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Background: Over the past decade, numerous treatment options have emerged for patients with locally advanced and metastatic neuroendocrine tumours (NETs). Nevertheless, the optimal timing of treatment interventions remains uncertain, given the highly variable disease course observed in these patients, even when patients have the same tumour stage and grade. The aim of the study was to evaluate the predictive role of standardized uptake values (SUVs) and volumetric parameters obtained from pretreatment [68Ga]Ga-DOTA-TATE for response to SSA therapy in patients with NET. In this retrospective study, we included 42 patients (21 women, 21 men; age range: 46-84 years) with histologically confirmed, metastatic, NET (G1 13, G2 28 cases); median Ki-67 index 5%, range 1-16) who received long acting SSA as a first line treatment and underwent [68Ga]Ga-DOTA-TATE PET/CT before SSA treatment. For all [68Ga]Ga-DOTA-TATE avid lesion SUVmax, SUVmean, somatostatin receptor expression tumour volume (STV), total lesion somatostatin receptor expression (TLD, STV multiplied by SUV mean) and Tmean/Smean (SUVmean of tumours/metastases divided by SUVmean of normal spleen) were measured. Finally, the sum of STV (total STV, TSTV) and TLD (total TLD, TTLD) was calculated for each patient and used in the analysis., Results: During the study period, 14 patients had stable disease (33.3%) and 28 patients experienced progression (66.7%), among whom 12 patients died. The median progression-free survival (PFS) and overall survival (OS) were 26.5 and 46.5 months, respectively. In the univariate and multivariate analysis, in the whole population study, Tmean/Smean ratio (HR 1.88, 95% CI 1.06-3.35, p = 0.03), Ki-67 index (HR 1.14, CI 1.03-1.26, p = 0.01) and pre-treatment chromogranin A serum concentration (HR 1.01, CI 1.0-1.03, p = 0.01) were significantly associated with PFS. Among patients with small intestinal NETs, TSTV (< 125.85 cm 3 vs. ≥ 125.85 cm 3 , p = 0.023) and TTLD (< 4168.95 vs. ≥ 4168.95, p = 0.026) were significantly associated with PFS in the univariate analyses. No significant correlation was found between measured volumetric parameters and OS., Conclusion: Volumetric parameters of pretreatment 68[Ga]Ga-DOTA-TATE PET/CT may be useful in prediction of the response to SSA (used in monotherapy as a first-line therapy) in patients with NET., Competing Interests: Declarations Ethics approval and consent to participate The study protocol was approved by the Commission for Ethics in Scientific Research at the Jagiellonian University Medical College in Krakow (approval no 118.0043.1.1.2024). Written informed consent for participation was not required for this study in accordance with the national legislation and the institutional requirements. Consent for publication. Not applicable. Competing interests The authors declare that they have no competing interests., (© 2024. The Author(s).)

Purpose: To evaluate the optimal tracer uptake time, the minimal amount of radioactivity and the inter-observer agreement for 11 C-choline positron emission tomography/computed tomography (PET/CT) in patients with primary hyperparathyroidism (pHPT)., Methods: Twenty-one patients with biochemically proven pHPT were retrospectively studied after injection of 6.3 ± 1.2 MBq/kg 11 C-choline. PET data of the first nine patients, scanned for up to 60 min, were reconstructed in 10-min frames from 10- to 60-min postinjection (p.i.), mimicking varying 11 C-choline uptake times. Parathyroid adenoma to background contrast ratios were calculated and compared, using standardized uptake values (SUVs). Data was reconstructed with varying scan durations (1, 2.5, 5, and 10 min) at 20-30-min p.i. (established optimal uptake time), mimicking less administered radioactivity. To establish the minimal required radioactivity, the SUVs in the shorter scan durations (1, 2.5, and 5 min) were compared to the 10-min scan duration to determine whether increased variability and/or statistical differences were observed. Four observers analyzed the 11 C-choline PET/CT in four randomized rounds for all patients., Results: SUVpeak of the adenoma decreased from 30 to 40 p.i. onwards. All adenoma/background contrast ratios did not differ from 20- to 30-min p.i. onwards. The SUVs of adenoma in the scan duration of 1, 2.5, and 5 min all differed significantly from the same SUV in the 10-min scan duration (all p = 0.012). However, the difference in absolute SUV adenoma values was well below 10% and therefore not considered clinically significant. The inter-observer analysis showed that the Fleiss' kappa of the 1-min scan were classified as "moderate," while these values were classified as "good" in the 2.5-, 5-, and 10-min scan duration. Observers scored lower certainty scores in the 1- and 2.5-min scans compared to the 5- and 10-min scan durations., Conclusion: The optimal time to start PET/CT scanning in patients with pHPT is 20 min after mean injection of 6.3 MBq/kg 11 C-choline, with a recommended scan duration of at least 5 min. Alternatively, the radioactivity dose can be lowered by 50% while keeping a 10-min scan duration without losing the accuracy of 11 C-choline PET/CT interpretation.