Fatigue is the most common side effect of cancer treatment with chemotherapy, radiation therapy, or selected biologic response modifiers. Cancer treatment–related fatigue generally improves after therapy is completed, but some level of fatigue may persist for months or years following treatment. Research indicates that for at least a subset of patients, fatigue may be a significant issue long into survivorship.[2,3] Fatigue is also seen as a presenting symptom in cancers that produce problems such as anemia, endocrine changes, and respiratory obstruction and is common in people with advanced cancer who are not receiving active cancer treatment. Cancer treatment–related fatigue is reported in 14% to 96% of patients undergoing cancer treatment [4,5,6,7,8,9,10] and in 19% to 82% of patients posttreatment.[1,2]
Several studies have documented significantly worse fatigue in cancer survivors compared with noncancer populations, as described in one review article. For example, a Norwegian cross-sectional study  compared the prevalence of fatigue in long-term survivors of testicular cancer (n = 1,431) at an average of 11 years posttreatment with the prevalence of fatigue in age-matched men in the general Norwegian population (n = 1,080). The prevalence of chronic cancer-related fatigue (CRF) was 17.1% (95% confidence interval [CI], 15.2–19.1%) among testicular cancer survivors, compared with 9.7% (95% CI, 8.0–11.5%) in the general population. Chronic CRF was also associated with multiple psychosocial problems, somatic complaints, and poor quality of life.
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Fatigue, like pain, is viewed as a self-perceived state. Patients may describe fatigue as feeling tired, weak, exhausted, lazy, weary, worn-out, heavy, slow, or like they do not have any energy or any get-up-and-go. Health professionals have included fatigue within concepts such as asthenia, lassitude, malaise, prostration, exercise intolerance, lack of energy, and weakness. Research on fatigue in people with cancer has included primarily self-reports of fatigue, with fewer but increasing data exploring biologic or physiologic correlates. Such correlates have included measures of muscle weakness, maximal oxygen uptake, cytokines, and cortisol.