Study design
This prospective dosimetry study focused on measuring the personal equivalent dose Hp (0.07) received during a specific 18F-FDG dose drawing procedure. This was performed in a paired design with two operational EDs fitted on the palm side of the index finger, namely in the P1 and P3 positions.
The study was approved by several committees in 2012, i.e., the Institutional Review Board (CPP Sud-Ouest Outre-Mer), the French Consultative Committee for Data Processing in Health Research (CCTIRS), and the French National Commission on Information Technology and Civil Liberties (CNIL).
Participants
The study participants were ten NMTs, all right-handed, who have worked regularly in 18F-FDG preparation in the NM departments of the Bordeaux University Hospital and the Côte Basque-Bayonne Hospital, each employing five study subjects. Taking into account that whatever the used hand (dominant or non-dominant), the gesture is realized identically, because of the hot cell layout and the kind of device used. All participants signed an informed consent before enrollment. Demographic data were collected from each participant.
Dosimetry procedures
We used Unfors® NED electronic operational dosimeters (Unfors Instruments AB, Billdal, Sweden) equipped with semiconductor sensor technology, whose technical measuring characteristics are fully adapted to γ radiation: dose rate range 0.18 mSv/h–9.0 Sv/h, dose range 50 nSv–9999 Sv, start trigger level 0.27 mSv/h, and end trigger level 0.18 mSv/h. The EDs underwent mandatory annual calibration by the manufacturer. The uncertainty measure of the ED stipulated on the calibration certificate issued by the manufacturer was 6 %.
The advantages of the EDs consisted of a direct reading of the measured dose and lack of reading delay typical of TLDs, along with the latter’s risk of loss, damage, and negative results displayed when below detection threshold (i.e., 100 μSv with ring TLDs).
The radiation measurement focused on the process of 18F-FDG dispensing by manual drawing into a syringe (Fig. 1).
The radiation measuring process comprised the following working steps: the sensors were fitted onto the palm side of the operator’s P1 and P3 (Fig. 2); the two dosimeters were turned on; the operator’s hands, protected by disposable non-sterile gloves, were placed into the glove port of the high-energy hot cell (Lemer Pax in Bayonne; Medisystem in Bordeaux); the tungsten shield syringe complex was fitted onto the 30-cm extension tube (the opposite end of which had been previously connected to the needle inserted into the multidose vial into which a vent needle fitted to a vent filter had also been inserted prior to the radiation measurement phase); the dose was dispensed by manual drawing into the syringe; the excess air was purged; the syringe was disconnected from the extension tube; the three-way stopcock-Y-connector combo was fitted onto the syringe; the operator’s hands were withdrawn from the glove port; and the two dosimeters were turned off and the study dosimetry ended. The choice fell on this truncated manipulation sequence for reasons of maximal reproducibility between the two investigational centers. While the utilized material and the modus operandi were identical in the two departments, there were minor layout differences, namely in the transfer of 18F-FDG out of the hot cell, hence the decision to stop the study measurement after the fitting the three-way stopcock.
Into the case report form, we transcribed the dose displayed by the EDs and additional parameters of interest for their potential impact on the disparity between Hp (0.07)P3 and Hp (0.07)P1: volume and activity of the multidose 18F-FDG vial, volume and activity of the withdrawn 18F-FDG dose, degree of discomfort caused by wearing the two sensors, the hand used for connecting the three-way stopcock to the syringe, and the duration exposure of each detector.
Statistical analysis
The study was powered to detect a minimum difference of 35 μSv between Hp (0.07)P1 and Hp (0.07)P3, with a variance of 120 μSv, according to a pilot study realized with only one ED and a NMT (unpublished data), a two-tailed type I error (α) of 0.05 and a power (1-β) of 90 %; the conservative hypothesis of no correlation between the doses received by the two phalanges was assumed. The calculation resulted in a requirement of at least 126 paired measurements (nQuery Advisor v6.0; Statistical Solutions Ltd., Cork, Ireland), but this number was brought to 200, in order to allow analysis of the explanatory variables impacting the disparity in radiation incurred by the two phalanges.
The disparities between Hp (0.07)P3 and Hp (0.07)P1 were estimated by a mixed-effects linear regression model. Logarithmic transformation of the measured dose was required in order to respect the model’s assumption of normal distribution of residuals. The factors associated with the disparity between the doses received by the two phalanges were selected by univariable analysis (at p value of <0.2) and were entered into the multivariable mixed model. All non-significant factors were eliminated by backward stepwise selection (at p value of <0.05) and verification that those factors were not confounders. The regression model is described by the following equation:
$$ \log \left(\mathrm{H}\mathrm{p}\ {(0.07)}_{\mathrm{P}3}\right)- \log \left(\mathrm{H}\mathrm{p}\ {(0.07)}_{\mathrm{P}1}\right)=\mathrm{intercept}+{\beta}_1\times {\mathrm{factor}}_1+\dots +{\beta}_i\times {\mathrm{factor}}_i $$
However, it is more straightforward when it is rewritten as
$$ \frac{\mathrm{Hp}\ {(0.07)}_{\mathrm{P}3}}{\mathrm{Hp}\ {(0.07)}_{\mathrm{P}1}}={e}^{\mathrm{intercept}}\times {e}^{\beta_1\times {\mathrm{factor}}_1}\times \dots \times {e}^{\beta_i\times {\mathrm{factor}}_i} $$
Thus, the exponentiated regression coefficients (β
1
,…, β
i
) estimate the per unit increases in the value of the Hp (0.07)P3/Hp (0.07)P1 ratio.
All statistical analyses were performed with SAS software, version 9.3 (SAS Institute, Cary, NC, USA).
We had performed several preliminary studies that verified a good ED reliability.
Operators routinely perform the manipulation sequence almost identically regardless of their handedness, except in the last step of the study sequence when the right and left hands were interchangeably used to fit the three-way stopcock–Y-connector combo onto the syringe. Therefore, the hand used in that last step was included as a potential explanatory variable in the mixed-effects linear model.