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

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.

A gate evaluation of the sources of error in quantitative90 Y PET

PURPOSE

Accurate reconstruction of the dose delivered by ⁹⁰ Y microspheres using a postembolization PET scan would permit the establishment of more accurate dose-response relationships for treatment of hepatocellular carcinoma with ⁹⁰ Y. However, the quality of the PET data obtained is compromised by several factors, including poor count statistics and a very high random fraction. This work uses Monte Carlo simulations to investigate what impact factors other than low count statistics have on the quantification of⁹⁰ Y PET.

METHODS

PET acquisitions of two phantoms-a NEMA PET phantom and the NEMA IEC PET body phantom-containing either ⁹⁰ Y or ¹⁸ F were simulated using gate. Simulated projections were created with subsets of the simulation data allowing the contributions of random, scatter, and LSO background to be independently evaluated. The simulated projections were reconstructed using the commercial software for the simulated scanner, and the quantitative accuracy of the reconstruction and the contrast recovery of the reconstructed images were evaluated.

RESULTS

The quantitative accuracy of the ⁹⁰ Y reconstructions were not strongly influenced by the high random fraction present in the projection data, and the activity concentration was recovered to within 5% of the known value. The contrast recovery measured for simulated ⁹⁰ Y data was slightly poorer than that for simulated ¹⁸ F data with similar count statistics. However, the degradation was not strongly linked to any particular factor. Using a more restricted energy range to reduce the random fraction in the projections had no significant effect.

CONCLUSIONS

Simulations of ⁹⁰ Y PET confirm that quantitative ⁹⁰ Y is achievable with the same approach as that used for ¹⁸ F, and that there is likely very little margin for improvement by attempting to model aspects unique to ⁹⁰ Y, such as the much higher random fraction or the presence of bremsstrahlung in the singles data.

Fluence to local skin absorbed dose and dose equivalent conversion coefficients for monoenergetic positrons using Monte-Carlo code MCNP6

Conversion coefficients fluence to local skin equivalent dose, as introduced in ICRP Publication 116, 2010, are calculated for positrons of energies ranging from 10 keV to 10 MeV using the code MCNP6. Fluence to dose equivalent conversion coefficients H'(0.07,0°)/Φ are calculated for positrons of energy ranging between 20 keV and 10 MeV. A comparison between operational dose quantity H'(0.07,0°) and the Local-Skin equivalent Dose shows an overall good agreement between these two quantities, except between 60 keV and 100 keV.

Sci-Fri PM: Delivery - 10: Megavoltage x-ray imaging detector based on cerenkov effect

A Monte Carlo simulation was used to study imaging and dosimetric characteristics of a novel design of a megavoltage (MV) x-ray imaging detector. The proposed detector consists of a matrix of optical fibers aligned with the incident x-rays and coupled to an active matrix flat-panel imager (AMFPI) for image readout. The new design relies on Cerenkov effect for MV x-ray imaging and is named CPID (for Cerenkov Portal Imaging Device). When MV x-rays are incident on CPID, they interact within the volume of the detector primarily via Compton effect and pair-production, resulting in electrons and positrons. From these charged particles, those with sufficient energy, trigger production of optical light via Cerenkov effect. The light that is generated in the optical fibre cores within the acceptance angle of the fibers is guided towards the AMFPI. Properties, such as detection efficiency, modulation transfer function, zero frequency detective quantum efficiency (DQE), and energy response of the detector, have been investigated. It has been shown that the proposed detector can have a zero-frequency DQE more than an order of magnitude higher than that of current electronic portal imaging device (EPID) systems and yet a spatial resolution comparable to that of video-based EPIDs. In additional the proposed detector is less sensitive to scattered x-rays than current EPIDs.