I found when taking Dex as part of CDT treatment for 8 months last year I put on about 20 lbs and gained 6 inches around the waist. Fortunately all these gains all went within 8-10 weeks of stopping treatment so don’t worry, it will go.
After 9 months plateau I’m have now been on Dex again with Velcade and Cyclophosphamide and have gained almost a stone in 8 months but I’m quite confident that it will dissapear when the treatment stops in 5 weeks time.
Hope this helps to put your mind at rest a little.