Background: Surveillance by serial magnetic resonance imaging (MRI) is important in the management of diffuse low-grade gliomas (LGGs). Radiological interpretations of LGG scans, however, are typically qualitative and difficult to use clinically. Methods: We retrospectively compared radiological interpretations of LGG growth/stability to volume change measured by manual segmentation. Tumour diameter was measured to evaluate methods for assessing glioma progression, including RECIST criteria, Macdonald/RANO criteria, and mean tumour diameter/ellipsoid method. Results: Tumours evaluated as stable by radiologists grew a median 5.1 mL (11.1%) relative to the comparison scan. Those evaluated as having grown increased by 13.3 mL (23.7%). Diameter-based measurements corresponded well but tended to overestimate segmented volumes, and overestimation error increased with tumour size. Agreement with segmented volume improved from a mean difference of 17.6 to 4.5 to 3.9 mm for diameter and from 104.0 to 25.3 to 15.9 mL for volume with measurements in one, two, and three dimensions. Conclusions: Given evidence that LGG volume and growth are prognostic factors, lesions should be accurately measured. Current radiological reporting workflows fail to appreciate and communicate the true expansion of LGGs. Volumetric analysis remains the gold standard for growth assessment, but diametric measurements in three dimensions may be an acceptable alternative.