Why nobody knows how many skin cancers there are: the counting rule behind every figure

The rule came first. In 1999 the UK and Ireland Association of Cancer Registries decided that, because of the difficulty of registering multiple pathology reports accurately, only the first basal cell carcinoma or squamous cell carcinoma per patient would be reported (quoted in Venables 2019). That is one registration per person per lifetime for each of the two types, whatever happens afterwards. Since people who get one keratinocyte cancer very often get several, the effect is large and one-directional.

How large has now been measured three times, each by comparing the old rule against counting the first of each type per person per year instead. Over England in 2013 to 2015, the annual method found 410,716 basal cell carcinomas against 268,565 on the old rule, 53 percent more, and 104,529 squamous cell carcinomas against 76,977, 36 percent more; 51 percent more keratinocyte cancers in all, and the authors judged that even this was still 5 to 11 percent short of the true tumour count (Venables 2019). Over 2013 to 2018 the gap was 59.2 percent for basal cell carcinoma and 37.2 percent for squamous cell carcinoma, and the residual shortfall was about 14 basal cell carcinomas and 2 squamous cell carcinomas per 100 patients (Kwiatkowska 2021). Over 2013 to 2022 it was 67 percent and 42 percent (Mistry 2026). The direction is always the same: the more carefully anybody counts, the more there turn out to be.

The four UK nations do not do the same thing, so their figures are not comparable with each other. England registers the first of each type per person and imputes the rest from pathology records, publishing on the annual basis. Scotland collects only the first occurrence of a basal cell carcinoma, in its own words 'because they are so common', but registers every squamous cell carcinoma manually (Public Health Scotland, September 2025; Venables 2019). Wales moved to the annual method from 2016 data and is the most explicit about it; on that basis non-melanoma skin cancer was 40 percent of all cancer cases in Wales in 2020, 11,792 first occurrences against 17,461 of every other cancer combined (Public Health Wales, May 2024). Northern Ireland publishes about 3,852 cases a year for 2018 to 2022.

And then the whole group is taken out of the headline. The Office for National Statistics defines all cancers as ICD-10 C00 to C97 excluding C44, stating that it has been advised 'that non-melanoma skin cancer (NMSC) is greatly under-registered' and that the exclusion exists 'to ensure that the reported figures are meaningful'; its bulletins add that the available figures 'are known to be underestimates and unreliable for comparison purposes' because recording policies have varied between registries. So when a British source says there are just over 400,000 new cancers a year in the UK, the commonest cancer of all is not in that number: around 156,000 non-melanoma skin cancers are counted separately (Cancer Research UK), and the research estimate on the annual method is 234,861 a year across the UK for 2016 to 2018 (Kwiatkowska 2021).

None of this is a reason to distrust the figures. It is a reason to read them as the floor of a range. Two practical rules follow. A rising trend in these statistics may be a rising disease, a changing counting rule, or both, and a reader should check which is being compared. And a comparison between England and another country is very often a comparison of counting methods: the NDRS notes that English rates can look alarmingly high beside other countries' simply because other countries still count only the first tumour.