Vermassen T, et al. - Whether or not the use of cystatin C-estimated glomerular filtration rate (eGFR) is applicable to a patient cohort of oncology patients treated with targeted agents was determined. A decrease in the concentrations of cystatin C due to cathepsin D-mediated proteolysis was noted during targeted therapy, and therefore cystatin C-eGFR was not considered a suitable marker for assessing kidney function in oncology patients.