Important Group Differences on the Functional Assessment of Cancer Therapy–Kidney Symptom Index Disease-Related Symptoms in Patients with Metastatic Renal Cell Carcinoma

Background: The Functional Assessment of Cancer Therapy–Kidney Symptom Index Disease-Related Symptoms (FKSI-DRS) is important to gauge clinical benefit in metastatic renal cell carcinoma (mRCC). Objectives: To estimate important difference (ID) in FKSI-DRS scores that is considered to be meaningful when comparing treatment effect between groups, using mRCC trial data. Methods: Data were derived from two pivotal phase III mRCC trials comparing sunitinib versus interferon alfa (N = 750) in first-line mRCC, and axitinib versus sorafenib (N = 723) in second-line mRCC. The change from baseline in FKSI-DRS score was examined as a function of a set of anchors using the repeated-measures model. Several anchors were evaluated: FKSI item “I am bothered by side effects of treatment,” EuroQol five-dimensional questionnaire utility score, and adverse events. Results: When the “I am bothered by side effects of treatment” score was used as an anchor, the ID ranged between 1.2 and 1.3 points. When change in the EuroQol five-dimensional questionnaire utility score was used as an anchor, the FKSI-DRS ID ranged between 0.62 and 0.63 points. Selecting the adverse events that corresponded to a maximum worsening in the FKSI-DRS score in either trial, the ID ranged between 0.62 and 0.74 points. Conclusions: Among patients undergoing treatment for mRCC, between-group differences in FKSI-DRS scores as low as 1 point might be meaningful.