Study in Children and Adolescents of 177Lu-DOTATATE (Lutathera®) Combined With the PARP Inhibitor Olaparib for the Treatment of Recurrent or Relapsed Solid Tumours Expressing Somatostatin Receptor (SSTR) (LuPARPed).

Inclusion criteria: * 18 months - 18 years of age at the time of the initial diagnosis. * ≥ 3 years at the moment of inclusion in the trial. * Diagnosis: relapsed/refractory solid tumours with positive uptake on SSTR-PET (PET-CT or PET-MRI), performed in the previous three months before entering the study. The evaluation of SSTR expression will be classified according to a qualitative 4-point scale: SSTR expression V (visual score): * Score = 0: Below or equal to blood pool * Score = 1: Above blood pool and lower than liver * Score = 2: Equal to or above liver and lower than spleen * Score = 3: Equal to or above spleen Patients with scores ≥ 2 in the majority of the tumoral lesions will be considered to have a positive SSTR-PET and will be therefore eligible for the trial. Patients with a higher score are presumed to have a better response to the treatment. It is admissible to have non-measurable disease only (e.g., HR-NB with bone-only or bone-marrow-only active disease). * Performance status ≥ 50% according to Lansky scale (\<16 years old) or Karnofsky scale (for ≥16 years old). * Life expectancy of at least 3 months. * Availability of ability to swallow tablets or capsules. * Adequate organ function within 28 days prior to enrolment, as defined by: * Hb ≥10 g/dl (packed red blood transfusion is acceptable up to 24 hours prior starting treatment); * White blood cell (WBC) count ≥ 2500/μL (equivalent to 2.5 x 109/L) * Absolute Neutrophil Count (ANC) ≥ 1000/μl; …