Using Durometers to Quantify Stiffness as an Outcome Measure for Cutaneous Neurofibroma in Neurofibromatosis Type 1

Patients with neurofibromatosis type 1 develop multiple cutaneous neurofibromas (cNFs) for which no effective drug therapy exists; mainstay treatment remains physical removal. However, clinical trials testing new drugs are ongoing, and quantitative techniques based on cNF biology are needed to measure changes in cNF after treatment. cNF tumor bulk is composed of extracellular matrix and inflammatory cells that dictate their stiffness. No studies have reported measuring the stiffness change in neurofibromas, which would indicate shrinking of the tumor bulk. The goal of this study was to evaluate 2 different instruments-the Rex Gauge durometer (denoted as REX) and the Delfin SkinFibroMeter (denoted as DELFIN)-in reproducibly measuring cNF stiffness. Ninety-seven neurofibromas from patients with neurofibromatosis type 1 on different skin areas were measured at each of 2 visits about 2 weeks apart. The DELFIN had moderate within-tumor agreement (intraclass correlation coefficient = 0.607, 95% confidence interval = 0.512-0.691) and moderate within-visit agreement (intraclass correlation coefficient = 0.732, 95% confidence interval = 0.665-0.786), and the REX had moderate within-tumor agreement (intraclass correlation coefficient = 0.740, 95% confidence interval = 0.631-0.816) and excellent within-visit agreement (intraclass correlation coefficient = 0.937, 95% confidence interval = 0.913-0.953), while accounting for repeated visits and tumor measurements clustered within each patient. We found that both the DELFIN and REX are easy to use and reliable, providing consistent quantification of cNF stiffness.