Perhaps you were thinking of basal cell carcinoma, the most common type of skin cancer. There does seem to be a lack of experimental evidence wrt BCC: https://www.hindawi.com/journals/jsc/2012/480985/ The skincancer.org FAQ conspicuously skips around that hole.
Your "50% reduction in melanoma" link is interesting, but pretty small & low-powered - and followup comments (such as https://ascopubs.org/doi/10.1200/JCO.2011.35.5727) highlight potential major problems in study design. For example:
• Study wasn't originally designed to detect effect on melanomas - that endpoint was added later. (This, alone, suggests possible p-hacking.)
• Instructions thus didn't even direct sunscreen application to the areas melanomas most often occur. Yet, at followup both control & intervention groups had a disproportionate number of melanomas in exactly the areas where application had been directed (and most applied, by self-reports, in both groups). As the commenters note, this is actually consistent with the theory that sunscreen encourages overexposure moreso than it protects.
• Small changes in arbitrary study choices - like to what extent back/leg melanomas were considered (when those areas weren't part of the encouraged sunscreen-application area), or whether a 1, 2, or 4 year exclusion of early events applied – remove the claim of 'significance', suggesting a very fragile result.
If the protective relationship of sunscreen were real, it'd have lots of studies with robust results. Or, the massive increases in recent decades in SPF factors, sunscreen purchasing/use, and sun-avoidance messaging and practice would've reduced melanoma rates. Instead, melanoma rates keep going up. To me, that's far more powerful evidence than a repurposed study with 11 more melanomas (but fewer invasive ones!) in a slightly-less-sunscreen-using control group.
The squamous study is also murky, though not as much. Its abstract notes, "There were no significant differences in the incidence of first new skin cancers between groups randomly assigned daily sunscreen and no daily sunscreen (basal-cell carcinoma 2588 vs 2509 per 100,000; rate ratio 1.03 [95% CI 0.73-1.46]; squamous-cell carcinoma 876 vs 996 per 100,000; rate ratio 0.88 [0.50-1.56])." The same number of people got new skin cancers in both groups! It's only the secondary count of tumours where the control group had more. I'm unsure that's an important protective effect.
But going back, I really should have said: "no data showing more sunscreen reduces deadly skin cancers". There's a reasonable amount of evidence sunscreen reduces various kinds of mild skin-tumours, much like the obvious-to-the-naked-eye effectiveness against sunburns.
But for melanomas/death-by-skin-cancer – the top worry of most sunscreen users – any effect is either very weak, non-existent, or possibly even inverse.
Your "50% reduction in melanoma" link is interesting, but pretty small & low-powered - and followup comments (such as https://ascopubs.org/doi/10.1200/JCO.2011.35.5727) highlight potential major problems in study design. For example:
• Study wasn't originally designed to detect effect on melanomas - that endpoint was added later. (This, alone, suggests possible p-hacking.)
• Instructions thus didn't even direct sunscreen application to the areas melanomas most often occur. Yet, at followup both control & intervention groups had a disproportionate number of melanomas in exactly the areas where application had been directed (and most applied, by self-reports, in both groups). As the commenters note, this is actually consistent with the theory that sunscreen encourages overexposure moreso than it protects.
• Small changes in arbitrary study choices - like to what extent back/leg melanomas were considered (when those areas weren't part of the encouraged sunscreen-application area), or whether a 1, 2, or 4 year exclusion of early events applied – remove the claim of 'significance', suggesting a very fragile result.
If the protective relationship of sunscreen were real, it'd have lots of studies with robust results. Or, the massive increases in recent decades in SPF factors, sunscreen purchasing/use, and sun-avoidance messaging and practice would've reduced melanoma rates. Instead, melanoma rates keep going up. To me, that's far more powerful evidence than a repurposed study with 11 more melanomas (but fewer invasive ones!) in a slightly-less-sunscreen-using control group.
The squamous study is also murky, though not as much. Its abstract notes, "There were no significant differences in the incidence of first new skin cancers between groups randomly assigned daily sunscreen and no daily sunscreen (basal-cell carcinoma 2588 vs 2509 per 100,000; rate ratio 1.03 [95% CI 0.73-1.46]; squamous-cell carcinoma 876 vs 996 per 100,000; rate ratio 0.88 [0.50-1.56])." The same number of people got new skin cancers in both groups! It's only the secondary count of tumours where the control group had more. I'm unsure that's an important protective effect.
But going back, I really should have said: "no data showing more sunscreen reduces deadly skin cancers". There's a reasonable amount of evidence sunscreen reduces various kinds of mild skin-tumours, much like the obvious-to-the-naked-eye effectiveness against sunburns.
But for melanomas/death-by-skin-cancer – the top worry of most sunscreen users – any effect is either very weak, non-existent, or possibly even inverse.