Applying the Toxicity Index to Patient-Reported Symptom Data: An Example Using the European Organization for Research and Treatment of Cancer Colorectal Cancer–Specific Quality of Life Questionnaire

The toxicity index (TI) is a summary index that accounts for toxicity grades associated with cancer symptoms that is more sensitive than other toxicity systems to treatment differences. The TI can be used with patient-reported symptoms but requires that scores for different items represent equivalent severity. The purpose of this article is to provide an example of scoring patient-reported symptoms that satisfies the requirement of equivalent symptom severity. A sample of 1232 adults with rectal cancer from a Phase III clinical trial self-reported 18 symptoms on the European Organization for Research and Treatment of Cancer colorectal cancer measure using a 4-category response scale (not at all, a little bit, quite a bit, or very much). The participants were 22 to 85 years of age (mean age, 57 years), 30% were female, 85% were non-Hispanic white, 59% had stage II cancer, and 41% had stage III cancer. A recoded TI was created using item response theory category thresholds. The recoded TI had larger rank-order correlations than the original TI with Karnofsky performance status index, hemoglobin level, symptom bother, and other aspects of health-related quality of life. Recoding items based on category thresholds yielded a more valid TI score that can be used to summarize adverse events.