Shielding

For most hospitals the natural assumption will be that PET scanning, either on a dedicated ring system or a dual-headed coincidence gamma camera PET (GC-PET) system, will be performed in the Nuclear Medicine Department. It should be remembered that although there are similarities between nuclear medicine and PET, the latter makes use of radiotracers with a much higher photon energy than those usually encountered in nuclear medicine (511 keV compared to 140 keV). Local legislation in the United Kingdom [11, 12] sets the annual whole-body dose limit for unclassified radiation workers at 6 mSv. This is a significant reduction from the previous limit of 15 mSv, and consequently places a greater burden of responsibility on departments to ensure that staff members are provided with the means to keep their doses below this level. Doses to staff include not only the whole-body dose but also the dose to extremities. When handling unsealed sources the key extremity dose is that to the hands. The photon energy of 511 keV means that conventional amounts of shielding with lead or tungsten will not be sufficient and more emphasis on the inverse square law, as a means of reducing dose, may need to be employed.

Currently, [18F]-FDG is not produced in enormous quantities, and on arrival the activity in the multi-dose vial will range from 7 to 20 GBq. These multi-dose vials should be shielded within a lead pot with a minimum thickness of 18 mm, although thicker pots (34 mm) are being introduced by some departments particularly in the light of increasing tracer yields. The design of the dose preparation area should be similar to that used in nuclear medicine. An L-shield with a lead glass insert and a lead base should be used. The thickness of the base and the shield should be of the order of 6 cm and the lead glass window should have a 5.5 cm lead glass equivalent. Measurements of instantaneous dose rate taken from a shielded vial of [18F]-FDG (7-8 GBq) placed behind the L-shield range from 20 |Sv/hr immediately in front of the L-shield to 2 |Sv/hr at 2.0 m from the shield. Clearly, the dose rate immediately in front of the shield is quite high and, as this is the place where the technologist will be standing when drawing up the tracer, it is essential that a good, reliable and fast technique is developed so that the dose received is reduced to a minimum.

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