This is apocryphal (I don’t have a direct source or cared quite enough to find one), but basically my understanding of the origins of the treatment was that it

This is apocryphal (I don’t have a direct source or cared quite enough to find one), but basically my understanding of the origins of the treatment was that it was noticed back when Britain was still an empire, you’d get these upper class guys with cancer heading off to some tropical place “for one last adventure”, and that some would somehow come back free of it. Basically in addition to the environmental heat stress, they’d catch some horrible tropical disease and end up with a dangerously high fever. If they survived that then they found the cancerous lump on their neck or wherever would start shrinking. These effects have been known and studied for quite a long while, but was broadly ignored until the past few decades. And even now it’s still not a well known phenomenon. Back when I looked into it, it seems that regular sauna use may also be beneficial against cancer incidence, though there are a cascade of various things that go on under heat stress that can have positive effects. Prior to my current medication I found sitting in my vehicle in the sun and just miserably sweating it out for half and hour was a good way to “reset” myself for the day if I was having a bad depressive episode with an involuntary spiral of negative thoughts. I’d say it was just endorphins, but those don’t last all day.