First of all when it comes to the fiqh ruling on treatment scholars hold different opinions:
- Most scholars or a majority out of them say it is permissible (not obligatory).
- The shafi'is and some hanabalis say it is mustahabb (recommended).
- The majority of the hanbalis even say that leaving it is better.
- In case that the treatment or medicament has been shown to help it is even wajib (obligatory) according the view of the shafi'is and dhahiri's
The source of all these interpretations is the hadith that was compiled by many scholars such as imam Ahmad and al-Hakim in his al-Mustarak and the authors of the Sunan:
"Some Bedouins asked: 'O Messenger of Allah (s.a.w) shall we treat (our ill)?' He said: 'Yes, O worshipers of Allah! Use remedies. For indeed Allah did not make a disease but He made a cure for it' - or - 'a remedy. Except for one disease.' They said: 'O Messenger of Allah (s.a.w)! What is it?' He said: 'Old age.'" (Source Jami' at-Tirmidhi)
When it comes to the question :"Is stopping or turning down a treatment equal to suicide?" The answer is no, according to this fatwa -in Arabic- on islamweb #27266, but if the person knows of the benefit or healing of a treatment or medicament and is able to take it, this person would commit a sin.
In osol al-Fiqh doubt is the opposite of certainty and if there's a hint of doubt as you suggest about chemo therapy most scholars would say he is allowed to turn it down.
islamweb even has two concrete fatwa on cancer #179611 and #64214 -both in Arabic-. The main opinion held by the muftis there is that treatment is not obligatory and in case of it being known to be helpful it is better to take/follow it.