The first recorded use of the phrase 'Quality of Life' was in the 1920s, but it is only in the last thirty years that this phrase has become widely used. However, a clear definition of the term is still elusive.
Everyone is instinctively aware of his or her own level of QL, but it is a vague concept, which is highly individual. The World Health Organization has defined good QL as the absence of disease , but this is a one-sided definition that concentrates solely on the negative, and ignores the positive aspects. Calman  defined QL as 'the difference between the hopes and expectations of the individual and that individual's present experience.' Perhaps one ideal state of QL therefore might be considered to be an absence of disease plus physical, social, and psychological well being. Whatever definition is used, it is clear that QL is highly personal, and that its key components (lifestyle, relationships, job satisfaction, health, housing, etc.) and the relative importance of these components will differ between individuals. Certainly overall QL can rarely be extrapolated from a single component. Thus we should be wary of statements such as '... the quality of patients' lives has improved markedly as evidenced by the rapid reduction and eventual elimination of transfusion requirements and serious infections...
The ultimate aim of cancer treatments is to eradicate, or at least control, the disease and thus extend and improve life. When we measure QL in cancer trials we need to know about both the positive and negative effects of treatment. In general we are interested in 'health-related QL,' the aspects or domains that are likely to be changed by treatment, and also the impact and consequences of treatment and disease on a person's life. Although during cancer treatment an individual's overall QL might improve if, for instance, they win the lottery, this is not something that we expect to be directly related to treatment. Thus the assessment of QL in cancer treatment is usually shorthand for the assessment of health-related QL, and in a sufficiently large randomized trial, 'noise' such as that created by non-treatment-related effects, which may be substantial, should on average be equally distributed between treatment groups.
Health-related QL is complex and, as with overall QL, a single definition of the term remains elusive, although clearly it covers the impact of disease and treatment on physical status (symptoms and toxicity), psychological status (well-being, distress, self-esteem), functional status (self-care, shopping, work) and social functioning (relationships).
The aim of most randomized trials is to provide patients and clinicians with information to make informed treatment decisions, bearing in mind the individually different weighting that patients may attribute to the components of QL. It is therefore important to assess and track changes on all the pre-defined key domains and/or symptoms in order to build up a picture of the relative advantages and disadvantages of each treatment in a way which will be more helpful to future patients.
An example of the value an assessment of QL can add was in a randomized trial of cisplatin and vinblastine plus either hydrazine sulphate (HS) or a placebo for patients with advanced non-small cell lung cancer. Herndon et al.  reported similar results in terms of survival, response and weight gain. Although patients in the HS group experienced significantly more severe neuropathy this was only one of eighty types of toxicity recorded, and it was therefore felt that this alone did not provide a clear indication of which treatment was better. However, QL analyses revealed worse physical functioning, fatigue, lung-cancer-specific and cancer-specific symptoms for patients in the HS group, and the authors concluded that the QL assessment provided a unique viewpoint from which to compare the treatments.
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