NF2 and plexiform neurofibroma: However, Smith et al. (14) reported that plasma S100B was not a useful biomarker for tumor burden in neurofibromatosis patients (NF1-related schwannomatosis: n = 69; NF2-related schwannomatosis: n = 28; schwannomatosis: n = 30) because they did not find a relationship between the presence of internal neurofibromas or schwannomas and S100B plasma levels, or between whole-body tumor burden and S100B concentration.