Reduction of 68Ge activity containing liquid waste from 68Ga PET chemistry in nuclear medicine and radiopharmacy by solidification
PET with 68Ga from the TiO2- or SnO2- based 68Ge/68Ga generators is of increasing interest for PET imaging in nuclear medicine. In general, radionuclidic purity (68Ge vs. 68Ga activity) of the eluate of these generators varies between 0.01 and 0.001%. Liquid waste containing low amounts of 68Ge activity is produced by eluting the 68Ge/68Ga generators and residues from PET chemistry. Since clearance level of 68Ge activity in waste may not exceed 10 Bq/g, as stated by European Directive 96/29/EURATOM, our purpose was to reduce 68Ge activity in solution from >10 kBq/g to <10 Bq/g; which implies the solution can be discarded as regular waste. Most efficient method to reduce the 68Ge activity is by sorption of TiO2 or Fe2O3 and subsequent centrifugation. The required 10 Bq per mL level of 68Ge activity in waste was reached by Fe2O3 logarithmically, whereas with TiO2 asymptotically. The procedure with Fe2O3 eliminates ≥90% of the 68Ge activity per treatment. Eventually, to simplify the processing a recirculation system was used to investigate 68Ge activity sorption on TiO2, Fe2O3 or Zeolite. Zeolite was introduced for its high sorption at low pH, therefore 68Ge activity containing waste could directly be used without further interventions. 68Ge activity containing liquid waste at different HCl concentrations (0.05–1.0 M HCl), was recirculated at 1 mL/min. With Zeolite in the recirculation system, 68Ge activity showed highest sorption.
Keywords68Ge Waste Sorbent Generator
PET scintigraphy with 68Ga-labelled peptides is of increasing interest in PET imaging in nuclear medicine [1, 2, 3, 4, 5, 6, 7, 8] and performed at >40 centres in Europe (current situation, 2010). In general, radionuclidic purity (RNP) of the eluate of these generators varies between 0.01 and 0.001% (68Ge vs. 68Ga activity). Liquid waste containing low amounts of 68Ge activity (further referred as 68Ge waste) is produced by eluting the 68Ge/68Ga generators and residues from PET chemistry. For several reasons in our facility, solid 68Ge containing waste (t½ = 9 months) is preferred over liquid waste. Since clearance level of 68Ge activity in waste may not exceed 10 Bq/g, as stated by European Directive 96/29/EURATOM, our aim was to reduce 68Ge activity in solution from >10 kBq/g to <10 Bq/g; which implies the solution can be discarded as regular waste. As an example: when a 1,110 MBq 68Ge/68Ga generator with a RNP of 0.005%, is eluted with 6 mL 1 M HCl, the eluate contains 55.5 kBq 68Ge activity. Eluting a 1,110 MBq 68Ge/68Ga generator three times a day, 200 days per year, and for 1 year will result in 33.3 MBq in 3.6 L (9.25 kBq/g). Although national legislation within the EU may vary per country, the rules may only be more strict. For example in our country, The Netherlands, legislation does not allow to store radioactive waste with half-lives of more than 100 days at local institutes for more than 2 years. After 2 years it is obligatory to store the here described waste in a special external waste facility, and at additional costs. In this facility waste is preferably compressed, therefore, liquid content is limited to <1% (v/w). The purpose of this study was to quantify 68Ge activity in our 68Ge liquid waste, and to concentrate and to transform 68Ge liquid waste to solid waste. In short: 68Geliquid ⇒ 68Gesolid, initially on a small scale. Liquid 68Ge waste was transformed to 68Ge solid waste by sorption of TiO2, Fe2O3 (325 and 500 mesh, respectively, Sigma-Aldrich, Zwijndrecht, The Netherlands) and Zeolite (Na2Al2Si3O10·2H2O, Zeolyst international, Conshohocken, PA, USA). Elution of 68Ge/Ga generator is performed with HCl, therefore 68Ge waste is acidic. Zeolite was investigated because high sorption at low pH could be achieved (manuscript Kamalika Roy submitted).
68Ge-containing liquid waste
Fractionated elution of the generator revealed at the activity of 68Ge (eg. Bq per mL) was constant during elution, whereas the main 68Ga activity could be collected in a small volume (1 mL) . Eluate from generators and left-over after ion-exchange prior radiolabelling is acidic and contain a certain amount of 68Ge activity [4, 8]. Quantification of 68Ge activity was performed in a well-type gamma counter as described earlier . During 5 years of daily practice with 4TiO2- and 4SnO2-based 68Ge/68Ga generators (0.4–2 GBq per generator) the total amount of 68Ge activity was 160 MBq in 48 L.
TiO2 and Fe2O3 as sorbent
Sorption of 68Ge activity on variable amounts of TiO2 and Fe2O3 was investigated with 1 mL samples of 68Ge waste as f[pH] (pH range 1–10) and time (range 1–24 h). These results were used for further up scaling. Typical example: 150 mL of 68Ge waste was mixed with ±1.5 g of TiO2 or Fe2O3 and centrifuged at 200×g. One mL of supernatant was taken as a sample and 68Ge activity was quantified 24 h after sampling as described earlier .
Results and discussion
TiO2, Fe2O3 and Zeolite as sorbent, lower 68Ge-containing liquid waste down to 10 Bq/g. Recirculation procedure using Zeolite as sorbent is preferred since less intervention with 68Ge containing liquid waste has to be performed and solid waste mass is reduced.
The authors wish to thank Timo Kleijn for technical assistance.
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