Comorbidity, the presence of additional illnesses unrelated to the tumor, has a significant impact on the prognosis of patients with head and neck cancer. In these patients, tobacco and alcohol abuse contributes greatly to comorbidity. Several instruments have been used to quantify comorbidity including Adult Comorbidity Evaluation 27 (ACE 27), Charlson Index (CI) and Cumulative Illness Rating Scale. The ACE 27 and CI are the most frequently used indices. Information on comorbidity at the time of diagnosis can be abstracted from patient records. Self-reporting is less reliable than record review. Functional status is not a reliable substitute for comorbidity evaluation as a prognostic measure. Severity as well as the presence of a condition is required for a good predictive instrument. Comorbidity increases mortality in patients with head and neck cancer, and this effect is greater in the early years following treatment. In addition to reducing overall survival, many studies have shown that comorbidity influences disease-specific survival negatively, most likely because patients with high comorbidity tend to have delay in diagnosis, often presenting with advanced stage tumors, and the comorbidity may also prompt less aggressive treatment. The impact of comorbidity on survival is greater in younger than in older patients, although it affects both. For specific tumor sites, comorbidity has been shown to negatively influence prognosis in oral, oropharyngeal, laryngeal and salivary gland tumors. Several studies have reported higher incidence and increased severity of treatment complications in patients with high comorbidity burden. Studies have demonstrated a negative impact of comorbidity on quality of life, and increased cost of treatment with higher degree of comorbidity. Our review of the literature suggests that routine collection of comorbidity data will be important in the analysis of survival, quality of life and functional outcomes after treatment as comorbidity has an impact on all of the above. These data should be integrated with tumor-specific staging systems in order to develop better instruments for prognostication, as well as comparing results of different treatment regimens and institutions.
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