Chemotherapy may spread cancer and trigger more aggressive tumours

This has been common knowledge and is trivially obvious. Chemo includes carcinogenic chemicals. If you're at a point where chemo is the right treatment, you are out of safer options.

And yet not using it is still far more dangerous, than not treating cancer at all.

This has been obvious for a very long time, but denied by mainstream media and "modern science" cheerleaders, including here on HN. It's insanity to undergo chemo where so many highly effective alternative options exist. The fact that those often get shut down in a conversation actually kills people.

Unfortunately, so can radiotherapy. Both treatments are very hazardous options.