Rimstinger and UKJEFF/SIMPLE SIMON/BE THERE are embroiled in an intellectual battle of the greatest magnitude.
In technical terms, UKJEFF/SIMPLE SIMON/BE THERE, is correct. Either the cancer treatment works or does not work. There is more of a direct, causal relationship between cancer treatment and treatment success.
However, it appears that Rimstinger is discussing the impact of MODERATING variables on treatment. In research, moderating variables influence the strength of the relationship between predictor variables and criterion variables. As such, quality of life (mental state, attitude, social support, or diet, etc) may act as a moderating variable between the predictor and criterion variable; in other words, people may be more likely to be treatment compliant if they have a strong quality of life, thus moderating the relationship between treatment and treatment success. Thus, quality of life may have an indirect influence on treatment success (not a direct, causal one).