Purpose <p>NETTER-P, an open-label Phase II study, evaluated the safety and dosimetry of [¹⁷⁷Lu]Lu-DOTA-TATE (hereafter ¹⁷⁷Lu-DOTATATE) in adolescents with advanced, somatostatin receptor-positive, well-differentiated, Grade 1/2 gastroenteropancreatic neuroendocrine tumours (GEP-NET) or pheochromocytoma and paragangliomas (PPGL).</p> Methods <p>Patients (12–17 years old) received four cycles of ¹⁷⁷Lu-DOTATATE (7.4 GBq every 8 ± 1 weeks; cumulative administered activity: 29.6 GBq). Primary endpoints were absorbed dose (kidneys and bone marrow) and safety after first administration. Safety during treatment and comparative assessments of dosimetry and pharmacokinetics between adolescents and historical adult patients were evaluated.</p> Results <p>Eleven patients (4 GEP-NET, 7 PPGL; median age 15 [range, 13–17] years) were enrolled and received ≥ 1 administration of ¹⁷⁷Lu-DOTATATE. Median (range) cumulative administered activity was 28.2 (7.3–29.9) GBq. Lymphopenia/lymphocyte count decreased and headache were the most common adverse events (AEs) during Cycle 1 (each 4/11 [36%]). Cycle 1 Grade ≥ 3 AEs occurred in 4/11 patients (36%). During the treatment period, the most common AE was lymphopenia/lymphocyte count decreased (7/11 [64%]; Grade ≥ 3, 5/11 [45%]). No clinically meaningful impacts on safety biomarkers nor any treatment-related nephrotoxicities were observed. Projected median (range) cumulative absorbed doses (four administrations) were 21 (14–40) Gy in kidneys and 0.76 (0.55-1.0) Gy in bone marrow (using blood data). Dosimetry values were predicted to be within safety thresholds for adolescents and adults; pharmacokinetics were comparable in both populations.</p> Conclusion <p>No new safety signals attributable to ¹⁷⁷Lu-DOTATATE were identified in adolescents with GEP-NET or PPGL versus adults with GEP-NET. Long-term follow-up is ongoing.</p> Trial registration <p>ClinicalTrials.gov, NCT04711135. Registered 15 January 2021.</p>