Assessment of radionuclidic impurities in cyclotron produced ^99mTc

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• 作者：Ond?ej Lebeda ; Erik J. van Lier ; Jan ?tursa ; Jan R¨¢li? ; Alex ; er Zyuzin
• 关键词：Technetium-99  ; m ; Cyclotron ; Proton irradiation ; Radionuclidic impurities
• 刊名：Nuclear Medicine and Biology
• 出版年：2012
• 出版时间：November, 2012
• 年：2012
• 卷：39
• 期：8
• 页码：1286-1291
• 全文大小：419 K

文摘

Introduction

The commercial viability of cyclotron-produced ^99mTc as an alternative to generator-produced ^99mTc depends on several factors. These include: production yield, ease of target processing and recycling of ¹⁰⁰Mo, radiochemical purity, specific activity as well as the presence of other radionuclides, particularly various Tc radioisotopes that cannot be separated chemically and will remain in the final clinical preparation. These Tc radionuclidic impurities are derived from nuclear interactions of the accelerated protons with other stable Mo isotopes present in the enriched ¹⁰⁰Mo target. The aim of our study was to determine experimentally the yields of Tc radioisotopes produced from these stable Mo isotopes as a function of incident beam energy in order to predict radionuclidic purity of ^99mTc produced in highly enriched ¹⁰⁰Mo targets of known isotopic composition.

Methods

Enriched molybdenum targets of ⁹⁵Mo, ⁹⁶Mo, ⁹⁷Mo, ⁹⁸Mo and ¹⁰⁰Mo were prepared by pressing powdered metal into an aluminum target support. The thick targets were bombarded with 10 to 24 MeV protons using the external beam line of the U-120 M cyclotron of the Nuclear Physics Institute, ?e?. The thick target yields of ⁹⁴Tc, ^94mTc, ⁹⁵Tc, ^95mTc, ^96m + gTc and ^97mTc were derived from their activities measured by ¦Ã spectrometry using a high purity Ge detector. These data were then used to assess the effect of isotopic composition of highly enriched ¹⁰⁰Mo targets on the radionuclidic purity of ^99mTc as a function of proton beam energy. Estimates were validated by comparison to measured activities of Tc radioisotopes in proton irradiated, highly enriched ¹⁰⁰Mo targets of known isotopic composition.

Results

The measured thick target yields of ⁹⁴Tc, ^94mTc, ⁹⁵Tc, ^95mTc, ^96m + gTc and ^97mTc correspond well with recently published values calculated via the EMPIRE-3 code. However, the measured yields are more favourable with regard to achievable radionuclidic purity of ^99mTc. Reliability of the measured thick target yields was demonstrated by comparison of the estimated and measured activities of ⁹⁴Tc, ⁹⁵Tc, ^95mTc, and ^96m + gTc in highly enriched ¹⁰⁰Mo (99 % ) targets that showed good agreement, with maximum differences within estimated uncertainties. Radioisotopes ^94mTc and ^97mTc were not detected in the irradiated ¹⁰⁰Mo targets due to their low activities and measurement conditions; on the other hand we detected small amounts of the short-lived positron emitter ⁹³Tc (T_? = 2.75 h). In addition to ^99mTc and trace amounts of the various Tc isotopes, significant activities of ⁹⁶Nb, ⁹⁷Nb and ⁹⁹Mo were detected in the irradiated ¹⁰⁰Mo targets.

Conclusions

Radioisotope formation during the proton irradiation of Mo targets prepared from different, enriched stable Mo isotopes provides a useful data base to predict the presence of Tc radionuclidic impurities in ^99mTc derived from proton irradiated ¹⁰⁰Mo targets of known isotopic composition. The longer-lived Tc isotopes including ⁹⁴Tc (T_? = 4.883 h), ⁹⁵Tc (T_? = 20.0 h), ^95mTc (T_? = 61 d), ^96m + gTc (T_? = 4.24 d) and ^97mTc (T_? = 90 d) are of particular concern since they may affect the dosimetry in clinical applications. Our data demonstrate that cyclotron production of ^99mTc, using highly enriched ¹⁰⁰Mo targets and 19-24 MeV incident proton energy, will result in a product of acceptable radionuclidic purity for applications in nuclear medicine.