An effective technique for the storage of short lived radioactive gaseous waste
Lutz Schweiger
University of Aberdeen, School of Medical Sciences, John Mallard Scottish PET
Centre, Foresterhill, Aberdeen AB252ZD, UK
Received 7 May 2010; revised 13 April 2011; Accepted 29 April 2011. Available
online 7 May 2011.
Abstract
An effective technique is described to deal with volatile, short lived radioactive waste
generated as a result of the routinely produced positron emission tomography (PET)
radiopharmaceutical 2-deoxy-2-[18F]fluoro-D-glucose (FDG). All radioactive gases
and aerosols created during the synthesis are collected and stored safely in
commercially available TEDLAR gas sampling bags. Once these collected PET byproducts decay, the TEDLAR gas bags can be easily emptied and reused. This
improved technique is effective, safe, reliable and economical.
Highlights
Volatile radioactive waste generally discharged during radiopharmaceutical
production. We use gas bags to safely store radioactive gases and aerosols.
Consequence is no radioactive discharge into atmosphere.
Keywords: 2-Deoxy-2-[18F]fluoro-D-glucose;
Radioactive waste
Positron
emission
tomography;
1. Introduction
[18F] Fluorine (F-18) is one of the most popular short lived (t1/2=109.8 min) positron
emitting radioisotopes (Wilson et al., 2008), which is usually cyclotronically
generated in liquid form by means of a 18O(p,n)18F nuclear reaction. Its major
application as a label for radioactive tracers used in combination with the non
invasive, metabolic PET imaging technique is of high importance (Cai et al., 2008). In
recent years, one particular F-18 labelled radiopharmaceutical, FDG, has even been
elevated to the most successful and widely used clinical imaging PET tracer in
oncology, neurology and cardiology ( [Hoh, 2007] and [Yu, 2006] ). Besides being
well established, the production of the PET radioisotope as well as the labelled
product are routinely accompanied with the generation of volatile radioactive gaseous
and aerosolic by-products, e.g. [18F]HF, [13N]NH3, whose retainment, storage and
disposal has been shown to be financially and environmentally challenging ( [Pascali
et al., 1996] and [Calandrino et al., 2007] ). Although nowadays most commercially
available hot cells and automated synthesis modules are conceptualised and facilitated
with trapping devices, such as activated charcoal filters, chemical absorbents and
liquid nitrogen traps (Calandrino et al., 2007; Hot Cell Manufacturer COMECER,
personal communication, 2009), still, not all created radioactive gases and aerosols
are effectively immobilised. Hence worldwide many PET Centres attempt to deposit
radioactive waste using expensive, large gas compression storage systems that need
additional shielding or the generated by-products are simply discharged directly into
the atmosphere.
The disposal of radioactive waste gases in PET centres around Scotland is regulated
by the Scottish Environment Protection Agency (SEPA) by means of the Radioactive
Substances Act (1993). Medical cyclotron facilities such as the John Mallard Scottish
PET Centre in Aberdeen have to comply strictly with daily and annual gaseous
disposal limits as agreed and authorised by SEPA. However, prior to the disposal it is
the authorisation holder's responsibility and obligation to minimise radioactive
gaseous waste using the best practicable means. Therefore our permanent aim is not to
discharge but to safely retain as much radioactive waste possible, which is created
during any PET isotope and PET tracer production.
2. Materials and methods
2.1. Radiation Protection
In August 2010, SEPA authorised a daily disposal limit of 74 GBq as well as an
annual disposal limit of 500 GBq for any radionuclide (18F, 11C, 13N, 15O) generated
as a product or by-product of cyclotron operation taken together.
All components of the radiation stack monitoring and data management system
(CMS-PET) for continuous radioactive gas detection in the air extract system were
purchased from and installed by Lab Impex Systems. The ventilation outlets of the hot
cells were connected to the stack. The average stack flow rate was 2300 m 3/h. The
level of background radiation detected by the stack detector was on an average 76
kBq/m3. TEDLAR gas sampling bags were purchased from GRACE/ALLTECH
Associates. TEDLAR gas sampling bags (volume: 10 1 and 40 l) were connected via
PARFLEX or PTFE tubing and SWAGELOK fittings. On an average five FDG
syntheses could be carried out before they had to be emptied. The contents of all
TEDLAR bags were measured and checked using a contamination monitor (MiniMonitor—Series 900 Thermo Fisher) and only then were deflated every Monday
morning before FDG production. To avoid any malfunction, as part of our FDG
preventive maintenance plan, the gas bags were replaced every 3 months.
2.2. [18F]Fluoride production
[18F]Fluoride was prepared by proton bombardment of 97% enriched [ 18O]H2O
(Isotrade GmbH, Germany) by the 18O(p,n)18F nuclear reaction. The silver target (1.1
ml) was pressurised to 600 psi and irradiated with 11 MeV protons produced by the
CTI/SIEMENS RDS-111 cyclotron at the John Mallard Scottish PET Centre in
Aberdeen. Irradiations of 50 min with a beam current of 27 μA were typically used.
At the end of bombardment (EOB) the target was unloaded within 5 min using argon
gas. The 10 l Tedlar gas bag was filled up to 20%. On average a maximum surface
dose rate of 5000 μSv/h was measured after the end of the unloading process.
Unloading the target without the use of this 10 l gas bag would discharge on average
1.6 GBq.
2.3. FDG production
The John Mallard Scottish PET Centre is currently producing FDG on average once a
day, three times a week. On average, 34 GBq of [18F]fluoride were transferred with
argon gas to the remote controlled FDG synthesis module (GE Healthcare
TRACERlab FXFDG synthesiser) that was contained in a lead shielded mini hot cell
(Von Gahlen). The FDG synthesis was carried out according to Hamacher et al.
(1986) via nucleophilic substitution followed by acid hydrolysis. Chromafix,
Chromabond purification cartridges and pharmaceutical grade FDG production
reagent kits (K-320 TF) were purchased from ABX GmbH, Germany. FDG
production took ca. 35 min and yielded on average 17 GBq of FDG at end of
synthesis (EOS). The 40 l Tedlar gas bag was filled up to 20%. On average a
maximum surface dose rate of 1000 μSv/h was measured at EOS. Without the use of
this 40 l gas bag a radioactive discharge of on average 0.5 GBq would be detected
during every FDG synthesis.
3. Results
For the [18F]fluoride production the cyclotron target is loaded with 97% enriched
[18O]H2O, pressurised and then bombarded. Although mainly [18F]fluoride is being
formed during this process also 13N is created (Guillaume et al., 1991) via the
16
O(p,α)13N reaction. After EOB the target unload process starts. For this the initial
target pressure is vented and subsequently the aqueous [18F]HF bolus is transferred
via argon gas to the target collect vial of the FDG synthesis module (Fig. 1). It is
during this high pressure unloading process where radioactive waste gases and
aerosols, mainly in the form of [13N]NH3 and [18F]HF, would be discharged via the
vent of the target delivery vial into the hot cell. Therefore many PET Centres simply
discharge the waste through the ventilation system into the atmosphere or are
equipped with devices such as gas compression systems to collect, transfer and store
the waste in additional lead shielded tanks. However these devices are not only very
expensive but also consume a lot of space. We on the other hand use a practical,
effective, reliable and economical alternative. We connected a small, 10 l TEDLAR
gas sampling bag (Fig. 2) to the vent of the target delivery vial and positioned it on
top of the synthesis module. Consequently, during the unload process of the target, the
radiation monitoring system did not register any elevated radiation levels in the stack
(Fig. 3). Hence any potential radioactive discharge is safely deposited in the bag
inside the lead shielded hot cell.
Fig. 1: Graphical representation of the GE FxFDG synthesis module. The 10 l
TEDLAR gas sampling bag is connected to the vent of the target collect vial with
Paraflex tubing while the 40 l TEDLAR gas bag is connected to the exhaust of
the vacuum pump via PTFE tubing.
Fig. 2: Photograph of a 10 l TEDLAR gas sampling bag.
82
Radioactivity [kBq/m 3]
80
78
76
74
72
70
0
1000
2000
3000
4000
5000
6000
7000
8000
Time [s]
Fig. 3: Level of background radiation measured before and after working with
radioactivity is on average 76 kBq/m3. No elevated levels were detected during
bombardment (start of beam, SOB, and end of beam, EOB), unloading of the
target and production of the tracer (start of synthesis, SOS, and end of synthesis
(EOS)). Therefore, no radioactivity was discharged. All volatile radioactive byproducts were retained in the TEDLAR gas bag.
After the target is unloaded the FDG production programme can be started. As a
matter of fact the FDG synthesis contains two reaction steps during which volatile
radioactive substances are created. Both steps, the azeotropic distillation of the
water/acetonitrile solution and the nucleophilic substitution generate mainly
[18F]CH3F and [18F]fluoride as suggested by Calandrino et al. (2007). During both
steps, volatile radioactive contaminants are supposed to be trapped by a liquid
nitrogen trap that is usually installed between the vacuum pump and the reaction
vessel (Fig. 1). Although the liquid nitrogen does retain some levels of radioactivity,
most volatile substances channel through the vacuum pump system into the hot cell
and then are extracted into the ventilation system. We solved this problem by
connecting a 40 l TEDLAR gas bag to the exhaust of the vacuum pump. The gas bag
was this time positioned in the neighbour hot cell and as a result no radioactive
discharge was registered during the FDG radio tracer production (Fig. 3). This novel,
effective, simple, environment friendly technique contains radioactive gases and
aerosols generated during the synthesis of FDG without the necessity of using
additional space and shielding.
4. Conclusions
We found an effective solution for the deposit of volatile radioactive by-products
created during the FDG synthesis. Instead of environment unfriendly radioactive
discharge or expensive and spacious storage systems we opted for the installation of
commercially available TEDLAR gas sampling bags to safely retain the short lived
waste. The gas sampling bags benefit from chemical resistance, low permeability and
furthermore their high tensile strength resists puncture. In addition to this, a variety of
sizes and bag fittings makes them easy to connect to the ordinary synthesis equipment
as well as to position them in place. Our technique is simple, quick, safe, inexpensive
and environment friendly.
Acknowledgements
The author would like to thank Mr. Stuart Craib for his skilled technical assistance
and Dr Steve McCallum for helpful discussions.
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