptor radionuclide therapy (PRRT) in patients with advanced neuroendocrine tumors (NET) and an overview of possible strategies to combine PRRT with locoregional and systemic anticancer treatments.

RECENT FINDINGS

Research on combining PRRT with other treatments encompasses a wide variety or treatments, both local (transarterial radioembolization) and systemic therapies, chemotherapy (i.e., capecitabine and temozolomide), targeted therapies (i.e., olaparib, everolimus, and sunitinib), and immunotherapies (e.g., nivolumab and pembrolizumab). Furthermore, PRRT shows promising first results as a treatment prior to surgery. There is great demand to enhance the efficacy of PRRT through combination with other anticancer treatments. While research in this area is currently limited, the field is rapidly evolving with numerous ongoing clinical trials aiming to address this need and explore novel therapeutic combinations.

Towards Effective Targeted Alpha Therapy for Neuroendocrine Tumours: A Review

This review article explores the evolving landscape of Molecular Radiotherapy (MRT), emphasizing Peptide Receptor Radionuclide Therapy (PRRT) for neuroendocrine tumours (NETs). The primary focus is on the transition from β-emitting radiopharmaceuticals to α-emitting agents in PRRT, offering a critical analysis of the radiobiological basis, clinical applications, and ongoing developments in Targeted Alpha Therapy (TAT). Through an extensive literature review, the article delves into the mechanisms and effectiveness of PRRT in targeting somatostatin subtype 2 receptors, highlighting both its successes and limitations. The discussion extends to the emerging paradigm of TAT, underlining its higher potency and specificity with α-particle emissions, which promise enhanced therapeutic efficacy and reduced toxicity. The review critically evaluates preclinical and clinical data, emphasizing the need for standardised dosimetry and a deeper understanding of the dose-response relationship in TAT. The review concludes by underscoring the significant potential of TAT in treating SSTR2-overexpressing cancers, especially in patients refractory to β-PRRT, while also acknowledging the current challenges and the necessity for further research to optimize treatment protocols.

Radiosensitizing Favors Response to Peptide Receptor Radionuclide Therapy in Patients With Highly Proliferative Neuroendocrine Malignancies: Preliminary Evidence From a Clinical Pilot Study

AIM/INTRODUCTION

Peptide receptor radionuclide therapy (PRRT) represents a cornerstone of treatment regimens for patients with low proliferative neuroendocrine tumors (NETs). However, in patients experiencing somatostatin receptor-positive NET with higher proliferation rates, a value and potential therapeutic benefit of PRRT as part of multimodal treatment approaches and potentially with addition of radiosensitizing agents has not yet been established.

PATIENTS AND METHODS

In this study, 20 patients with histologically confirmed gastroenteropancreatic (GEP) NET with proliferation rates (Ki67) between 15% and 55% were treated either with PRRT only (n = 10) or with a combination therapy (n = 10) comprising PRRT and capecitabine/temozolomide (CAP/TEM) for at least 2 consecutive cycles.

RESULTS

Disease control rate in patients treated with PRRT alone was 60% (40% stable disease and 20% partial response). Strikingly, in patients treated with PRRT in combination with radiosensitization (CAP/TEM), the disease control rate was 90% (20% stable disease and 70% partial response). The median progression-free survival in the PRRT only group was 12 months, whereas the median progression-free survival in the PRRT + CAP/TEM group was 26 months and has not been yet reached for all patients in the group during the observation period. The median disease-specific survival for patients with PRRT alone was 51 months, whereas this end point was not yet reached in the PRRT + CAP/TEM group. Moreover, the PRRT + CAP/TEM group showed a significantly higher reduction of SSTR-PET-based metabolic tumor volume and chromogranin A levels compared with the PRRT only group. Importantly, adverse events of all grades did not differ between both groups.

CONCLUSIONS

PRRT + CAP/TEM represents a highly promising and well-tolerated therapeutic regimen for patients experiencing somatostatin receptor-positive NET with higher (Ki67 ≥ 15%) proliferation rate. Prospective randomized clinical trials are warranted.

Risk of treatment-altering haematological toxicity and its dependence on bone marrow doses in peptide receptor radionuclide therapy

BACKGROUND

Peptide receptor radionuclide therapy is effective in treating neuroendocrine tumours, but treatment may be limited by kidney and bone marrow toxicity. In this work, the absorbed dose burden to the bone marrow was estimated using image-based dosimetry and its potential use for predicting treatment-altering toxicity was studied. Peripheral blood samples taken before and after 229 treatments with 177Lu-DOTATATE in 59 patients were studied. In connection to the treatments, a total of 940 blood sample occasions provided data on white blood cell, neutrophil granulocyte, platelet, erythrocyte and haemoglobin concentrations. SPECT/CT image data were collected at two or three time points after each treatment. Absorbed doses to bone marrow were calculated from the activity concentration in a metastasis-free lumbar vertebra. The rate of delayed and aborted treatments was analysed based on medical records.

RESULTS

The average absorbed dose to the bone marrow was 0.42 Gy (median 0.33 Gy, SD 0.27 Gy) per treatment. Dose-response relationships between white blood cells, neutrophil granulocytes and haemoglobin concentrations were observed, most prominently at 31-45 days after each treatment. The correlations were stronger in patients with skeletal metastases. The rates of haematological toxicity-related delays and aborted treatments were 6% and 12%, respectively. None of the studied bone marrow dosimetric parameters could clearly predict treatment-related toxicity. However, patients with skeletal metastases had higher risk of treatment-altering toxicity (odds ratio = 6.0).

CONCLUSIONS

Treatment-altering haematological toxicity in peptide receptor radionuclide therapy is relatively rare and appears difficult to fully predict from post-therapeutic image-based dosimetry. However, for patients with skeletal metastases, the haematological dose-response relationships are stronger. Future studies may focus on this patient group, to further investigate the usefulness of dosimetry in predicting decreases in blood values.

Intra-arterial Peptide Receptor Radionuclide Therapy for the Treatment of Hepatic Neuroendocrine Tumor Metastases: Hope or Hype?

Peptide receptor radionuclide therapy (PRRT) confers significant progression-free survival advantage for patients with small bowel grade 1 and 2 well-differentiated neuroendocrine tumors (WD NET). PRRT may also be clinically beneficial for patients with NET of pancreatic, bronchial, and other sites of origin; patients with paragangliomas; as well as for patients with well-differentiated grade 3 NET. Direct intra-arterial (IA) administration of PRRT into the hepatic artery for patients with NET liver metastases may result in higher radiopharmaceutical dose and longer dwell time in the liver tumors while relatively sparing non-tumor liver tissue and other organs such as the kidneys and bone marrow when compared with intravenous (IV) administration. This review summarizes currently available data on IA and IV PRRT dose distribution, reports safety and efficacy of IA PRRT, and proposes future research questions.

The cycle effect quantified: reduced tumour uptake in subsequent cycles of [177Lu]Lu-HA-DOTATATE during peptide receptor radionuclide therapy

BACKGROUND

Clear evidence regarding the effect of reduced tumour accumulation in later peptide receptor radionuclide therapy (PRRT) cycles is lacking. Therefore, we aimed to quantify potential cycle effects for patients treated with [177Lu]Lu-HA-DOTATATE using a population pharmacokinetic (PK) modelling approach.

METHODS

A population PK model was developed using imaging data from 48 patients who received multiple cycles of [177Lu]Lu-HA-DOTATATE. The five-compartment model included a central, kidney, spleen, tumour and lumped rest compartment. Tumour volume and continued use of long-acting somatostatin analogues (SSAs) were tested as covariates in the model. In addition, the presence of a cycle effect was evaluated by relating the uptake rate in a specific cycle as a fraction of the (tumour or organ) uptake rate in the first cycle.

RESULTS

The final PK model adequately captured observed radioactivity accumulation in kidney, spleen and tumour. A higher tumour volume was identified to increase the tumour uptake rate, where a twofold increase in tumour volume resulted in a 2.3-fold higher uptake rate. Also, continued use of long-acting SSAs significantly reduced the spleen uptake rate (68.4% uptake compared to SSA withdrawal (10.5% RSE)). Lastly, a cycle effect was significantly identified, where tumour uptake rate decreased to 86.9% (5.3% RSE) in the second cycle and even further to 79.7% (5.6% RSE) and 77.6% (6.2% RSE) in the third and fourth cycle, respectively, compared to cycle one.

CONCLUSIONS

Using a population PK modelling approach, the cycle effect of reduced tumour uptake in subsequent PRRT cycles was quantified. Our findings implied that downregulation of target receptors is probably not the major cause of the cycle effect, due to a plateau in the decrease of tumour uptake in the fourth cycle.

The contest between internal and external-beam dosimetry: The Zeno's paradox of Achilles and the tortoise

Radionuclide therapy, also called molecular radiotherapy (MRT), has come of age, with several novel radiopharmaceuticals being approved for clinical use or under development in the last decade. External beam radiotherapy (EBRT) is a well-established treatment modality, with about half of all oncologic patients expected to receive at least one external radiation treatment over their disease course. The efficacy and the toxicity of both types of treatment rely on the interaction of radiation with biological tissues. Dosimetry played a fundamental role in the scientific and technological evolution of EBRT, and absorbed doses to the target and to the organs at risk are calculated on a routine basis. In contrast, in MRT the usefulness of internal dosimetry has long been questioned, and a structured path to include absorbed dose calculation is missing. However, following a similar route of development as EBRT, MRT treatments could probably be optimized in a significant proportion of patients, likely based on dosimetry and radiobiology. In the present paper we describe the differences and the similarities between internal and external-beam dosimetry in the context of radiation treatments, and we retrace the main stages of their development over the last decades.

Development and Utility of an Imaging System for Internal Dosimetry of Astatine-211 in Mice

In targeted radionuclide therapy, determining the absorbed dose of the ligand distributed to the whole body is vital due to its direct influence on therapeutic and adverse effects. However, many targeted alpha therapy drugs present challenges for in vivo quantitative imaging. To address this issue, we developed a planar imaging system equipped with a cadmium telluride semiconductor detector that offers high energy resolution. This system also comprised a 3D-printed tungsten collimator optimized for high sensitivity to astatine-211, an alpha-emitting radionuclide, and adequate spatial resolution for mouse imaging. The imager revealed a spectrum with a distinct peak for X-rays from astatine-211 owing to the high energy resolution, clearly distinguishing these X-rays from the fluorescent X-rays of tungsten. High collimator efficiency (4.5 × 10-4) was achieved, with the maintenance of the spatial resolution required for discerning mouse tissues. Using this system, the activity of astatine-211 in thyroid cancer tumors with and without the expression of the sodium iodide symporter (K1-NIS/K1, respectively) was evaluated through in vivo imaging. The K1-NIS tumors had significantly higher astatine-211 activity (sign test, p = 0.031, n = 6) and significantly decreased post-treatment tumor volume (Student's t-test, p = 0.005, n = 6). The concurrent examination of intratumor drug distribution and treatment outcome could be performed with the same mice.

EFOMP policy statement NO. 19: Dosimetry in nuclear medicine therapy - Molecular radiotherapy

The European Council Directive 2013/59/Euratom (BSS Directive) includes optimisation of treatment with radiotherapeutic procedures based on patient dosimetry and verification of the absorbed doses delivered. The present policy statement summarises aspects of three directives relating to the therapeutic use of radiopharmaceuticals and medical devices, and outlines the steps needed for implementation of patient dosimetry for radioactive drugs. To support the transition from administrations of fixed activities to personalised treatments based on patient-specific dosimetry, EFOMP presents a number of recommendations including: increased networking between centres and disciplines to support data collection and development of codes-of-practice; resourcing to support an infrastructure that permits routine patient dosimetry; research funding to support investigation into individualised treatments; inter-disciplinary training and education programmes; and support for investigator led clinical trials. Close collaborations between the medical physicist and responsible practitioner are encouraged to develop a similar pathway as is routine for external beam radiotherapy and brachytherapy. EFOMP's policy is to promote the roles and responsibilities of medical physics throughout Europe in the development of molecular radiotherapy to ensure patient benefit. As the BSS directive is adopted throughout Europe, unprecedented opportunities arise to develop informed treatments that will mitigate the risks of under- or over-treatments.

On the Use of Artificial Intelligence for Dosimetry of Radiopharmaceutical Therapies

Routine clinical dosimetry along with radiopharmaceutical therapies is key for future treatment personalization. However, dosimetry is considered complex and time-consuming with various challenges amongst the required steps within the dosimetry workflow. The general workflow for image-based dosimetry consists of quantitative imaging, the segmentation of organs and tumors, fitting of the time-activity-curves, and the conversion to absorbed dose. This work reviews the potential and advantages of the use of artificial intelligence to improve speed and accuracy of every single step of the dosimetry workflow.

Data reconciliation connected to guard bands to set specification limits related to risk assessment for radiopharmaceutical activity

Radiopharmaceuticals have been used to diagnose several diseases, particularly because the procedure is non-invasive. However, it is important that the correct amount of radiopharmaceutical is used to avoid inaccurate diagnostic results and suboptimal therapeutic outcomes. The amount of the radiopharmaceutical is measured when produced (by the supplier) and a second time (by the receiver), before it's use. When measured at the receiver, the result is corrected for its normal radioactivity decay. Even then, it is possible that both measurements should be considered nominal different or even statistically different when compared through various statistical tools. This research combines two innovative techniques in the field of clinical metrology. The first technique is data reconciliation, which not only enhances measurement accuracy but also reduces measurement uncertainty. The second technique involves using uncertainty information to establish specification limits for compliance assessments. In this way, our proposal aimed to minimize the risk of making incorrect decisions regarding the conformity of the concentration of radiopharmaceutical activity, that is, rejecting an item or batch that is within specification or accepting an item or batch that is outside of specification. A spreadsheet, based on these metrology fundamentals, is available to help the user with the calculations, presenting numerical and graphical results for some common radioisotopes. Reliable specification limits can be calculated and used to determine if the radiopharmaceutical is in accordance with its proposed application.

Radiopharmaceuticals as combinatorial partners for immune checkpoint inhibitors

Immune checkpoint inhibitors (ICIs) have revolutionized the treatment of multiple cancer types. However, only a fraction of patients with cancer responds to ICIs employed as stand-alone therapeutics, calling for the development of safe and effective combinatorial regimens to extend the benefits of ICIs to a larger patient population. In addition to exhibiting a good safety and efficacy profile, targeted radionuclide therapy (TRT) with radiopharmaceuticals that specifically accumulate in the tumor microenvironment has been associated with promising immunostimulatory effects that (at least in preclinical cancer models) provide a robust platform for the development of TRT/ICI combinations. We discuss preclinical and clinical findings suggesting that TRT stands out as a promising partner for the development of safe and efficient combinatorial regimens involving ICIs.

Patient-specific biokinetics and hybrid 2D/3D approach integration in OEDIPE software: Application to radioiodine therapy

BACKGROUND

The progression of targeted radionuclide therapy requires the development of dosimetry software accounting for patient-specific biokinetics. New functionalities were thus developed in the OEDIPE software, to deal with multiple 3D images or multiple planar images and a SPECT image.

MATERIEL & METHOD

Methods were implemented to recover patient biokinetics in volumes of interest. If several 3D SPECT images are available, they are registered to a reference CT scan. When several planar images and a single SPECT are available, the planar images are registered to the SPECT and counts of the planar images converted to activity. To validate these developments, six SPECT/CT and planar images of a Jaszczak phantom containing I-131 were acquired at different dates. Cumulated activity was estimated in each sphere using the SPECT/CT images only or the planar series associated to one SPECT/CT. Biokinetics and doses in lesions and in the lungs of a patient treated with I-131 for differentiated thyroid cancer were then estimated using four planar images and a SPECT/CT scan. Whole-body retention data were used to compare the biokinetics obtained from the planar and SPECT data.

RESULTS

Activities and cumulated activities estimated using OEDIPE in the phantom spheres agreed well with the reference values for both approaches. Results obtained for the patient compared well with those derived from whole-body retention data.

CONCLUSION

The implemented features allow automatic evaluation of patient-specific biokinetics from different series of patient images, enabling patient-specific dosimetry without the need for external software to estimate the cumulated activities in different VOIs.

Predicting [177Lu]Lu-HA-DOTATATE kidney and tumor accumulation based on [68Ga]Ga-HA-DOTATATE diagnostic imaging using semi-physiological population pharmacokinetic modeling

BACKGROUND

Prediction of [177Lu]Lu-HA-DOTATATE kidney and tumor uptake based on diagnostic [68Ga]Ga-HA-DOTATATE imaging would be a crucial step for precision dosing of [177Lu]Lu-HA-DOTATATE. In this study, the population pharmacokinetic (PK) differences between [177Lu]Lu-HA-DOTATATE and [68Ga]Ga-HA-DOTATATE were assessed and subsequently [177Lu]Lu-HA-DOTATATE was predicted based on [68Ga]Ga-HA-DOTATATE imaging.

METHODS

A semi-physiological nonlinear mixed-effects model was developed for [68Ga]Ga-HA-DOTATATE and [177Lu]Lu-HA-DOTATATE, including six compartments (representing blood, spleen, kidney, tumor lesions, other somatostatin receptor expressing organs and a lumped rest compartment). Model parameters were fixed based on a previously developed physiologically based pharmacokinetic model for [68Ga]Ga-HA-DOTATATE. For [177Lu]Lu-HA-DOTATATE, PK parameters were based on literature values or estimated based on scan data (four time points post-injection) from nine patients. Finally, individual [177Lu]Lu-HA-DOTATATE uptake into tumors and kidneys was predicted based on individual [68Ga]Ga-HA-DOTATATE scan data using Bayesian estimates. Predictions were evaluated compared to observed data using a relative prediction error (RPE) for both area under the curve (AUC) and absorbed dose. Lastly, to assess the predictive value of diagnostic imaging to predict therapeutic exposure, individual prediction RPEs (using Bayesian estimation) were compared to those from population predictions (using the population model).

RESULTS

Population uptake rate parameters for spleen, kidney and tumors differed by a 0.29-fold (15% relative standard error (RSE)), 0.49-fold (15% RSE) and 1.43-fold (14% RSE), respectively, for [177Lu]Lu-HA-DOTATATE compared to [68Ga]Ga-HA-DOTATATE. Model predictions adequately described observed data in kidney and tumors for both peptides (based on visual inspection of goodness-of-fit plots). Individual predictions of tumor uptake were better (RPE AUC -40 to 28%) compared to kidney predictions (RPE AUC -53 to 41%). Absorbed dose predictions were less predictive for both tumor and kidneys (RPE tumor and kidney -51 to 44% and -58 to 82%, respectively). For most patients, [177Lu]Lu-HA-DOTATATE tumor accumulation predictions based on individual PK parameters estimated from diagnostic imaging outperformed predictions based on population parameters.

CONCLUSION

Our semi-physiological PK model indicated clear differences in PK parameters for [68Ga]Ga-HA-DOTATATE and [177Lu]Lu-HA-DOTATATE. Diagnostic images provided additional information to individually predict [177Lu]Lu-HA-DOTATATE tumor uptake compared to using a population approach. In addition, individual predictions indicated that many aspects, apart from PK differences, play a part in predicting [177Lu]Lu-HA-DOTATATE distribution.

Tumor dosimetry using 177Lu: influence of background activity, measurement method and reconstruction algorithm

BACKGROUND

Image-based tumor dosimetry after radionuclide therapy, using the isotope ¹⁷⁷Lu, finds application e.g., for tumor-to-organ dose comparison and for dose response evaluation. When the tumor extent is not much larger than the image resolution, and when ¹⁷⁷Lu is found in nearby organs or other tumors, an accurate determination of tumor dose is particularly challenging. Here a quantitative evaluation of three different methods for determining the ¹⁷⁷Lu activity concentration in a phantom is performed, and the dependence on a variety of parameters is described. The phantom (NEMA IEC body phantom) has spheres of different size in a background volume, and sphere-to-background ¹⁷⁷Lu activity concentration ratios of infinity, 9.5, 5.0 and 2.7 are applied. The methods are simple to implement and well-known from the literature. They are based on (1) a large VOI encompassing the whole sphere, without background activity and with volume information from other sources, (2) a small VOI located in the sphere center, and (3) a VOI consisting of voxels with voxel value above a certain percentage of the maximum voxel value.

RESULTS

The determined activity concentration varies significantly with sphere size, sphere-to-background ratio, SPECT reconstruction method and method for determining the concentration. Based on the phantom study, criteria are identified under which the activity concentration can be determined with a maximal error of 40% even in the presence of background activity.

CONCLUSIONS

Tumor dosimetry is feasible in the presence of background activity using the above-mentioned methods, provided appropriate SPECT reconstructions are applied and tumors are selected for dosimetry analysis according to the following criteria for the three methods: (1) solitary tumor with diameter > 15 mm, (2) tumor diameter > 30 mm and tumor-to-background ratio > 2, and (3) tumor diameter > 30 mm and tumor-to-background ratio > 3.

From bench to bedside: 64Cu/177Lu 1C1m-Fc anti TEM-1: mice-to-human dosimetry extrapolations for future theranostic applications

The development of diagnostic and therapeutic radiopharmaceuticals is an hot topic in nuclear medicine. Several radiolabeled antibodies are under development necessitating both biokinetic and dosimetry extrapolations for effective human translation. The validation of different animal-to-human dosimetry extrapolation methods still is an open issue. This study reports the mice-to-human dosimetry extrapolation of ⁶⁴Cu/¹⁷⁷Lu 1C1m-Fc anti-TEM-1 for theranostic application in soft-tissue sarcomas. We adopt four methods; direct mice-to-human extrapolation (M1); dosimetry extrapolation considering a relative mass scaling factor (M2), application of a metabolic scaling factor (M3) and combination of M2 and M3 (M4). Predicted in-human dosimetry for the [⁶⁴Cu]Cu-1C1m-Fc resulted in an effective dose of 0.05 mSv/MBq. Absorbed dose (AD) extrapolation for the [¹⁷⁷Lu]Lu-1C1m-Fc indicated that the AD of 2 Gy and 4 Gy to the red-marrow and total-body can be reached with 5-10 GBq and 25-30 GBq of therapeutic activity administration respectively depending on applied dosimetry method. Dosimetry extrapolation methods provided significantly different absorbed doses in organs. Dosimetry properties for the [⁶⁴Cu]Cu-1C1m-Fc are suitable for a diagnostic in-human use. The therapeutic application of [¹⁷⁷Lu]Lu-1C1m-Fc presents challenges and would benefit from further assessments in animals' models such as dogs before moving into the clinic.

EANM enabling guide: how to improve the accessibility of clinical dosimetry

Dosimetry can be a useful tool for personalization of molecular radiotherapy (MRT) procedures, enabling the continuous development of theranostic concepts. However, the additional resource requirements are often seen as a barrier to implementation. This guide discusses the requirements for dosimetry and demonstrates how a dosimetry regimen can be tailored to the available facilities of a centre. The aim is to help centres wishing to initiate a dosimetry service but may not have the experience or resources of some of the more established therapy and dosimetry centres. The multidisciplinary approach and different personnel requirements are discussed and key equipment reviewed example protocols demonstrating these factors are given in the supplementary material for the main therapies carried out in nuclear medicine, including [131I]-NaI for benign thyroid disorders, [177Lu]-DOTATATE and 131I-mIBG for neuroendocrine tumours and [90Y]-microspheres for unresectable hepatic carcinoma.

Recent Therapeutic Advances in Pituitary Carcinoma

Pituitary carcinoma (PC) is a rare, aggressive malignancy that comprises 0.1-0.2% of all pituitary tumors. PC is defined anatomically as a pituitary tumor that metastasizes outside the primary intrasellar location as noncontiguous lesions in the central nervous system or as metastases to other organs. Similar to pituitary adenoma, PC originates from various cell types of the pituitary gland and can be functioning or nonfunctioning, with the former constituting the majority of the cases. Compression of intricate skull-based structures, excessive hormonal secretion, impaired pituitary function from therapy, and systemic metastases lead to debilitating symptoms and a poor survival outcome in most cases. PC frequently recurs despite multimodality treatments, including surgical resection, radiotherapy, and biochemical and cytotoxic treatments. There is an unmet need to better understand the pathogenesis and molecular characterization of PC to improve therapeutic strategies. As our understanding of the role of signaling pathways in the tumorigenesis of and malignant transformation of PC evolves, efforts have focused on targeted therapy. In addition, recent advances in the use of immune checkpoint inhibitors to treat various solid cancers have led to an interest in exploring the role of immunotherapy for the treatment of aggressive refractory pituitary tumors. Here, we review our current understanding of the pathogenesis, molecular characterization, and treatment of PC. Particular attention is given to emerging treatment options, including targeted therapy, immunotherapy, and peptide receptor radionuclide therapy.

Gastro-Enteric-Pancreatic Neuroendocrine Tumor Treatment: 177Lu-DOTATATE

177Lu-DOTA-TATE therapy is a highly effective therapy in metastatic, well-differentiated, somatostatin receptor-positive GEP-neuroendocrine tumors (NETs) with mostly tolerable adverse effects. Guidelines generally refer to peptide receptor radionuclide therapy as a second-line therapy after SSA in gastroenteric and second- or third-line therapy in pancreatic NETs to improve survival rates and quality of life. Although we do not have sufficient data, 177Lu-DOTA-TATE therapy may also have a role in high-grade NET therapy, mostly in combination with other treatments such as chemotherapy.

Heterogeneity of absorbed dose distribution in kidney tissues and dose–response modelling of nephrotoxicity in radiopharmaceutical therapy with beta-particle emitters: A review

Absorbed dose heterogeneity in kidney tissues is an important issue in radiopharmaceutical therapy. The effect of absorbed dose heterogeneity in nephrotoxicity is, however, not fully understood yet, which hampers the implementation of treatment optimization by obscuring the interpretation of clinical response data and the selection of optimal treatment options. Although some dosimetry methods have been developed for kidney dosimetry to the level of microscopic renal substructures, the clinical assessment of the microscopic distribution of radiopharmaceuticals in kidney tissues currently remains a challenge. This restricts the anatomical resolution of clinical dosimetry, which hinders a thorough clinical investigation of the impact of absorbed dose heterogeneity. The potential of absorbed dose-response modelling to support individual treatment optimization in radiopharmaceutical therapy is recognized and gaining attraction. However, biophysical modelling is currently underexplored for the kidney, where particular modelling challenges arise from the convolution of a complex functional organization of renal tissues with the function-mediated dose distribution of radiopharmaceuticals. This article reviews and discusses the heterogeneity of absorbed dose distribution in kidney tissues and the absorbed dose-response modelling of nephrotoxicity in radiopharmaceutical therapy. The review focuses mainly on the peptide receptor radionuclide therapy with beta-particle emitting somatostatin analogues, for which the scientific literature reflects over two decades of clinical experience. Additionally, detailed research perspectives are proposed to address various identified challenges to progress in this field.

Theranostic cobalt-55/58m for neurotensin receptor-mediated radiotherapy in vivo: A pilot study with dosimetry

Neurotensin receptor 1 (NTSR1) can stimulate tumor proliferation through neurotensin (NTS) activation and are overexpressed by a variety of cancers. The high binding affinity of NTS/NTSR1 makes radiolabeled NTS derivatives interesting for cancer diagnosis and staging. Internalization of NTS/NTSR1 also suggests therapeutic application with high LET alpha particles and low energy electrons. We investigated the therapeutic efficacy of [58mCo]Co-NOTA-NT-20.3 in vivo using murine models xenografted with NTSR1-positive HT29 human colorectal adenocarcinoma cells, and utilized [55Co]Co-NOTA-NT-20.3 for dosimetry.

METHODS

Targeting properties and cytotoxicity of [55/58mCo]Co-NOTA-NT-20.3 were assessed with HT29 cells. Female nude mice were xenografted with HT29 tumors and administered [55Co or 58mCo]Co-NOTA-NT-20.3 to evaluate pharmacokinetics or for therapy, respectively. Dosimetry calculations followed the Medical Internal Radiation Dose (MIRD) formalism and human absorbed dose rate per unit activity were obtained from OpenDose. The pilot therapy study consisted of two groups (each N = 3) receiving 110 ± 15 MBq and 26 ± 6 MBq [58mCo]Co-NOTA-NT-20.3 one week after tumor inoculation, and control (N = 3). Tumor sizes and masses were measured twice a week after therapy. Complete blood count and kidney histology were also performed to assess toxicity.

RESULTS

HPLC measured radiochemical purity of [55,58mCo]Co-NOTA-NT-20.3 > 99 %. Labeled compounds retained NTS targeting properties. [58mCo]Co-NOTA-NT-20.3 exhibited cytotoxicity for HT29 cells and was >15× more potent than [58mCo]CoCl2. Xenografted tumors responded modestly to administered doses, but mice showed no signs of radiotoxicity. Absorbed dose to tumor and kidney with 110 MBq [58mCo]Co-NOTA-NT-20.3 were 0.6 Gy and 0.8 Gy, respectively, and other organs received less than half of the absorbed dose to tumor. Off-target radiation dose from cobalt-58g was small but reduces the therapeutic window.

CONCLUSION

The enhanced in vitro cytotoxicity and high tumor-to-background led us to investigate the therapeutic efficacy of [58mCo]Co-NOTA-NT-20.3 in vivo. Although we were unable to induce tumor response commensurate with [177Lu]Lu-NT127 (NLys-Lys-Pro-Tyr-Tle-Leu) studies involving similar time-integrated activity, the absence of observed toxicity may constitute an opportunity for targeting vectors with improved uptake and/or retention to avoid the aftereffects of other high-LET radioactive emissions. Future studies with higher uptake, activity and/or multiple dosing regimens are warranted. The theranostic approach employed in this work was crucial for dosimetry analysis.

A comparison of simplified protocols of personalized dosimetry in NEN patients treated by radioligand therapy (RLT) with [177Lu]Lu-DOTATATE to favor its use in clinical practice

The role of internal dosimetry is usually proposed for investigational purposes in patients treated by RLT, even if its application is not yet the standard method in clinical practice. This limited use is partially justified by several concomitant factors that make calculations a complex process. Therefore, simplified dosimetry protocols are required.

METHODS

In our study, dosimetric evaluations were performed in thirty patients with NENs who underwent RLT with [¹⁷⁷Lu]Lu-DOTATATE. The reference method (M0) calculated the cumulative absorbed dose performing dosimetry after each of the four cycles. Obtained data were employed to assess the feasibility of simplified protocols: defining the dosimetry only after the first cycle (M1) and after the first and last one (M2).

RESULTS

The mean differences of the cumulative absorbed doses between M1 and M0 were - 10% for kidney, - 5% for spleen, + 34% for liver, + 13% for red marrow, and + 37% for tumor lesions. Conversely, differences lower than ± 10% were measured between M2 and M0.

CONCLUSION

Cumulative absorbed doses obtained with the M2 protocol resembled the doses calculated by M0, while the M1 protocol overestimated the absorbed doses in all organs at risk, except for the spleen.

Quantification and dosimetry of small volumes including associated uncertainty estimation

BACKGROUND

Accurate quantification of radioactivity in a source of interest relies on accurate registration between SPECT and anatomical images, and appropriate correction of partial volume effects (PVEs). For small volumes, exact registration between the two imaging modalities and recovery factors used to correct for PVE are unreliable. There is currently no guidance relating to quantification or the associated uncertainty estimation for small volumes.

MATERIAL AND METHODS

A method for quantification of small sources of interest is proposed, which uses multiple oversized volumes of interest. The method was applied to three Na[¹³¹I]I activity distributions where a Na[¹³¹I]I capsule was situated within a cylindrical phantom containing either zero background, uniform background or non-uniform background and to a scenario with small lesions placed in an anthropomorphic phantom. The Na[¹³¹I]I capsule and lesions were quantified using the proposed method and compared with measurements made using two alternative quantification methods. The proposed method was also applied to assess the absorbed dose delivered to a bone metastasis following [¹³¹I]mIBG therapy for neuroblastoma including the associated uncertainty estimation.

RESULTS

The method is accurate across a range of activities and in varied radioactivity distributions. Median percentage errors using the proposed method in no background, uniform backgrounds and non-uniform backgrounds were - 0.4%, - 0.3% and 1.7% with median associated uncertainties of 1.4%, 1.4% and 1.6%, respectively. The technique is more accurate and robust when compared to currently available alternative methods.

CONCLUSIONS

The proposed method provides a reliable and accurate method for quantification of sources of interest, which are less than three times the spatial resolution of the imaging system. The method may be of use in absorbed dose calculation in cases of bone metastasis, lung metastasis or thyroid remnants.

Alpha-peptide receptor radionuclide therapy using actinium-225 labeled somatostatin receptor agonists and antagonists

Peptide receptor radionuclide therapy (PRRT) has over the last two decades emerged as a very promising approach to treat neuroendocrine tumors (NETs) with rapidly expanding clinical applications. By chelating a radiometal to a somatostatin receptor (SSTR) ligand, radiation can be delivered to cancer cells with high precision. Unlike conventional external beam radiotherapy, PRRT utilizes primarily β or α radiation derived from nuclear decay, which causes damage to cancer cells in the immediate proximity by irreversible direct or indirect ionization of the cells' DNA, which induces apoptosis. In addition, to avoid damage to surrounding normal cells, PRRT privileges the use of radionuclides that have little penetrating and more energetic (and thus more ionizing) radiations. To date, the most frequently radioisotopes are β^- emitters, particularly Yttrium-90 (⁹⁰Y) and Lutetium-177 (¹⁷⁷Lu), labeled SSTR agonists. Current development of SSTR-targeting is triggering the shift from using SSTR agonists to antagonists for PRRT. Furthermore, targeted α-particle therapy (TAT), has attracted special attention for the treatment of tumors and offers an improved therapeutic option for patients resistant to conventional treatments or even beta-irradiation treatment. Due to its short range and high linear energy transfer (LET), α-particles significantly damage the targeted cancer cells while causing minimal cytotoxicity toward surrounding normal tissue. Actinium-225 (²²⁵Ac) has been developed into potent targeting drug constructs including somatostatin-receptor-based radiopharmaceuticals and is in early clinical use against multiple neuroendocrine tumor types. In this article, we give a review of preclinical and clinical applications of ²²⁵Ac-PRRT in NETs, discuss the strengths and challenges of ²²⁵Ac complexes being used in PRRT; and envision the prospect of ²²⁵Ac-PRRT as a future alternative in the treatment of NETs.

Optimisation of radioligand therapy in neuroendocrine tumours: Current and evolving evidence

Treatment of neuroendocrine tumours (NETs) with radioligand therapy (RLT) for example, ¹⁷⁷ Lu-DOTATATE is generally well-tolerated and prolongs time to progression in most patients. However, approximately 20% of patients are nonresponders. In addition, complete responses are rare (<5% of patients), and durable responses beyond 3-4 years are uncommon. This article will discuss factors which may improve the outcomes of PRRT by using biomarkers to identify patients at high risk to be nonresponders (imaging and liquid biomarkers) and will examine mechanisms to potentially improve/optimise current RLT treatment strategies. These include mechanisms to potentiate the effects of RLT, increase tumour absorbed dose, overcoming radio-resistance and upregulation of somatostatin receptors, although larger studies will be required to demonstrate which techniques are going to be most efficacious in clinical practice.

Personalized Dosimetry in the Context of Radioiodine Therapy for Differentiated Thyroid Cancer

The most frequent thyroid cancer is Differentiated Thyroid Cancer (DTC) representing more than 95% of cases. A suitable choice for the treatment of DTC is the systemic administration of 131-sodium or potassium iodide. It is an effective tool used for the irradiation of thyroid remnants, microscopic DTC, other nonresectable or incompletely resectable DTC, or all the cited purposes. Dosimetry represents a valid tool that permits a tailored therapy to be obtained, sparing healthy tissue and so minimizing potential damages to at-risk organs. Absorbed dose represents a reliable indicator of biological response due to its correlation to tissue irradiation effects. The present paper aims to focus attention on iodine therapy for DTC treatment and has developed due to the urgent need for standardization in procedures, since no unique approaches are available. This review aims to summarize new proposals for a dosimetry-based therapy and so explore new alternatives that could provide the possibility to achieve more tailored therapies, minimizing the possible side effects of radioiodine therapy for Differentiated Thyroid Cancer.

Individualization of Radionuclide Therapies: Challenges and Prospects

Simple Summary

Currently, patient-specific treatment plans and dosimetry calculations are not routinely performed for radionuclide therapies. In external beam radiotherapy, it is quite the opposite. As a result, a small fraction of patients receives optimal radioactivity. This conservative approach provides “radiation safety” to healthy tissues but delivers a lower than indicated absorbed dose to the tumors, resulting in a lower response rate and a higher disease relapse rate. Evidence shows that better and more predictable outcomes can be achieved with patient-individualized dose assessment. Therefore, the incorporation of individual planning into radionuclide therapies is a high priority for nuclear medicine physicians and medical physicists alike. Internal dosimetry is used in tumor therapy to optimize the absorbed dose to the target tissue. The main reasons for the difficulties in incorporating patients’ internal dosimetry into routine clinical practice are discussed. The article presents the prospects for the routine implementation of personalized radionuclide therapies.

Abstract

The article presents the problems of clinical implementation of personalized radioisotope therapy. The use of radioactive drugs in the treatment of malignant and benign diseases is rapidly expanding. Currently, in the majority of nuclear medicine departments worldwide, patients receive standard activities of therapeutic radiopharmaceuticals. Intensively conducted clinical trials constantly provide more evidence of a close relationship between the dose of radiopharmaceutical absorbed in pathological tissues and the therapeutic effect of radioisotope therapy. Due to the lack of individual internal dosimetry (based on the quantitative analysis of a series of diagnostic images) before or during the treatment, only a small fraction of patients receives optimal radioactivity. The vast majority of patients receive too-low doses of ionizing radiation to the target tissues. This conservative approach provides “radiation safety” to healthy tissues, but also delivers lower radiopharmaceutical activity to the neoplastic tissue, resulting in a low level of response and a higher relapse rate. The article presents information on the currently used radionuclides in individual radioisotope therapies and on radionuclides newly introduced to the therapeutic market. It discusses the causes of difficulties with the implementation of individualized radioisotope therapies as well as possible changes in the current clinical situation.

Milestones in dosimetry for nuclear medicine therapy

Nuclear Medicine therapy has reached a critical juncture with an unprecedented number of patients being treated and an extensive list of new radiopharmaceuticals under development. Since the early applications of these treatments dosimetry has played a vital role in their development, in both aiding optimisation and enhancing safety and efficacy. To inform the future direction of this field, it is useful to reflect on the scientific and technological advances that have occurred since those early uses. In this review, we explore how dosimetry has evolved over the years and discuss why such initiatives were conceived and the importance of maintaining standards within our practise. Specific milestones and landmark publications are highlighted and a thematic review and significant outcomes during each decade are presented.

Optimized cocktail of 90Y/177Lu for radionuclide therapy of neuroendocrine tumors of various sizes: a simulation study

BACKGROUND AND OBJECTIVES

There is significant interest and potential in the treatment of neuroendocrine tumors via peptide receptor radionuclide therapy (PRRT) using one or both of 90Y and 177Lu-labeled peptides. Given the presence of different tumor sizes in patients and differing radionuclide dose delivery properties, the present study aims to use Monte Carlo simulations to estimate S-values to spherical tumors of various sizes with 90Y and 177Lu separately and in combination. The goal is to determine ratios of 90Y to 177Lu that result in the largest absorbed doses per decay of the radionuclides and the most suitable dose profiles to treat tumors of specific sizes.

MATERIAL AND METHODS

Particle transfer calculations and simulations were performed using the Monte Carlo GATE simulation software. Spherical tumors of different sizes, ranging from 0.5 to 20 mm in radius, were designed. Activities of 177Lu and 90Y, individually and in combination, were homogeneously placed within the total volume of the tumors. We determined the S-values to the tumors, and to the external volume outside of the tumors (cross-dose) which was used to approximate background tissue. The dose profiles were obtained for each of the different tumor sizes, and the uniformity of dose within each tumor was calculated.

RESULTS

For all tumor sizes, the self-dose and cross-dose per decay from 90Y were higher than that from 177Lu. We observed that 177Lu had the most uniform dose distribution within tumors with radii less than 5 mm. For tumors greater than 5 mm in radius, a ratio of 25% 90Y to 75% 177Lu resulted in the most uniform doses. When the ratio of 177Lu to 90Y was smaller, the uniformity improved more with increasing tumor size. The cross-dose stayed approximately constant for tumors larger than 15 mm for all ratios of 177Lu to 90Y. Finally, as the size of the tumor increased, differences in the S-values between different ratios of 177Lu to 90Y decreased.

CONCLUSION

Our work showed that to achieve a more uniform dose distribution within the tumor, 177Lu alone is more effective for small tumors. For medium and large tumors, a ratio of 90Y to 177Lu with more or less 177Lu, respectively, is recommended.

Know thy tumour: Biomarkers to improve treatment of molecular radionuclide therapy

Molecular radionuclide therapy (MRT) is an effective treatment for both localised and disseminated tumours. Biomarkers can be used to identify potential subtypes of tumours that are known to respond better to standard MRT protocols. These enrolment-based biomarkers can further be used to develop dose-response relationships using image-based dosimetry within these defined subtypes. However, the biological identity of the cancers treated with MRT are commonly not well-defined, particularly for neuroendocrine neoplasms. The biological heterogeneity of such cancers has hindered the establishment of dose-responses and minimum tumour dose thresholds. Biomarkers could also be used to determine normal tissue MRT dose limits and permit greater injected doses of MRT in patients. An alternative approach is to understand the repair capacity limits of tumours using radiobiology-based biomarkers within and outside patient cohorts currently treated with MRT. It is hoped that by knowing more about tumours and how they respond to MRT, biomarkers can provide needed dimensionality to image-based biodosimetry to improve MRT with optimized protocols and personalised therapies.

Phase II trial demonstrates the efficacy and safety of individualized, dosimetry-based 177Lu-DOTATATE treatment of NET patients

PURPOSE

Radionuclide therapy with ¹⁷⁷Lu-DOTATATE is well established for patients with advanced somatostatin receptor-positive neuroendocrine tumors with a standard schedule of 7.4 GBq at four occasions. However, this approach does not consider individual variability affecting the tumor radiation dose or dose to organs at risk. Therefore, it is important to assess more personalized strategies. The aim of this phase II trial was to evaluate individualized ¹⁷⁷Lu-DOTATATE for which the number of cycles varied based on renal dosimetry.

METHODS

Patients were eligible if they had a progressive, somatostatin receptor-positive neuroendocrine tumor with a Ki 67 labeling index < 20%. They received cycles of 7.4 GBq of ¹⁷⁷Lu-DOTATATE at 10 ± 2-week intervals until a predefined radiation dose to the kidneys was reached. The primary endpoint was objective tumor response (RECIST v 1.1). Secondary endpoints included progression-free survival (PFS), overall survival (OS), and toxicity (CTCAE v. 4.0).

RESULTS

Ninety-six patients who had received a median of 5 cycles (range 1-9) were evaluable for efficacy. The objective tumor response was 16% partial response, 66% stable disease, and 19% progressive disease. The median PFS and OS were 29 months and 47 months, respectively, and were significantly associated with kidney dose, performance status, and Ki 67 levels but not with tumor origin. The overall toxicity was mild, and the most common events were grade 1-2 anemia, thrombocytopenia, fatigue, nausea, and diarrhea. Grade 3-4 toxicity occurred in < 10% of patients and was mostly hematological, with no grade 3-4 renal toxicity.

CONCLUSION

Individualized treatment with ¹⁷⁷Lu-DOTATATE based on renal dosimetry is clearly feasible with low toxicity and promising efficacy, showing the potential to further improve outcome beyond the standard approach, and should be further assessed in randomized trials.

TRIAL REGISTRATION

EudraCT 2011-000,240-16. NCT01456078. https://clinicaltrials.gov/ct2/show/NCT01456078.

Comparison of absorbed dose extrapolation methods for mouse-to-human translation of radiolabelled macromolecules

Background

Extrapolation of human absorbed doses (ADs) from biodistribution experiments on laboratory animals is used to predict the efficacy and toxicity profiles of new radiopharmaceuticals. Comparative studies between available animal-to-human dosimetry extrapolation methods are missing. We compared five computational methods for mice-to-human AD extrapolations, using two different radiopharmaceuticals, namely [¹¹¹In]CHX-DTPA-scFv78-Fc and [⁶⁸Ga]NODAGA-RGDyK. Human organ-specific time-integrated activity coefficients (TIACs) were derived from biodistribution studies previously conducted in our centre. The five computational methods adopted are based on simple direct application of mice TIACs to human organs (M1), relative mass scaling (M2), metabolic time scaling (M3), combined mass and time scaling (M4), and organ-specific allometric scaling (M5), respectively. For [⁶⁸Ga]NODAGA-RGDyK, these methods for mice-to-human extrapolations were tested against the ADs obtained on patients, previously published by our group. Lastly, an average [⁶⁸Ga]NODAGA-RGDyK-specific allometric parameter α_new was calculated from the organ-specific biological half-lives in mouse and humans and retrospectively applied to M3 and M4 to assess differences in human AD predictions with the α = 0.25 recommended by previous studies.

Results

For both radiopharmaceuticals, the five extrapolation methods showed significantly different AD results (p < 0.0001). In general, organ ADs obtained with M3 were higher than those obtained with the other methods. For [⁶⁸Ga]NODAGA-RGDyK, no significant differences were found between ADs calculated with M3 and those obtained directly on human subjects (H) (p = 0.99; average M3/H AD ratio = 1.03). All other methods for dose extrapolations resulted in ADs significantly different from those calculated directly on humans (all p ≤ 0.0001). Organ-specific allometric parameters calculated using combined experimental [⁶⁸Ga]NODAGA-RGDyK mice and human biodistribution data varied significantly. ADs calculated with M3 and M4 after the application of α_new = 0.17 were significantly different from those obtained by the application of α = 0.25 (both p < 0.001).

Conclusions

Available methods for mouse-to-human dosimetry extrapolations provided significantly different results in two different experimental models. For [⁶⁸Ga]NODAGA-RGDyK, the best approximation of human dosimetry was shown by M3, applying a metabolic scaling to the mouse organ TIACs. The accuracy of more refined extrapolation algorithms adopting model-specific metabolic scaling parameters should be further investigated.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13550-022-00893-z.

EANM dosimetry committee recommendations for dosimetry of 177Lu-labelled somatostatin-receptor- and PSMA-targeting ligands

The purpose of the EANM Dosimetry Committee is to provide recommendations and guidance to scientists and clinicians on patient-specific dosimetry. Radiopharmaceuticals labelled with lutetium-177 (177Lu) are increasingly used for therapeutic applications, in particular for the treatment of metastatic neuroendocrine tumours using ligands for somatostatin receptors and prostate adenocarcinoma with small-molecule PSMA-targeting ligands. This paper provides an overview of reported dosimetry data for these therapies and summarises current knowledge about radiation-induced side effects on normal tissues and dose-effect relationships for tumours. Dosimetry methods and data are summarised for kidneys, bone marrow, salivary glands, lacrimal glands, pituitary glands, tumours, and the skin in case of radiopharmaceutical extravasation. Where applicable, taking into account the present status of the field and recent evidence in the literature, guidance is provided. The purpose of these recommendations is to encourage the practice of patient-specific dosimetry in therapy with 177Lu-labelled compounds. The proposed methods should be within the scope of centres offering therapy with 177Lu-labelled ligands for somatostatin receptors or small-molecule PSMA.

Post-treatment three-dimensional voxel-based dosimetry after Yttrium-90 resin microsphere radioembolization in HCC

Background

Post-therapy [⁹⁰Y] PET/CT-based dosimetry is currently recommended to validate treatment planning as [^99mTc] MAA SPECT/CT is often a poor predictor of subsequent actual [⁹⁰Y] absorbed dose. Treatment planning software became available allowing 3D voxel dosimetry offering tumour-absorbed dose distributions and dose-volume histograms (DVH). We aim to assess dose–response effects in post-therapy [⁹⁰Y] PET/CT dosimetry in SIRT-treated HCC patients for predicting overall and progression-free survival (OS and PFS) and four-month follow-up tumour response (mRECIST). Tumour-absorbed dose and mean percentage of the tumour volume (V) receiving ≥ 100, 150, 200, or 250 Gy and mean minimum absorbed dose (D) delivered to 30%, 50%, 70%, and 90% of tumour volume were calculated from DVH’s. Depending on the mean tumour -absorbed dose, treated lesions were assigned to a < 120 Gy or ≥ 120 Gy group.

Results

Thirty patients received 36 SIRT treatments, totalling 43 lesions. Median tumour-absorbed dose was significantly different between the ≥ 120 Gy (n = 28, 207 Gy, IQR 154–311 Gy) and < 120 Gy group (n = 15, 62 Gy, IQR 49–97 Gy, p <0 .01). Disease control (DC) was found more frequently in the ≥ 120 Gy group (79%) compared to < 120 Gy (53%). Mean tumour-absorbed dose optimal cut-off predicting DC was 131 Gy. Tumour control probability was 54% (95% CI 52–54%) for a mean tumour-absorbed dose of 120 Gy and 90% (95% CI 87–92%) for 284 Gy. Only D30 was significantly different between DC and progressive disease (p = 0.04). For the ≥ 120 Gy group, median OS and PFS were longer (median OS 33 months, [range 8–33 months] and median PFS 23 months [range 4–33 months]) than the < 120 Gy group (median OS 17 months, [range 5–33 months] and median PFS 13 months [range 1–33 months]) (p < 0.01 and p = 0.03, respectively).

Conclusions

Higher 3D voxel-based tumour-absorbed dose in patients with HCC is associated with four-month DC and longer OS and PFS. DVHs in [⁹⁰Y] SIRT could play a role in evaluative dosimetry.

Implications of physics, chemistry and biology for dosimetry calculations using theranostic pairs

Theranostics is an emerging paradigm that combines imaging and therapy in order to personalize patient treatment. In nuclear medicine, this is achieved by using radiopharmaceuticals that target identical molecular targets for both imaging (using emitted gamma rays) and radiopharmaceutical therapy (using emitted beta, alpha or Auger-electron particles) for the treatment of various diseases, such as cancer. If the therapeutic radiopharmaceutical cannot be imaged quantitatively, a “theranostic pair” imaging surrogate can be used to predict the absorbed radiation doses from the therapeutic radiopharmaceutical. However, theranostic dosimetry assumes that the pharmacokinetics and biodistributions of both radiopharmaceuticals in the pair are identical or very similar, an assumption that still requires further validation for many theranostic pairs. In this review, we consider both same-element and different-element theranostic pairs and attempt to determine if factors exist which may cause inaccurate dose extrapolations in theranostic dosimetry, either intrinsic (e.g. chemical differences) or extrinsic (e.g. injecting different amounts of each radiopharmaceutical) to the radiopharmaceuticals. We discuss the basis behind theranostic dosimetry and present common theranostic pairs and their therapeutic applications in oncology. We investigate general factors that could create alterations in the behavior of the radiopharmaceuticals or the quantitative accuracy of imaging them. Finally, we attempt to determine if there is evidence showing some specific pairs as suitable for theranostic dosimetry. We show that there are a variety of intrinsic and extrinsic factors which can significantly alter the behavior among pairs of radiopharmaceuticals, even if they belong to the same chemical element. More research is needed to determine the impact of these factors on theranostic dosimetry estimates and on patient outcomes, and how to correctly account for them.

Pretherapeutic estimated glomerular filtration rate predicts development of chronic kidney disease in patients receiving PSMA-targeted radioligand therapy

BACKGROUND

Prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) may be associated with renal toxicity. We aimed to identify predictive parameters for the development of chronic kidney disease (CKD) in patients with metastatic castration resistant prostate cancer (mCRPC) undergoing RLT.

METHODS

In 46 mCRPC patients scheduled for Lu-177-PSMA-RLT, pretherapeutic estimated glomerular filtration rate (eGFR [ml/min/1.73 m² ]), Tc-99m-mercaptoacetyltriglycine (Tc-99m-MAG3) clearance and baseline Ga-68-PSMA-ligand positron emission tomography (PET)-derived renal cortical uptake and PSMA-tumor volume (TV) were determined. We tested the predictive capability of these parameters and clinical risk factors for the occurrence of CKD (defined as CTCAE vers. 5.0 grade 2 or higher) during follow-up.

RESULTS

After 4 ± 3 cycles of RLT average eGFR declined from 76 ± 17 to 72 ± 20 ml/min/1.73 m² (p = 0.003). Increased estimated renal radiation dose (eRRD) was significantly associated with renal functional decline (p = 0.008). During follow-up, 16/46 (30.4%) developed CKD grade 2 (no grade 3 or higher). In receiver operating characteristic (ROC) analysis, pretherapeutic eGFR was highly accurate in identifying the occurrence of CKD vs no CKD with an area under the curve (AUC) of 0.945 (p < 0.001; best threshold, 77 ml/min/1.73 m² ), followed by Tc-99m-MAG3-derived tubular extraction rate (TER; AUC, 0.831, p < 0.001; best threshold, 200 ml/min/1.73 m² ). Renal PET signal (p = 0.751) and PSMA-TV (p = 0.942), however, were not predictive. Kaplan-Meier analyses revealed adverse renal outcome for patients with lower eGFR (p = 0.001) and lower scintigraphy-derived TER (p = 0.009), with pretherapeutic eGFR emerging as the sole predictive parameter in multivariate analysis (p = 0.007).

CONCLUSION

Serious adverse renal events are not a frequent phenomenon after PSMA-targeted RLT. However, in patients developing moderate CKD after RLT, pretherapeutic eGFR is an independent predictor for renal impairment during follow-up.

Dosimetry of nuclear medicine therapies: current controversies and impact on treatment optimization

Nuclear medicine therapeutic procedures have considerably expanded over the last few years, and their number is expected to grow exponentially in the future. Internal dosimetry has significantly developed as well, but has not yet been uniformly accepted as a valuable tool for prediction of therapeutic efficacy and toxicity. In this paper, we briefly summarize some of the arguments about the implementation of internal dosimetry in clinical practice. In addition, we provide a few examples of radionuclide anticancer therapies for which internal dosimetry demonstrated a significant impact on treatment optimization and patient outcome.

From the target cell theory to a more integrated view of radiobiology in Targeted radionuclide therapy: The Montpellier group's experience

Targeted radionuclide therapy (TRT) is used to treat disseminated or metastatic tumours in which conventional external beam radiotherapy (EBRT) would have unacceptable side effects. Unlike EBRT, TRT delivers low doses at a continuous low dose rate. In EBRT, the effect increases progressively with the dose rate, and biological effects (tumour control and normal tissue damage) are related to the dose according to a sigmoid curve model. This model is part of the so-called quantitative radiobiology that is mostly based on the target cell theory, according to which cell death is due to (lethal) radiation hits to vital cellular targets. This model was developed for EBRT, but was adapted to low dose-rate situations by including a parameter that reflects the time needed to repair tissue damage. However, a growing body of evidence indicates that the model should take into account also the biological effects, which are due to intercellular communications (bystander effects) and amplify the effects of radiation, as well as the immune system. Moreover, extranuclear targets must be considered, although induced intracellular and intercellular signalling pathways may ultimately result in DNA damage. It is likely that bystander effects and immune response always contribute to the overall response to TRT at different levels, and that dose and dose rate are key parameters in controlling their real contribution. We hypothesize that the dose rate is the key determinant in the balance between the physical and DNA-centred response on one side, and the biological response that integrates all subcellular compartments and intercellular signalling pathways on the other side.

Imaging and dosimetry for alpha-particle emitter radiopharmaceutical therapy: improving radiopharmaceutical therapy by looking into the black box

Radiopharmaceutical therapy using α-particle emitting radionuclides (αRPT) is a novel treatment modality that delivers highly potent alpha-particles to cancer cells or their environment. We review the advantages and challenges of imaging and dosimetry in implementing αRPT for cancer patients.

Haematotoxicity during peptide receptor radionuclide therapy: Baseline parameters differences and effect on patient's therapy course

BACKGROUND

Mainly severe (CTCAE grade 3-4) haematotoxicity during peptide receptor radionuclide therapy (PRRT) is reported in literature due to major clinical impact, however moderate (CTCAE grade 2) haematotoxicity is common and could affect therapy management. The aim of this study was to evaluate the haematotoxicity course during PRRT and to compare baseline parameters between haematotoxicity grades.

METHODS

In this retrospective study, 100 patients with a neuroendocrine tumour treated with PRRT were included. Patients were treated with an aimed number of four cycles with 7.4 GBq [177Lu]Lu-DOTA-TATE administered every 10 weeks. Haematological assessment was performed at baseline and frequently up to 10 weeks after the fourth cycle. The lowest haematological value was graded according to CTCAE v5.0, and patients were classified using the highest observed grade. Differences in baseline parameters, including [68Ga]Ga-DOTA-TATE positive tumour volume, were evaluated between CTCAE grades.

RESULTS

Four cycles were completed by 86/100 of patients, 4/100 patients discontinued due to haematotoxicity, and 10/100 patients due to progressive disease. The treatment course was adjusted due to haematotoxicity in 24/100 patients, including postponed next cycle (n = 17), reduced administered activity (n = 13), and both adjustments (n = 10). The most observed haematotoxicity grade was grade 0-1 in 54/100 patients, grade 2 in 38/100 and grade 3-4 in 8/100. Significant differences in baseline leucocyte, neutrophil and platelet counts were observed between grade 0-1 and grade 2. However, the correlation between baseline and lowest observed values was poor to moderate. No differences between haematotoxicity grades and baseline parameters or somatostatin receptor positive tumour volume was observed.

CONCLUSIONS

The incidence of severe haematotoxicity was low with extensive screening and monitoring. The vast majority of patients (96/100) was not restricted in treatment continuation by haematotoxicity; therefore, our selection criteria appeared appropriate for safe PRRT treatment. Baseline parameters showed limited correlation with the degree of decline in haematological values.

Practical kidney dosimetry in peptide receptor radionuclide therapy using [177Lu]Lu-DOTATOC and [177Lu]Lu-DOTATATE with focus on uncertainty estimates

Background

Kidney dosimetry after peptide receptor radionuclide therapy using ¹⁷⁷Lu-labelled somatostatin analogues is a procedure with multiple steps. We present the SPECT/CT-based implementation at Aarhus University Hospital and evaluate the uncertainty of the various steps in order to estimate the total uncertainty and to identify the major sources of uncertainty. Absorbed dose data from 115 treatment fractions are reported.

Results

The total absorbed dose with uncertainty is presented for 59 treatments with [¹⁷⁷Lu]Lu-DOTATOC and 56 treatments with [¹⁷⁷Lu]Lu-DOTATATE. For [¹⁷⁷Lu]Lu-DOTATOC the mean and median specific absorbed dose (dose per injected activity) is 0.37 Gy/GBq and 0.38 Gy/GBq, respectively, while for [¹⁷⁷Lu]Lu-DOTATATE the median and mean are 0.47 Gy/GBq and 0.46 Gy/GBq, respectively. The uncertainty of the procedure is estimated to be about 13% for a single treatment fraction, where the absorbed dose calculation is based on three SPECT/CT scans 1, 4 and 7 days post-injection, while it increases to about 19% if only a single SPECT/CT scan is performed 1 day post-injection.

Conclusions

The specific absorbed dose values obtained with the described procedure are comparable to those from other treatment sites for both [¹⁷⁷Lu]Lu-DOTATOC and [¹⁷⁷Lu]Lu-DOTATATE, but towards the lower end of the range of reported values. The estimated uncertainty is also comparable to that from other reports and judged acceptable for clinical and research use, thus proving the kidney dosimetry procedure a useful tool. The greatest reduction in uncertainty can be obtained by improved activity determination, partial volume correction and additional SPECT/CT scans.