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

Abstract

Background: 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.

Materials and methods: 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.

Results: 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.

Conclusions: 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.

Keywords: Manipulation of radionuclides; PET radiopharmaceuticals; Positrons; Skin dose.