Wholebody Dose

The whole-body doses to staff in PET are slightly higher than those typically seen in nuclear medicine.

Benatar et al looked at the whole-body doses received by staff in a dedicated clinical PET centre and then used measured data to estimate the doses which might be received by staff in departments using [18F]-FDG, but not originally designed to do so [13]. The results showed that in a dedicated unit the average whole-body dose received by each technologist was around 5.5 |Sv per patient study and approximately 14.0 |Sv per day. It should be noted that the new annual dose limit for unclassified workers of 6 |Sv equates to an average daily dose of 24 |Sv . With the exception of same-day rest/stress [99mTc]-Myoview studies [14], the average dose per patient study in nuclear medicine is only 1.5 |Sv [15], which is a lot lower than for PET studies. However, the throughput per technologist in a PET unit is likely to be lower than that in nuclear medicine and therefore, the daily whole-body dose received is comparable. This work was carried out in a dedicated purpose-built PET unit where the technologists are shielded from any radioactive sources for the majority of the working day and only had significant contact during injection and positioning of the patient. In this unit the control room is separate to, and shielded from, the scanning area and the room where the patients wait after injection prior to the scan. This contrasts with the set-up commonly seen in nuclear medicine where the control console is often sited adjacent to the gamma camera with no shielding between the two. In order to make estimates of the doses likely to be received by technologists using [18F]-FDG in an existing department, Benatar et al measured the instantaneous dose rates at 0.1 m, 0.5 m, 1.0 m, and 2.0 m from the anterior chest wall in patients immediately following their [18F]-FDG injection. The average dose rates per MBq injected at each of the four distances were used to estimate the whole-body dose that could be received by technologists. Figure 15.6 shows the whole-body dose received during a one-hour scan commencing one hour after injection based on an injected activity of between 100 and 200 MBq. These injected activities were used to try to reproduce the likely usage on a gamma camera PET system. As can be seen, at a distance of 2 m with an average injected activity of 200 MBq the whole-body dose received is of the order of 12 |Sv. Although in isolation this dose may not seem particularly high, it must be remembered that this is the dose from each individual PET patient whilst each technologist is likely to scan many more than this in a year, as well as a number of nuclear medicine patients. Five hundred PET patients in a year would take the technologist close to the 6 | Sv annual limit. Therefore, if PET is going to be introduced into existing departments, some thought needs to be given to the positioning of the control console relative to the scanner itself. Figure 15.7 shows the instantaneous dose rates measured from a patient one hour after being injected with [18F]-FDG.

The dose rate immediately next to the bed is quite high (120 |Sv.hr-1). At one meter from the bedside it drops to 15 |Sv.hr-1 but significantly, at the foot end of the bed the dose rate is only 4.0 |Sv.hr-1. Obviously, the dose received by a technologist can be reduced by careful planning even within a pre-existing unit. An alternative, if space is a constraint and the operator's console must be included in the scanning room, is to use a mobile lead shield. A practical shield, on wheels, has been reported as reducing the exposure rate by 90% from approximately 20 |Sv.hr-1 at an operator's console roughly one meter from a patient who had been injected with 370 MBq of [18F]-FDG to 2 |Sv.hr-1 [16]. The comparable exposure rate from a typical [99mTc]-MDP bone scan patient at a similar distance is 6 |Sv.hr-1. Consideration must also be given where a patient who has been injected with a positron-emitting tracer waits during the uptake period. With [18F]-FDG it is important that the patient is able to rest, preferably

Figure 45.6. Estimated whole body doses likely to be received by technologists during 1 hour scan beginning 60 minutes after [18F]-FDG injection.

Figure 45.6. Estimated whole body doses likely to be received by technologists during 1 hour scan beginning 60 minutes after [18F]-FDG injection.

Figure 15.7. Dose rates measured from a patient in the scanner 55 minutes after injection of 334MBq of [18F-FDG. (Reproduced from Valk PE, Bailey DL, Townsend DW, Maisey MN. Positron Emission Tomography: Basic Science and Clinical Practice. Springer-Verlag London LTD, 2003, p. 789.)

lying down, and should not be allowed to leave the department during this time. Ideally a separate room should be provided for this but in the event that this is not available and that they will have to share a room it should be remembered that the dose rates from patients immediately following injection can be quite high and may present a radiation hazard. Benatar et al [13] used the dose rate data to estimate the doses which might be received by people coming into contact with injected patients during the one-hour uptake period. Figure 15.8 shows that, following administration of [18F]-FDG, the dose received during the following hour can range from just over 0.3 |Sv if sitting right next to the patient down to 0.16 |Sv at 2 m.

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