Body composition, defined as the proportions and distribution of lean and fat tissues in the human body, is an emergent theme in clinical oncology. Severe muscle depletion (sarcopenia) is most easily overlooked in obese patients; the advent of secondary analysis of oncologic images provides a precise and specific assessment of sarcopenia. Here, we review the definitions, prevalence and clinical implications of sarcopenic obesity (SO) in medical and surgical oncology. Reported prevalence of SO varies due to the heterogeneity in the definitions and the variability in the cut points used to define low muscle mass and high fat mass. Prevalence of SO in advanced solid tumor patient populations average 9% (range 2.3%–14.6%) overall, and one in four (24.7%, range 5.9%–39.2%) patients with body mass index ≥ 30 kg/m2 are sarcopenic. SO is independently associated with higher mortality and higher rate of complications in systemic and surgical cancer treatment, across multiple cancer sites and treatment plans. These associations remain unexplained, however, it has been hypothesized that patients with sarcopenia are generally unfit and unable to tolerate stress. Another proposed mechanism relates to increased exposure to antineoplastic therapy, i.e. a large fat mass would be expected to inflate drug dose in BSA-based treatments, causing an increased rate of dose-limiting toxicity. Pharmacokinetic data are needed to confirm or refute this hypothesis. Old age, deconditioning, cancer progression, acute or chronic nonmalignant disease and drug side-effects are suggested causes of muscle loss, and it is unknown the degree to which this can be reversed. Sarcopenia can be readily detected before start of cancer treatment, however, clinical management protocols for SO patients require development. Studies of cancer treatment dose-modulation are in progress.
