The goal of this observational study is to investigate the beneficial value of complementary surgery for appendiceal neuro-endocrine tumours in children.

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Aim Current guidelines recommend complementary right-sided hemicolectomy for high-risk (pT2(with risk factors)/pT3) neuro-endocrine tumors (NET) of the appendix (based on adult studies). In contrast to adults, high-risk NET of the appendix in children seems to be a relatively benign disease with high disease-free survival (100% versus 70-80% in adults), but high quality data are lacking. Therefore these recommendations are now being questioned. We aim to investigate the value of complementary right-sided hemicolectomy for children with high-risk NET of the appendix. Ultimately leading to the development of a consensus guideline and solid information for patients/parents.

Plan of investigation In order to generate big data, an international historical cohort study is planned to compare complementary right-sided hemicolectomy with appendectomy alone for children with high-risk NET of the appendix. Results will be CONFIDENTIAL used by an international expert group to formulate treatment recommendations. Subsequently, these recommendations will be tested in an international Delphi study in order to develop a consensus guideline on the treatment of pediatric high-risk NET of the appendix.

Expected results The cohort study will generate high quality information on overall/disease-free survival, recurrence, complications, costs, and hr-QoL. Recommendations made will be tested in a Delphi study; not only on the beneficial value of complementary right-sided hemicolectomy, but also on follow-up protocols and preoperative work-up. Ultimately, an international consensus guideline that redefines low-risk and high-risk NET of the appendix will be developed, leading to global de-escalation and uniformity of treatment.

Relevance for childhood cancer Results are relevant for pediatric oncologists/surgeons/gastro-enterologists across the world, as redefining low-risk and high-risk patient groups, will lead to de-escalation of treatment. Furthermore, QoL of child and parents can be improved by reducing exposure to complications after complementary right-sided hemicolectomy, and by reducing the fear of recurrence by obtaining high-quality data to accurately inform patients and parents.

Inclusion Criteria:

• All patients that were treated for an appendiceal NET before the age of 18 years old
• Time period: 1990-2020

Exclusion Criteria:

• Other appendiceal malignancies/tumours, for example:
• goblet cell carcinoma
• adenocarcinoma
• neuroendocrine carcinoma

All patients treated for an appendiceal neuro endocrine tumour before the age of 18 during 1990 and 2020

individual participant data will not be shared with other researchers. Collaborating institution will however enter participant data in a coded/pseudonimyzed manner in Castor EDC database.