This study investigated the possible impact of the amount of peptide, administered in the 177Lu-DOTATATE preparation, on the absorbed dose in tumors and normal tissues at the first PRRT cycle, and in relation to the patients’ total tumor SSTR expression (tTSSTRE). The patients who underwent PRRT on the day of 177Lu delivery received a 177Lu-DOTATATE-preparation with an amount of peptide in the lower range, whereas patients who underwent PRRT 4 days after 177Lu delivery received a preparation with an amount of peptide in the higher range. The patients’ tumor load was assessed on the SPECT-examination performed 24 h after 177Lu-DOTATATE-infusion. In order to assure that almost all tumors were included in the field-of-view of the abdominal SPECT, patients with mainly liver metastases were selected. In this retrospective setting, the 24 h SPECT examination was utilized for the calculation of the tTSSTRE, although at this time point, it is uncertain what the remaining tumor radioactivity might represent, regarding the process of ligand-receptor dissociation and recycling and the metabolism of the 177Lu-DOTATATE molecule [12, 26,27,28]. Thus, in a prospective study, the optimal time point to calculate the tTSSTRE would be approximately 3 h after 177Lu-DOTATATE-infusion when the tumor uptake is the highest [29].

In this study, it was not possible to demonstrate any influence from the administered amount of peptide in the 177Lu-DOTATATE-preparation on the absorbed dose in tumors or in the normal tissues (kidney, spleen, liver), or on the corresponding tumor-to-normal tissue ratios. No relation was found between the administered amount of peptide and the patients’ total tumor somatostatin receptor expression (tTSSTRE) either. Our findings are in contrast to two recent reports, one comparing standard peptide amounts versus both high and low amounts of peptide administered in the 177Lu-DOTA-3-iodo-Tyr3-octreotate (177Lu-HA-DOTATATE) preparation [30] and another comparing radioactive uptake of 177Lu-HA-DOTATATE with or without postponing the long-term SSA medication [31]. Siebinga et al. studied 13 patients receiving 15 cycles with a high peptide amount of 346 ± 33 μg (mean ± SD) and found decreased uptake in tumors, spleen and kidney as compared to 15 cycles administered with a standard peptide amount of 178 ± 8.8 μg, and with similar uptake in liver, blood and bone marrow. In 15 patients receiving 15 cycles with low peptide amount of 109 ± 6.6 μg, decreased uptake was found in tumors and increased uptake was found in spleen, as compared to 15 cycles administered with a standard peptide amount of 202 ± 15 μg, and with similar uptake in kidney, liver, blood and bone marrow [30]. In line with their findings, Veerman et al. reported a clear decline in liver and spleen uptake of 177Lu-HA-DOTATATE in patients continuing long-acting SSAs during PRRT, as compared to those who stopped SSAs before treatment start, although the uptake in tumors, kidneys, bone marrow and blood pool was similar between groups. These conflicting results are likely explained by the different methodology regarding the choice of PRRT cycles, peptide amounts, effect metrics (uptake versed dosimetry), NET types and number of patients. While Siebinga et al. retrospectively compared different PRRT cycles in the same patients, our study exclusively focuses on the first PRRT cycle in order not to risk confounding factor of therapy effects on the dose response. In contrast to the present study, which compared the absorbed doses in the tumors and normal tissues based on 7-day dosimetry, both Siebinga et al. and Veerman et al. reported differences in tissue uptake on the 24 h SPECT/CT. Neither did Siebinga et al. or Veerman et al. relate their results to the patient’s total SSTR expression, as in the present study.

Our findings contrast those in earlier imaging studies, such as SPECT with 111In-pentreotide [22] and 68Ga-DOTATOC PET/CT [23] where generally lower amounts of injected peptide are used compared to PRRT, which is a probable confounder in the comparison.

Further, except for the differences in the administered amounts of peptide between imaging studies and PRRT reports (also including the present one), the time point for measurements is diverse. Thus, PET/CT imaging at one-hour post-injection of 68Ga-DOTATOC/TATE differs very much from the SPECT/CT registrations starting at 24 h after initiation of PRRT. Considering the time frame for the receptors to resurface (7 to 24 h) [12, 27, 28], 68Ga-DOTATOC/TATE-PET/CT mainly registers the influx of the ligand-receptor complex before any dissociation or metabolization has occurred. SPECT/CT performed after 24 h will, by contrast, encounter processes of receptor ligand dissociation completed within 6 h, and both ligands and receptors recirculate [12]. It is further uncertain how much of the radioactivity that is left in the tumor cells 24 h post injection, as it was shown by Anderson et al. that only 50% of the initially incorporated radioactivity remains after 12 h when using carcinoid cells cultures in vitro [10]. Consequently, the radioactivity registered on SPECT at 24 h most likely represented a fraction of the initial radioactivity internalized with the receptors. This will consequently affect the assessment of tTSSTRE, based on 24-h SPECT/CT, as compared to a similar estimation using 68Ga-DOTA-SSA-PET/CT at 1 h, and also impact the measurements of tissue uptake at 24 h SPECT [30] versus that of absorbed dose based on subsequent SPECT 1 to 7 days as in the present study. Additional factors adding to the differences between the PET-experiments and the therapy setting are the intravenous amino acid infusion during PRRT for kidney protection that may have impacted the 177Lu-DOTATATE biodistribution at PRRT versus that of 68Ga-DOTATOC/TATE at PET/CT, and the difference between the two preparations regarding the radiometal and the peptide, since any change of the radioligand alters its affinity and behavior [8].

Dosimetry during PRRT was performed according to procedures developed at our center and have been shown to be reliable for normal organ dosimetry [4]. Up to three of the largest tumors with the highest 177Lu-DOTATATE uptake on SPECT per patient were included for dosimetry. The precision in these tumor measurements, performed with a technique primarily developed for normal tissues, with homogenous distribution, may not have been as high as desirable for the present assessment. Although homogeneous tumor areas were chosen for tTSSTR analysis, minor irregularities in the tumor uptake may have influenced the study results. Further, the influence of the partial volume effects is yet a factor of concern. Probably, this predominately affected the group of patients for whom low cut-off, rather than high cut-off SUV VOIs, were applied in the SPECT examinations for tumor delineation. Visually, it was however clear that many small tumors were excluded from the tumor VOIs when the SUV cut-off step was applied. There were wide variations in tumor load and SSTR expression between our P-NET and SI-NET patients, and also within each tumor group. Thus, in order for the semiautomated soft-ware delineation (tumor VOIs) to correspond to the morphological tumor burden on CT, the tTSSTRE calculations required the use of two different SUV cut-off thresholds, in P-NET and SI-NET-patients, with large tumor load, but similar cut-off was feasible for all patients with a low tumor burden. This was not unexpected, considering our previous findings of different absorbed doses to tumor in P-NETs and SI-NETs [32]. Thus, the fact that it was not possible to apply one SUV cut-off threshold for all patients, may accordingly have introduced a bias in the tTSSTRE data.

To achieve the tTSSTR, we applied 42% iso-contour VOIs, originally adapted to delineate tumors on FDG-PET to accomplish a VOI size that fits the tumor size on CT. There is thus no support that the 42% iso-contour tumor VOIs are optimal in the present setting of 177Lu-DOTATATE-SPECT and other iso-contour percentages may be more appropriate. In the present evaluation, the tumor VOIs on SPECT fairly well-corresponded to tumor size on CT, and 42% iso-contour VOIs were therefore applied as a starting point for our assessment.

To our knowledge, this is the first study to examine the potential influence of the amount of administered peptide in the 177Lu-DOTATATE preparation, on the absorbed dose in the tumors, and also taking the patient’s tTSSTRE into account. Limitations of this study, including 40% (203/510) of our P-NET and SI-NET patients, are its retrospective design over a decade with use of different SPECT/CT gamma cameras, the non-standardized SUV cut-off applied for tumor delineation on the SPECT/CT examinations, assessment of tTSSTRE at 24 h rather than during the tumor uptake peak at 3–4 h, and that the dosimetry technique developed for normal organ dosimetry was applied to also calculate the absorbed dose in the tumors [29].

In conclusion, the amount of administered peptide in the 177Lu-DOTATATE preparation did not correlate to the absorbed dose in the tumors and normal organs and was unrelated to the patients’ total tumor somatostatin receptor expression (tTSSTRE). Given the sparse evidence in the literature of the impact of the administered peptide mass at PRRT in well-differentiated NETs, our findings warrant further investigation.