Peritumoral Edema in Canine Extra-Axial Brain Tumours: Effect of Steroids.

This multicenter retrospective study evaluated the effects of a time delay and steroids on the volume of peritumoral edema (VPTE) in dogs with extra-axial brain tumours. The hypothesis is that VPTE will decrease between the diagnostic (MRI-1) and RT planning (MRI-2) MRIs following the administration of steroids. Inclusion required paired MRI acquisitions within 3 months, with VPTE contouring for each MRI registered to the RT planning CT. No edema was defined as < 0.2 cm³, increased edema was > 30% VPTE increase and decreased edema was > 30% VPTE decrease. Forty-four dogs of which 34 (77%) received steroids between MRIs were included. The median time between the MRIs was 22 days (range: 8-74 days). Nine (20%) had no edema on both MRIs. The median MRI-1/VPTE: 0.83 cm³ (IQR: 0.15-2.06 cm³) and median MRI-2/VPTE: 0.40 cm³ (IQR: 0.06-1.12 cm³) were significantly different (p = 0.048). Compared to MRI-1/VPTE: 17 (39%) VPTE decreased, eight were stable and 10 increased. The median VPTE difference was -21%, range: -100 to +6287. With steroids, VPTE decreased in 15/34 (44%) and increasedin 6/34 (18%) (median VPTE diff: -60%) compared to no steroids (median VPTE diff: +25%). Steroids use was associated with change in VPTE (p = 0.009). Two dogs had clinical deterioration and were on steroids with documented VPTE increase (+86% and +1880%) without tumour progression. The change in VPTE is highly variable but reduction is associated with steroids. Notably, subjective improvement of clinical signs can be seen without significant decrease to the VPTE on imaging.