Introduction
In the Nordic countries, approximately 660 children below 15 years are diagnosed with cancer annually [1]. During the past decades, improvements in surgical interventions, radiation and/or chemotherapy have contributed to a major improvement in the five-year survival now exceeding 80% for all childhood cancers combined in most high-income countries [1]. Due to improved survival, the population of childhood cancer survivors is steadily growing and it has been estimated that approximately one in 1000 adults in the general population is now a childhood cancer survivor [2].
Cancer survivors face a life with an increased risk of a wide range of somatic and mental late effects related to their cancer diagnosis and therapeutic exposures with a cumulative burden nearly two-fold greater than controls [3, 4]. Several clinical studies of late effects of childhood cancer have been conducted during the past decade. To obtain relevant, comparable and generalisable results from the clinical studies, the population must be representative of the survivors. Studies with low participation might lack adequate statistical precision and thus might be inconclusive or subject to selection bias. Factors known to influence recruitment procedures and response rates have repeatedly been of interest [5, 6]. Potential barriers for participation in clinical late effect studies were analyzed in the Childhood Cancer Survivor Study (CCSS) [4], which identified distance to the hospital and fear of bringing back old memories of being sick as the most important barriers. Among motivating factors were re-visiting clinicians who had been involved in their treatment, to learn more about their own health, and helping out other children with cancer.
Recruitment of especially adolescents and young adults (AYAs), however, can be challenging. A study investigating recruitment of AYAs between 15-39 years in the United States through a survey achieved a response rate of 43% [7]. Although this survey invitation was sent close to the date of diagnosis, it required extensive resources to achieve a sufficiently high participation rate in this age group of young people being mobile and difficult to follow due to educational and employment opportunities, marriage and other personal life changes.
A systematic review assessed participation reporting quality, participation percentage and characteristics in psychosocial oncology studies including both adult and pediatric patients [8]. Of the 156 pediatric studies included, 121 (78%) reported sufficient data to calculate a participation rate, which was higher in studies conducted near the time of diagnosis compared to later in the treatment trajectory.
Willingness to participate in a clinical study seems to vary markedly among childhood cancer survivors [9] [10], which might be due to a variety of different factors influencing participation. However, this has not previously been systematically investigated in childhood cancer survivors. Thus, our aims were to outline the participation rate among childhood cancer survivors and to investigate potential factors influencing the participation rate, including cancer type of the survivor, time since diagnosis, and duration of the clinical examination. We investigated wether survivors with a cancer diagnosis carrying a high risk of late effects such as hematologic malignancies and central nervous system tumors, long time since diagnosis, and a long duration of the clinical examination would lead to a lower participation rate.