Completeness of registration has improved during the period covered by these data, for two main reasons. The level of completeness in the general (all-ages) regional and national cancer registries that cover the whole of the UK, and which all contribute to the National Registry of Childhood Tumours (NRCT), has increased over time. The UKCCSG (UK Children’s Cancer Study Group, now Children’s Cancer and Leukaemia Group, CCLG) has been an increasingly important source of data since it was founded in 1977, and nowadays more than 90% of children with cancer are notified to the NRCT from pediatric oncology centers affiliated to the CCLG. The overall level of completeness of NRCT registration is thought to have been around 95% for children diagnosed with cancer in the early 1980s, and is probably near 100% for recent years.
The proportion of all cancers developing in childhood that are actually diagnosed before the fifteenth birthday (the upper end of the age range covered by the NRCT) has probably also increased over time. Since 1970 there have been major advances in diagnostic methods for cancer, including imaging by ultrasound, CT and MRI. These methods have made it possible to recognize cancer at a less advanced stage of disease: perhaps as much as several years earlier for some very slow-growing brain tumors, for example. So the recent data probably include some children who previously would not have been diagnosed until after the fifteenth birthday, or even at all (if the child died and the death was attributed to non-cancer causes, or if the cancer regressed spontaneously without ever causing symptoms).
Accuracy of diagnosis has also improved over time. Since 1970 specific tests have been developed for many types of childhood cancer. Some children in certain diagnostic groups in the earlier years would probably have been allocated to a different group if they had been diagnosed more recently.