Second Symposium of the European Working Group on the Radiobiology of Molecular Radionuclide Therapy

Molecular radionuclide therapy is a relatively novel anticancer treatment option using radiolabeled, tumor-specific vectors. On binding of these vectors to cancer cells, radioactive decay induces DNA damage and other effects, leading to cancer cell death. Treatments, such as with [177Lu]Lu-octreotate for neuroendocrine tumors and [177Lu]Lu-PSMA for prostate cancer, are now being implemented into routine clinical practice around the world. Nonetheless, research into the underlying radiobiologic effects of these treatments is essential to further improve them or formulate new ones. The purpose of the European Working Group on the Radiobiology of Molecular Radiotherapy is to promote knowledge, investment, and networking in this area. This report summarizes recent research and insights presented at the second International Workshop on Radiobiology of Molecular Radiotherapy, held in London, U.K., on March 13 and 14, 2023. The symposium was organized by members of the Cancer Research U.K. RadNet City of London and the European Working Group on the Radiobiology of Molecular Radiotherapy.

Relationship of In Vitro Toxicity of Technetium-99m to Subcellular Localisation and Absorbed Dose

Auger electron-emitters increasingly attract attention as potential radionuclides for molecular radionuclide therapy in oncology. The radionuclide technetium-99m is widely used for imaging; however, its potential as a therapeutic radionuclide has not yet been fully assessed. We used MDA-MB-231 breast cancer cells engineered to express the human sodium iodide symporter-green fluorescent protein fusion reporter (hNIS-GFP; MDA-MB-231.hNIS-GFP) as a model for controlled cellular radionuclide uptake. Uptake, efflux, and subcellular location of the NIS radiotracer [99mTc]TcO4- were characterised to calculate the nuclear-absorbed dose using Medical Internal Radiation Dose formalism. Radiotoxicity was determined using clonogenic and γ-H2AX assays. The daughter radionuclide technetium-99 or external beam irradiation therapy (EBRT) served as controls. [99mTc]TcO4- in vivo biodistribution in MDA-MB-231.hNIS-GFP tumour-bearing mice was determined by imaging and complemented by ex vivo tissue radioactivity analysis. [99mTc]TcO4- resulted in substantial DNA damage and reduction in the survival fraction (SF) following 24 h incubation in hNIS-expressing cells only. We found that 24,430 decays/cell (30 mBq/cell) were required to achieve SF0.37 (95%-confidence interval = [SF0.31; SF0.43]). Different approaches for determining the subcellular localisation of [99mTc]TcO4- led to SF0.37 nuclear-absorbed doses ranging from 0.33 to 11.7 Gy. In comparison, EBRT of MDA-MB-231.hNIS-GFP cells resulted in an SF0.37 of 2.59 Gy. In vivo retention of [99mTc]TcO4- after 24 h remained high at 28.0% ± 4.5% of the administered activity/gram tissue in MDA-MB-231.hNIS-GFP tumours. [99mTc]TcO4- caused DNA damage and reduced clonogenicity in this model, but only when the radioisotope was taken up into the cells. This data guides the safe use of technetium-99m during imaging and potential future therapeutic applications.

A wearable radiation measurement system for collection of patient-specific time-activity data in radiopharmaceutical therapy: system design and Monte Carlo simulation results

Purpose: A high level of personalization in Molecular Radiotherapy (MRT) could bring advantages in terms of treatment effectiveness and toxicity reduction. Individual organ‐level dosimetry is crucial to describe the radiopharmaceutical biodistribution expressed by the patient, to estimate absorbed doses to normal organs and target tissue(s). This paper presents a proof‐of‐concept Monte Carlo simulation study of “WIDMApp” (Wearable Individual Dose Monitoring Apparatus), a multi‐channel radiation detector and data processing system for in vivo patient measurement and collection of radiopharmaceutical biokinetic data (i.e., time‐activity data). Potentially, such a system can increase the amount of such data that can be collected while reducing the need to derive it via nuclear medicine imaging.

Methods: a male anthropomorphic MIRD phantom was used to simulate photons (i.e., gamma‐rays) propagation in a patient undergoing a 131I thyroid treatment. The administered activity was set to the amount usually administered for the treatment of differentiated carcinoma while its initial distribution in different organs was assigned following the ICRP indications for the 131I biokinetics. Using this information, the simulation computes the Time‐dependent Counts Curves (TCCs) that would have been measured by seven WIDMApp‐like sensors placed and oriented to face each one of five emitting organs plus two thyroid lobes. A deconvolution algorithm was then applied on this simulated data set to reconstruct the Time‐Activity Curve (TAC) of each organ. Deviations of the reconstructed TACs parameters from values used to generate them were studied as a function of the deconvolution algorithm initialization parameters and assuming non‐Poisson fluctuation of the TCCs data points.

Results: This study demonstrates that it is possible, at least in the simple simulated scenario, to reconstruct the organ cumulated activity by measuring the time dependence of counts recorded by several detectors placed at selected positions on the patient's body. The ability to perform in vivo sampling more frequently than conventional biokinetic studies increases the number of time points and therefore the accuracy in TAC estimates. In this study, an accuracy on cumulated activity of 5% is obtained even with a 20% error on the TCC data points and a 50% error on the initial guess on the parameters of the deconvolution algorithm.

Conclusions: the WIDMApp approach could provide an effective tool to characterize more accurately the radiopharmaceutical biokinetics in MRT patients, reducing the need of resources of nuclear medicine departments, such as technologist and scanner time, to perform individualized biokinetics studies. The relatively simple hardware for the approach proposed would allow its application to large numbers of patients. The results obtained justify development of an actual prototype system to characterize this technique under realistic conditions.