Imaging technologies for radionuclide dosimetry

Astatine-211 imaging by a Compton camera for targeted radiotherapy

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

The production of [124I]iodine and [86Y]yttrium

The use of paired tracers such as (124)I/(131)I and (86)Y/(90)Y allows pretherapy PET imaging with positron emitting radioisotopes of the same element as used for therapy. Whereas nowadays most therapy nuclides are produced by reactors or generators, the production of the corresponding PET isotopes requires the irradiation of adequate targets using particle accelerators such as cyclotrons. This paper describes the production routes for (124)I and (86)Y.

Prospects and problems in the production of 143Pr for radionuclide therapy applications

Praseodymium-143 (T 1/2=13.57 d and E β(max)= 0.937 MeV) is the beta decay product of 143Ce (T 1/2=33.04 h) and would provide an ideal complement to the widely used 32P (T 1/2= 14.3 d, E β(max)=1.71 MeV), for some applications in radionuclide therapy (RNT), as it has a lower beta energy, but comparable half-life. For the production of 143Pr, 10-500 mg of two cerium compounds, ceric ammonium sulphate and ceric oxide, were irradiated in the reactor for 7 days at a neutron flux of ∼1×1013 n cm−2 s−1. After five days of post irradiation cooling, the samples were dissolved in appropriate reagents and aliquots drawn were subjected to high resolution gamma ray spectrometry for assay of activity and radionuclide purity. 7.4 MBq 143Ce per mg ceric oxide was produced under the aforesaid conditions along with other long-lived isotopes of cerium. An effort was made for the separation of no-carrier added (nca) 143Pr in order to study its utility for radionuclide therapy (RNT). A maximum yield of ∼450 MBq of 143Pr was obtained at calibration time defined as 7 d from end of irradiation (EOI). Hydroxyapatite (HA) was labeled with 143Pr at pH 5–6 and greater than 99.5% of 143Pr activity remained bound to HA even after 90 h post labeling. Despite the low cross-section of 142Ce(n,γ) reaction (0.1 barn) and isotopic abundance of 142Ce in natural targets (11.114%) being inherent disadvantages in large scale production of 143Pr, therapies needing low to medium doses would benefit from the use of ‘nca’ 143Pr.

Clinical radionuclide therapy dosimetry: the quest for the "Holy Gray"

Introduction

Radionuclide therapy has distinct similarities to, but also profound differences from external radiotherapy.

Review

This review discusses techniques and results of previously developed dosimetry methods in thyroid carcinoma, neuro-endocrine tumours, solid tumours and lymphoma. In each case, emphasis is placed on the level of evidence and practical applicability. Although dosimetry has been of enormous value in the preclinical phase of radiopharmaceutical development, its clinical use to optimise administered activity on an individual patient basis has been less evident. In phase I and II trials, dosimetry may be considered an inherent part of therapy to establish the maximum tolerated dose and dose-response relationship. To prove that dosimetry-based radionuclide therapy is of additional benefit over fixed dosing or dosing per kilogram body weight, prospective randomised phase III trials with appropriate end points have to be undertaken. Data in the literature which underscore the potential of dosimetry to avoid under- and overdosing and to standardise radionuclide therapy methods internationally are very scarce.

Developments

In each section, particular developments and insights into these therapies are related to opportunities for dosimetry. The recent developments in PET and PET/CT imaging, including micro-devices for animal research, and molecular medicine provide major challenges for innovative therapy and dosimetry techniques. Furthermore, the increasing scientific interest in the radiobiological features specific to radionuclide therapy will advance our ability to administer this treatment modality optimally.

Tissue-specific dosimetry for radioiodine therapy of the autonomous thyroid nodule

A tissue-specific dosimetric method based on gamma camera acquisitions was developed to determine the 131I activity to administer to patients with autonomous thyroid nodules (ATN) to deliver 200 Gy to the nodule and to evaluate the correspondent dose to extranodular tissue. Twenty patients with ATN were given 111 MBq of 123I i.v. and their neck was imaged 2, 4, 24, 48, and 120 hours after administration to evaluate separate iodine kinetics for nodule and contralateral lobe. The volumes of nodule and lobe were measured on the 4 hour scintigraphic image, after optimization of the method on a thyroid phantom. Three simplified dosimetric methods were then considered and compared to the reference method in terms of 131I activity: (a) three point method, based on 4, 24, and 120 h acquisitions, (b) fixed T1/2 method, that measures only the 24 h uptake and assumes an effective half-life of 5 days for the nodule, (c) fixed activity method, based on the administration of 413 MBq of 131I. The mean 131I activity to administer to the 20 patients was 413 MBq (range 65-1327) and the mean dose to the contralateral lobe was 43 Gy (range 11-121). The percentage differences in 131I activity between the reference method and the simplified methods were in the ranges: (a) -14%, 13%, (b) -42%, 74%, (c) -69%, 533%. The relevant dose to extranodular tissue and the great interpatient variability of the radioiodine activity required to give a predetermined dose to ATN suggest that a tissue specific dosimetric approach based on gamma camera acquisitions is fundamental. A simple method based on only three uptake measurements is a reliable alternative to the five point method when the clinical workload of a Nuclear Medicine department is particularly heavy.

Kinetics of 76Br-labeled anti-CEA antibodies in pigs; aspects of dosimetry and PET imaging properties

A monoclonal antibody labeled with the positron-emitting radionuclide 76Br (T(1/2) 16.2 h) has previously been shown useful for positron emission tomography (PET) imaging of experimental tumors. Our aim in the present study was to investigate the effects of the complex decay scheme of this radionuclide on normal organ dosimetry and PET image quality. Three mini-pigs were injected intravenously with 46-75 MBq of the 76Br-labeled anti-CEA antibody 38S1, and the whole-body kinetics followed by PET imaging for 19 h. From PET data, absorbed doses in human organs were estimated using the MIRDOSE 3.0 software. The highest 76Br concentrations were found in lungs, after a correction for the air volume in this organ. The lungs received the highest absorbed dose (mGy/MBq, mean+/-maximum error), 0.84+/-0.16, followed by liver, 0.74+/-0.28, and small intestine, 0.55+/-0.05, while the effective dose equivalent was 0.41+/-0.03 mSv/MBq. The PET imaging properties of 76Br in a two-dimensional 2D PET camera, including central area resolution and scattering effects, were investigated in phantoms and compared to those of 18F. In a 0.97 g/cm3 material, approximating soft tissue density, the FMHW ("full width at half-maximum") value of the point spread function was 7.7+/-0.2 mm for 76Br and 6.0+/-0.1 mm for 18F. In conclusion, radioimmuno PET using 76Br-labeled antibodies resulted in a fairly even distribution of the radiation dose, where the highest absorbed organ doses were only about two to three times higher than the mean absorbed body dose. The high energy beta+ spectrum in the 76Br decay had only minor effects on the resolution, but may decrease the quantification accuracy, especially in organs with a lower density such as a lung.

Radionuclides and carrier molecules for therapy

Although radionuclide therapy has been around for a long time, this modality of cancer treatment has been limited mainly to the use of [32P]-phosphate and [131I]-sodium iodide. The last few years, however, have seen an increased interest in this area due to new developments of radionuclides and carrier molecules that may provide selective targeting of tumour sites. The potential of this technique can be further realized if the radionuclide is carefully selected to match both the localization of the carrier molecule and tumour morphology. This paper briefly reviews radionuclides in current use and potential candidates for targeted therapy. Decay characteristics, production methods and relevant chemical properties are discussed.

Targeted therapy using alpha emitters

Radionuclides such as 211At and 212Bi which decay by the emission of alpha-particles are attractive for certain applications of targeted radiotherapy. The tissue penetration of 212Bi and 211At alpha-particles is equivalent to only a few cell diameters, offering the possibility of combining cell-specific targeting with radiation of similar range. Unlike the beta-particles emitted by radionuclides such as 131I and 90Y, alpha-particles are radiation of high linear energy transfer and thus greater biological effectiveness. Several approaches have been explored for targeted radiotherapy with 212Bi- and 211At-labelled substances including colloids, monoclonal antibodies, metabolic precursors, receptor-avid ligands and other lower molecular weight molecules. An additional agent which exemplifies the promise of alpha-emitting radiopharmaceuticals is meta-[211At]astatobenzylguanidine. The toxicity of this compound under single-cell conditions, determined both by [3H]thymidine incorporation and by limiting dilution clonogenic assays, for human neuroblastoma cells is of the order of 1000 times higher than that of meta-[131I] iodobenzylguanidine. For meta-[211At] astatobenzylguanidine, the Do value was equivalent to only 6-7 211At atoms bound per cell. These results suggest that meta-[211At] astatobenzylguanidine might be valuable for the targeted radiotherapy of micrometastatic neuroblastomas.