To determine the reproducibility of the volumetric MRI analysis in tumor bearing mice, one observer (ED) applied the method to the tumors of 10 randomly selected mice on three different days. We obtained mouse MRIs at age of 4, 6, 8 10 and 12 months for a natural history study; at age of 6, 7 and 9 months for the RAD001 treatment groups; and at age 5, 7, 9 and 11 months for the Sorafenib treatment groups. On the above described 3D RARE sequence, the plexiform neurofibromas appear bright in contrast to most other tissues. Medx software (Medical Numerics, Inc., Germantown, MD) was used for volumetric analysis. One observer (ED) manually outlined lesions on each MRI slice containing tumor. Tumor on the left and right side of each mouse was measured separately at the cervical and thoracic levels, where the bulk of tumor was identified. Smaller cauda equine tumors were not included in the analysis, as the resolution in this region limited accuracy. Measurement error increases for tumors with volumes <10mm3, therefore, we report only tumors with volumes >10mm3. The tumor criteria were based on image resolution and MRI section slice thickness similar to those described previously [3 (link)]. The program calculated tumor volume from the area of graphic outline and MRI slice thickness. All volumes are reported as combined tumor volume in an individual mouse.