Methods
Literature search
A comprehensive search strategy was developed from systematic reviews in
collaboration with a health science librarian. We combined MeSH headings
for cancer, children, survivor/long-term and Nordic countries
supplemented with free text words. Further details on search strategies
are provided in Supplemental Material. Relevant studies were identified
by searching the databases PubMed, EMBASE and Web of Science and by
manual review of reference lists in full-text articles as well as the
clinical trials register, clinicaltrials.gov. Studies were screened
based on title and abstract according to inclusion and exclusion
criteria as described below.
Study selection criteria
We searched for studies in English published from January
1st 2010 - March 1st 2020 conducted
within the Nordic countries (Denmark, Norway, Sweden, Finland and
Iceland) and including an invitation to a clinical examination for
assessment of late effects after childhood cancer. Participants had to
be younger than 18 years at time of the cancer diagnosis, whereas no
requirements to the minimum size of the study population was set up. The
clinical examination should have been performed at least three years
after the cancer diagnosis and could include any type of clinical
examination such as echocardiography, endocrinologic or neurocognitive
testing or radiologic imaging. Studies were excluded if no clinical
examination was performed in the study population, and if data were
solely based on questionnaires, blood samples, telephone interviews,
online focus group interviews, or routine out-patient follow-up visits.
We also excluded studies if part of the study population was above 18
years of age at diagnosis, or if the study was conducted outside the
Nordic countries. Furthermore, reviews, surveys, guidelines or editorial
letters were excluded in addition to posters and conference abstracts,
since they typically do not describe the recruitment process in deltail.
If more studies on the same cohort were published, only the first study
published on the cohort was included.
Data extraction
Eligible full text studies were included if they met the predefined
inclusion criteria. For each study, the following data were abstracted
for the study population: participation rate, cancer type, sex
distribution, time since diagnosis or stem cell transplantation (SCT),
duration of the examination (estimated to more or less than three
hours), the person who contacted the participant (doctor, nurse or
other) and how the participant was approached (postal letter, mail or
telephone). Four predefined diagnostic groups of childhood cancer were
used: Hematological malignancies, central nervous system tumors, solid
tumors, and mixed tumors defined as more than one diagnostic group. In
case of missing data, corresponding or senior authors were contacted to
obtain additional information.
Definitions of participation rate
Invited participants were defined as survivors receiving an invitation
to participate in a late effect study including a clinical examination.
Invited participants were further divided into accepted participants
(invited participants who accepted the invitation), study
non-participants (invited participants who declined or did not respond
to the invitation), study dropouts (invited participants who accepted
the invitation but did not complete the clinical examination) and study
completers (invited participants who accepted the invitation and
completed the clinical examination).
Statistical analysis
The study participation rate (SPR) was calculated as the ratio between
the number of accepted and invited participants. In case-control
studies, only the SPR for the cases was calculated and not for the
controls. To address the potential effect of participant
characteristics, we used a random-effect logistic regression model for
the logit-transformed (log odds) participation rates. For studies with a
100% participation rate, the number of non-participants was set to 0.5.
We investigated cancer type (hematological malignancies, central nervous
system tumors, solid tumors or mixed diagnosis), time since diagnosis
(more or less than ten years) and duration of the examination (more or
less than three hours), separately. The results were presented in forest
plots in which the logit-transformed participation rates were
transformed back to the participation rate-scale with corresponding 95%
confidence intervals (CI).