Equivalent Dose Rate 1 Meter from Neuroendocrine Tumor Patients Exiting the Nuclear Medicine Department After Undergoing Imaging

DISCUSSION

The present study found that EDR-1m was less than 20 μSv/h in all patients, even though the highest values with ¹⁸F-FDOPA or ¹⁸F-FDG were close to that limit (Table 2). Actually, there is no constraint threshold for this dose rate in the European Union or anywhere else in the world. Furthermore, the few thresholds that have been proposed concerned patients leaving the nuclear medicine department after a therapeutic procedure, not after a diagnostic procedure. The lowest and most recently proposed limit was 20 μSv/h (8). In 2010, the practical rules of the Heads of the European Radiological Protection Competent Authorities recommended a limit of 40 μSv/h (9).

When exiting the department, patients injected with ⁶⁸Ga-DOTATOC or ¹²³I-MIBG delivered an average EDR-1m roughly half that delivered by patients injected with other radiopharmaceuticals. For an injected activity of the same order of magnitude, the shorter physical half-life of photons emitted by PET radiopharmaceuticals compensated for their higher energy because the patient had to remain longer before exiting. This finding is a complementary argument for replacing SPECT by PET for imaging of somatostatin receptors.

There were two major differences between PET and SPECT in routine practice. First, a fixed activity was used for patients referred for SPECT, in accord with the scheduled injected activities of the 2 SPECT radiopharmaceuticals in Table 1 and as revealed by the SDs in Table 2. In contrast, the injected activity for PET radiopharmaceuticals was more variable, depending not only on patient body mass (in accord with the scheduled injected activities of the 3 PET radiopharmaceuticals in Table 1) but also on logistic constraints (the delivered activity in the multidose vial for fluorinated radiopharmaceuticals; the eluted activity from the ⁶⁸Ge/⁶⁸Ga generator and the labeling yield for ⁶⁸Ga-DOTATOC).

The second major difference between PET and SPECT in routine practice is that patients who were referred for SPECT left the department a few minutes after injection and came back later for imaging, whereas patients referred for PET did not leave the department until after imaging, at least 1 h after injection (Table 2). This difference in the delay between injection and measurement explains why bladder EDR-1m was significantly greater than sternal EDR-1m for PET radiopharmaceuticals but not for SPECT radiopharmaceuticals. Because all 5 radiopharmaceuticals are excreted through the kidneys, urinary excretion and bladder accumulation of SPECT radiopharmaceuticals are just beginning when the patient exits the department, whereas for the PET radiopharmaceuticals, measured more than 1 h after injection, the bladder has become a radiation source. In this context, the variability of individual biologic half-lives between patients and the frequency of voiding enhance differences in individual equivalent dose rates.

Both factors (more homogeneous injected activity and shorter delay after injection) explain why the EDR-1m values had a much narrower range for SPECT radiopharmaceuticals than for PET radiopharmaceuticals (Table 2).

To the best of our knowledge as derived from an extensive bibliographical survey, no data have yet been published concerning EDR-1m at the time patients leave the nuclear medicine department after undergoing ⁶⁸Ga-DOTATOC or ¹⁸F-FDOPA PET/CT. This lack enhances the interest of the present ⁶⁸Ga-DOTATOC data derived from a rather large sample (n = 53) representative of all NET indications, with a growing demand. The sample size was more limited for ¹⁸F-FDOPA, but those data are probably the first of their kind. Very few data have been published concerning ¹⁸F-FDG (Table 3), ¹¹¹In-pentetreotide (Table 4), or ¹²³I-MIBG (Table 5). Overall, all series are in accordance with the fact that the patient may leave the nuclear medicine department according to the current practice, that is, just after injection for ¹¹¹In-pentetreotide or ¹²³I-MIBG, without risk of exposing the public to excessive radiation. It is also clear that new developments in PET/CT scanners, particularly time-of-flight technology, have allowed a reduction in ¹⁸F-FDG injected activity and consequently a reduced EDR-1m (Table 3). In contrast, Demir et al. (12), who injected on average 3 times more ¹⁸F-FDG than we did, reported EDR-1m measurements of well above 20 μSv/h. Even when it was possible to normalize EDR-1m per megabecquerel of injected activity, a rather large variation in results between studies of similar design could be observed. One possible explanation is the small number of patients in most studies, including ours (apart from ⁶⁸Ga-DOTATOC). Differences in individual functional processes lead to variability in EDR-1m enhanced by PET patients’ lengthened stay in the department compared with SPECT patients. Nevertheless, this factor seems insufficient to explain a ratio of 3 in EDR-1m for ¹¹¹In-pentetreotide between the series of Kurtaran et al. (15) and ours: the waiting time is short and the injected activities are not in the same proportion. A similar question can be raised when comparing EDR-1m values after administration of ¹²³I-MIBG; these values were significantly lower in the series of Ofluoglu et al. (10) than in ours, although the injected activity was almost twice greater. An alternative hypothesis involves the detectors themselves. Although our two detectors, produced by two different manufacturers, gave similar EDR-1m values, one cannot exclude the possibility that different dose rate detectors give discrepant results for the same measurements, particularly when the measurements have been performed over 14 y or more.