The next skin cancer: second primaries after a keratinocyte cancer

Marcil and Stern reviewed 17 studies covering 26 tumour combinations and pooled them. The three-year cumulative risk of a subsequent squamous cell carcinoma after an index squamous cell carcinoma was 18 percent, at least a ten-fold increase over the incidence of first tumours in a comparable general population. For basal cell carcinoma the three-year cumulative risk of a subsequent one was 44 percent, also at least a ten-fold increase. The risk of a basal cell carcinoma in someone with a prior squamous cell carcinoma was about the same as in someone with a prior basal cell carcinoma, while the risk of a squamous cell carcinoma after a basal cell carcinoma was low at 6 percent. Those are cumulative risks in the cohorts reviewed, not a prediction for an individual, and they depend on the number of prior tumours of that type.

The British Association of Dermatologists puts the squamous cell carcinoma figure higher over a longer window in its patient leaflet: within the five years after treatment for a low-risk squamous cell carcinoma there is about a 40 percent chance of another one, and after a higher-risk squamous cell carcinoma the risk may be as high as 80 percent. This is the reason both the BAD and Cancer Research UK ask people to check their own skin monthly rather than wait for appointments, and the reason a dermatologist may treat sun-damaged skin that does not look like cancer.