Comparing the sensitivity of linear and volumetric MRI measurements to detect changes in the size of vestibular schwannomas in patients with neurofibromatosis type 2 on bevacizumab treatment

To compare the sensitivity of linear and volumetric measurements on MRI in detecting schwannoma progression in patients with neurofibromatosis type 2 on bevacizumab treatment as well as the extent to which this depends on the size of the tumour. We compared retrospectively, changes in linear tumour dimensions at a range of thresholds to volumetric tumour measurements performed using Brainlab iPlan(®) software (Feldkirchen, Germany) and classified for tumour progression according to the Response Evaluation in Neurofibromatosis and Schwannomatosis (REiNS) criteria. Assessment of 61 schwannomas in 46 patients with a median follow-up of 20 months (range 3-43 months) was performed. There was a mean of 7 time points per tumour (range 2-12 time points). Using the volumetric REiNS criteria as the gold standard, a sensitivity of 86% was achieved for linear measurement using a 2-mm threshold to define progression. We propose that a change in linear measurement by 2 mm (particularly in tumours with starting diameters 20-30 mm, the majority of this cohort) could be used as a filter to identify cases of possible progression requiring volumetric analysis. This pragmatic approach can be used if stabilization of a previously growing schwannoma is sufficient for a patient to continue treatment in such a circumstance. We demonstrate the real-world limitations of linear vs volumetric measurement in tumour response assessment and identify limited circumstances where linear measurements can be used to determine which patients require the more resource-intensive volumetric measurements.