Page 12 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Childhood cancer types
Cancer starts when abnormal cells in the body grow uncontrollably, often forming a mass of tissue (i.e., a tumor) or affecting the blood or immune system. Childhood cancer is different from cancer in adults considering pathological characteristics, underlying biology, treatment approach, and survival opportunities. In adults, many cancer types are related to lifestyle or environmental risk factors, such as smoking, unhealthy diet, or consuming alcohol [11, 12]. Childhood cancer types are commonly not caused by lifestyle-related risk factors but develop for example by mutations in early fetal development or through the presence of genetic predisposition [13]. However, much uncertainty remains over what causes childhood cancer.
Childhood cancer comprises a heterogeneous group of tumor types and can evolve in various body sites and organs. With classification depending on their cell of origin, childhood cancer types can be distinguished in hematological malignancies and solid cancers. They are mostly unique for children and are rarely presented in adults. Incidence rates for hematological malignancies are highest for leukemia and lymphoma [14]. Most frequently diagnosed solid tumors are central nervous system (CNS) tumors, neuroblastomas, sarcomas, Wilms’ tumors, and retinoblastomas. Neuroblastomas originate most frequently in the adrenal and retroperitoneal regions, but can also develop in the thoracic, pelvis or neck region [15]. Sarcomas can be divided in soft tissue sarcomas and bone sarcomas [16], and can be located at various sites. Wilms’ tumors and retinoblastomas are site-specific and originate in the kidney and eye, respectively. This distribution of cancers varies among age groups [4, 14]. In infants, the most predominant cancer is neuroblastoma, while leukemia is more common in 1-4-year-old children, and CNS tumors in 5-9-year-old children. For older children, bone tumors, lymphomas, and carcinomas are more common.
Treatment options
Since childhood cancer types differ from adult cancer types, the approach and response to treatment varies. Each type of cancer requires a different treatment approach. The three main components of treatment are surgery, chemotherapy and radiotherapy. Childhood cancers seem to respond better to high doses of chemotherapy. Secondly, pediatric tissues are still in development and their organs have lower tolerance to radiation than adults [17, 18]. Childhood cancer treatment is challenged by the small disease population. For most childhood cancer types, collaborative groups, such as the Children’s Oncology Group (COG) [19], the International Society of Paediatric Oncology (SIOP) [20], and Paediatric Radiation Oncology Society (PROS) [21], have developed child-specific diagnostic procedures and treatment protocols. Depending on histology, risk group, and/or stage of cancer, children are treated according to these proposed protocols. However, due to the rarity of the disease and patient-specific needs, a multidisciplinary team needs to work intensively together to create the optimal treatment plan for each individual child. The optimal treatment approach should aim to minimize adverse health- related outcomes and to maximize quality of life outcomes [8].
Historically, the use of radiotherapy has contributed to improved survival rates of several childhood cancers [22]. However, since the late treatment-related adverse events from radiotherapy in childhood cancer survivors are well known and documented [9, 10], effort has been made to reserve the use of
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