The relative contribution of photons and positrons to skin dose in the handling of PET radiopharmaceuticals

Despite recommendations to use syringe and vial shields to reduce exposure of the hands of staff when manipulating PET radiopharmaceuticals, operators sometimes prefer to work without shields, believing that the faster handling limits the equivalent dose. The aim of this work is to show that this approach does not properly consider the contribution of positrons to the dose. Using the Varskin+ code, skin doses were calculated for syringes of various sizes, filled with 18F, 11C or 68Ga solution. Syringes without shielding, or shielded with 2 mm and 10 mm of tungsten were considered. Dose rate values in mSv/s per MBq, averaged on a 1 cm2 surface at a depth of 0.07 mm were calculated for all the above conditions. For example, in the case of 3 mL 18F syringe at 1 mm from the skin, the dose rate without shielding is 1.32E-02 and 8.63E-04 for positrons and photons respectively. For 11C, the corresponding dose rates are 4.70E-02 and 8.90E-04 respectively, and for 68Ga, 8.52E-02 and 9.48E-04. Our results show that the dose due to positrons is the principal component of skin irradiation, by a factor of 3–100, depending on the conditions. The use of shields for syringes and vials is necessary to avoid unjustified skin exposures, that may challenge dose limits. In our opinion, automatic systems for dispensing and allowing injection with shielded syringes, or automatic injectors, are economically justified and should be adopted in PET. •Positrons are not fully self-absorbed in the radiopharmaceutical volume or by the syringe walls.•Skin dose due to positrons is much higher than that due to photons.•Shielding of syringes and vials is necessary, otherwise dose limits may be exceeded.•Even thin shields, optimized for 99mTc, are better than no shield, as they remove positrons.•Automatic syringe dispensers are recommended in PET radiopharmacy.