Known data limitations

This page documents the known limitations with the data.

Due to the coronavirus illness (COVID-19) disruption during 2020, these data demonstrate a change in the patterns of cancer diagnoses and deaths, with a significant reduction in the number and rates of diagnosis compared to 2019 (approximately an 11% fall over all cancers). Therefore, trend data should be interpreted with care over the COVID-19 period.

A small number of surgery cases were missing from the 2019 treatment data released on 03/11/2022. This affected the data from April 2019 onwards, reducing the proportion reported as treated with surgery. This effect is usually no more than a 2% reduction, but greater for non-melanoma skin cancers. This issue has been resolved in the treatment data released on 01/06/2023, but please note that the treatment data downloaded from the website between 03/11/2022 and 01/06/2023 has minor inaccuracies.

The treatment statistics for certain skin tumours, namely keratinocyte cancers (BCC and cSCC), are currently experimental. NDRS has improved the underlying methodology to identify skin surgical treatments using HES outpatient data between the 03/11/2022 release and the 01/06/2023 release.
Keratinocyte skin cancer is very common. Due to the burden of work processing these cases, they are generally not manually reviewed, but are run through an auto-processor. This creates a cancer registration for the first BCC and cSCC per patient. Subsequent tumours are imputed based on the presence of pathology, following the Z Venables 1stPPPA methodology. This is described in more detail in the full documentation, and has been validated as a good measure of skin incidence, but these cases do not have the full detail of complete cancer registrations.

Every year there are around 10-30 liver neuroendocrine tumours that are classified into the ‘Other liver’ group. A sample of these liver neuroendocrine tumours was reviewed and most of them were secondary liver cancers, rather than primary, and so would usually be excluded from the cohort. These tumours are being reviewed and a QA process is being put in place to improve our data quality in the future. In the data released on 1st June 2023, the size of the ‘Other liver’ group is likely to be slightly inflated.

Get Data Out aims to publish data that reflects the true incidence of cancer, but improvements in coding can artificially affect changes over time. For example, poorly coded data could make greater use of the ‘not otherwise specified’ code. As data quality improves, the incidence of specific types of cancer may appear to increase, while the incidence of NOS cancers can decrease. When interpreting changes over time for very specifically coded cancers, reviewing the incidence of adjacent ‘NOS’ groups may be helpful.

The National Disease Registry has been one organisation since 2013, which is why Get Data Out statistics start in this year. However, the training and development of staff to work consistently and to a high quality in one organisation was not a binary process. Because of this in the early years of data rapid improvement in fields such as stage completeness and more specific coding may be observed.

The registration of 2019 tumours were being completed during the COVID-19 pandemic. This led to reduced access to the usual data sources, and despite the registry’s best efforts, a noticeable decrease in data quality in some fields. This is most seen in an increase in ‘stage unknown’ tumours, and a corresponding decrease in other stage groups. This should be noted when undertaking time-series analysis on the data.
Get Data Out reports on many statistics by stage. Different cancers are staged in different systems, where the main staging systems used for each site are described in that sites grouping document. However, as understanding of cancers improve, the staging system that cancers are registered in continues to change and improve. In the GDO data, this is most often seen as a move from using TNM 7 to TNM 8, usually between 2017 and 2018 diagnoses. Please note that although groups may seem the same either side of this change, there may be subtle changes in their definitions between the staging systems. The main changes that may be seen are the size of a group may change, or the survival of patients inthe group may change. Times of staging system changes can be identified from the table below.
The table below lists the percentage of tumours staged in each system for all staged tumours at each cancer site, diagnosed 2013-2020 by year.

There were 96 rows of inaccurate data in the GDO_data_wide.csv file released on 01/06/2023. For all years, 2013-2019, some bladder groups had data published for treatment, survival, and routes to diagnosis, where they should have been given a “.j” flag for “Data are not available as the grouping is new and not all statistics have been calculated yet (statistics are calculated annually)”. These groups are the four stages (Stage localised, Stage locally advanced, Stage metastatic, and Stage unknown) of the group Renal Pelvis and Ureter > Malignant and in situ > Muscle-invasive, accounting for four rows of data per year. Also the same four stages of the Bladder > Malignant and in situ > Urothelial > Muscle-invasive group, accounting for another four rows of data per year. This issue was fixed in the data and re-released on the website on 09/06/2023.