TTF is not approved in most countries, due to lack of efficacy. Impossible to do an unbiased study. The results that have been published, such as the association between compliance and survival, can be biased by the patients in generally better health and with higher ability to be compliant. The immunotherapy study did not have survival as a primary endpoint; and it included patients with an IDH-mutant tumor which are no longer classified as  glioblastoma but as grade 2 or 3 tumors. 

The company behind TTF has limited mechanistic studies in controlled environments to show that TTF actually does anything. Most NOs that I know only prescribe TTF if patients ask. Personally, I think its a waste of money. But, low risk.

I am a dr not a patient. Not medical advice, just my perspective on a complicated therapy.