Theoretical assessment of specific radioactivity: the effect of target burn-up, isotope dilution and target purity and the application for 177Lu production
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Date
2009-11-29
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Asia-Pacific Symposium on Radiochemistry '09
Abstract
The wide expansion of targeting radiotherapy depends a lot on the availability of metallic radionuclide with high specific radioactivity (SA). As an example, as high as 20 Ci / mg SA of 177Lu is required to formulate the targeting radiopharmaceuticals for different cancer treatments. High SA nuclide can be produced using a neutron capture reaction of the target nuclide of high cross section (such as 176Lu (n, gamma) 177Lu, б = 2300 b). This direct route could be successfully performed in the nuclear reactor of rather high neutron flux which is accessible in only a handful of countries in the world. However, large burn-up of the target nuclide during high neutron flux irradiation may cause a problem of degrading the SA value of the produced nuclide if the target contains isotopic impurities. No-carrier- added (n.c.a) radioisotopes can be produced via a process of neutron capture-followed-by-beta particle decay (such as 176Yb (n, gamma) 177Yb ( β- decay) 177Lu). In this case, the same reduction in SA was also experienced if the target contains the isotopic and/or elemental impurities, one radionuclide of which is expected to be produced. The 177Lu production was reported in many publications 1-3, but until now the product quality assessment, especially the evaluation of 177Lu specific radioactivity in the product is not sufficiently performed. The SA assessment was based on the calculation of nuclear reaction and radioactive transformation yield as a function of thermal neutron flux, target nuclide burn-up, isotopic composition and irradiation and post-activation processing time. Among the factors affecting SA the target composition (elemental and isotopic impurities), target nuclide burn-up and the irradiation and processing time are most accounted for. Theoretical SA assessment has definitely given us a firm ground to set up an optimized process for the production of clinically applicable radioactive product. The specific radioactivity of radionuclide formed via neutron capture followed by radioactive transformation and its application for 177Lu production using indirect route 176Yb (n, gamma) 177Yb (β - decay) 177Lu was evaluated. In contrast to the direct route involving the 176Lu (n, gamma) 177Lu reaction, the indirect production route of 177Lu using 176Yb enriched target gives a product of high SA. However, the product is never carrier free due to the elemental Lu and isotopic 174Yb impurities in the 176Yb enriched target. 175Lu isotope generated from 174Yb (n, gamma) 175Yb (β - decay) 175Lu process and elemental Lu impurities remaining in the 177Lu product (which is chemically separated from 176Yb target) makes the 177Lu SA value strongly decreased. Based on the above mentioned assessment method the effect of several factors (such as elemental Lu impurity, isotopic 174Yb content and isotope dilution) on the 177Lu SA degradation was evaluated. As a result obtained, the specific radioactivity of n.c.a 177Lu produced from 176Yb target was strongly affected by irradiation time and impurities of the target. The production yield and desired specific activity should be compromised to achieve a cost effective production of clinically applicable 177Lu product. The experimental results reported in our previous publications have well agreed with theoretical calculation results.
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Keywords
Radioactivity, Isotope dilution, Lutetium 177, Ytterbium 177, Radioisotopes, Radiopharmaceuticals, Neoplasms, Lutetium 176 target, Ytterbium 176 targets
Citation
Le V.S. (2009). Theoretical assessment of specific radioactivity: the effect of target burn-up, isotope dilution and target purity and the application for 177Lu production. Presentation to the Asia-Pacific Symposium on Radiochemistry '09 (APSORC'09), 29th November - 4th December 2009. Napa, California, U.S.A.