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Although all tumours are rare in childhood, there are some particularly rare paediatric cancers which have not benefited from advances made by the international paediatric oncology network. To establish a shared definition and produce a list of these entities, the European Union Joint Action on Rare Cancers (JARC) promoted a consensus effort. The definition was based on the incidence rates estimated using the information network on rare cancers (RARECAREnet) database, pooling data from 94 population-based cancer registries and 27 countries. The RARECAREnet list of cancers was used to estimate the incidence rates. This list groups cancers by combining the International Classification of Diseases for Oncology, third edition, morphology and topography codes. According to the consensus, very rare paediatric cancers were identified as those with an annual incidence <2/1000000 and corresponded to 11% of all cancers in patients aged 0-14 years. Two subgroups were identified: tumour types typical of childhood (i.e. hepatoblastoma, pleuropulmonary blastoma, pancreatoblastoma) and those typical of adult age (i.e. carcinomas, melanoma). The threshold of 2/1000000 could also be adopted in populations aged 0-19 years: in this case, three tumour types had an incidence rate which was >2/1000000 (i.e. thyroid and testicular cancers and skin melanoma), but the consensus experts considered them as 'very rare' according to their clinical needs (e.g. shortage of knowledge and clinical expertise as the other rare paediatric cancers). The JARC consensus produced a definition and a list of very rare paediatric cancers which may represent a starting point for prioritising research on these tumours, based on data and patients' clinical needs., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Background: The German Childhood Cancer Registry (GCCR) annually registers approximately 2,000 children diagnosed with a malignant disease (completeness of registration >95%). While most pediatric cancer patients are diagnosed and treated according to standardized cooperative protocols of the German Society for Pediatric Oncology and Hematology (GPOH), patients with rare tumors are at risk of not being integrated in the network including trials and reference centers., Procedure: A retrospective analysis of all rare extracranial solid tumors reported to the GCCR 2001-2010 (age <18 years) was undertaken using a combination of the International Classification of Childhood Cancer (ICCC-3) and the International Classification of Diseases-Oncology (ICD-O-3). Tumors accounting for <0.3% of all malignancies were defined as rare (approx. 6 cases/year and registered malignancy)., Results: According to this definition 1,189 rare extracranial solid tumors (18.2% of all malignant extracranial solid tumors) were registered, among these 232 patients (19.5% of rare tumor cases), were not included in preexisting GPOH studies/registries. Within 10 years, the number of registered non-GPOH-trial patients with a rare tumor increased., Conclusions: Though most of the GCCR-registered patients with rare malignant tumors are treated within GPOH trials, there is a considerable number of patients that have been diagnosed and treated outside the structures of the GPOH. These patients should be reported to the recently founded German Pediatric Rare Tumor Registry (STEP). Active data accrual and the development of appropriate structures will allow for better registration and improvement of medical care in these patients., (© 2014 Wiley Periodicals, Inc.)

The low incidence and the heterogeneity of very rare tumors (VRTs) demand for international cooperation. In 2008, EXPeRT (European Cooperative Study Group for Pediatric Rare Tumors) was founded by national groups from Italy, France, United Kingdom, Poland and Germany. The first aims of EXPeRT were to agree on a uniform definition of VRTs and to develop the currently most relevant scientific questions. Current initiatives include international data exchange, retrospective and prospective studies of specific entities, and the development of harmonized and internationally recognized guidelines. Moreover, EXPeRT established a network for expert consultation to assist in clinical decision in VRTs., (© Georg Thieme Verlag KG Stuttgart · New York.)

Malignant peritoneal mesothelioma is extremely rarely seen in young patients.A 16 year-old girl presented with appendicitis-like acute abdominal pain. Intra-operatively, multiple confluent peritoneal nodules were seen on the entire greater omentum and in the pelvis infiltrating the uterus and both ovaries. Biopsies were obtained and interpreted as serous ovarian carcinoma. Radical surgical resection and hyperthermic intraperitoneal chemotherapy -(HIPEC) with carboplatin was performed and followed by 2 cycles of carboplatin/paclitaxel. Histological reevaluation showed characteristic features of epithelioid peritoneal mesothelioma and ruled out serous ovarian cancer. Therapy was continued with 6 cycles of pemetrexed/cisplatin.3 months after end of chemotherapy vital tumor tissue was found in the recess behind the liver, which could be resected completely. The patient is currently disease-free 17 months after initial diagnosis.Malignant peritoneal mesothelioma in young female patients might be under-recognized and possibly misdiagnosed as ovarian serous carcinoma in some cases. International and interdisciplinary cooperation is necessary in order to provide evidence based guidelines for diagnosis and treatment in the future., (© Georg Thieme Verlag KG Stuttgart · New York.)

In comparison to cancer in adults, virtually all cancers of childhood and adolescence are rare. Nevertheless, there is a rather ill-defined group of tumors that are not only exceptionally rare but also do not fall into the major clinical categories of childhood cancers. Thus, a substantial proportion of these exceptionally rare tumors are not registered within clinical registries or prospective therapy optimization studies. Only recently, major attention has been drawn to the diagnostic assessment and treatment of children and adolescents with such orphan diseases. In 2006, the RARE TUMOR GROUP has been established within the German Society of Pediatric Oncology and Hematology (GPOH). This working group includes experts from Pediatric Oncology, Pediatric Surgery, Pediatric Pathology, Medical, Dermatologic and Radiation Oncology as well as Pediatric Epidemiology. The major aim of the rare tumor group is to integrate these patients into the diagnostic and therapeutic network successfully established in the pediatric oncologic society. Thus, the group aims at fostering the exchange of experience in the treatment of rare tumors between medical centers and to include patients in the diagnostic and therapeutic reference network. In addition, an information platform shall be established that will be accessible to treating physicians, patients and their parents. More information and better registration shall be established by active data accrual on a regular basis and by the implementation of a data base including diagnostic and therapeutic data of patients with rare tumors. These efforts as presented in this article as well as an intensified international collaboration will allow us to provide children and adolescents with rare tumors the best possible care.