I lost 110 pounds just from chemo. I was down to 130lbs from 245lbs in a few weeks from cancer cachexia, also known as wasting syndrome, and other illnesses. I'm now between 195-200lbs, but that took almost 4 years to gain back, although I was in several treatments between. My highest weight ever was 275lbs at 5'10", so I'm at the point if I want to bulk up I can, and started exercising with weights recently, and I get big just looking at weights lol, and only go occasionally. I'm comfortable at this weight, and should not gain more due to kidney disease, again from chemo. Muscle, fat has memory, so you will gain back the weight to your highest level faster than someone who wasn't, and used to do it many times since I was 16, losing weight, gaining weight at will, sometimes losing 20, 30 a week, and up to 100lbs in a year, and back up again, but losing gets was harder and harder if you want that one day. It's more difficult to gain after Tx with the body fight cancer, malabsorbtion in the digestive track and stomach, metabolism changes, and altered diet.
There are ways to increase your appetite, and weight with prescription drugs, OTC vitamins, exercise and diet. Even sight, taste, smelling foods, food combination, colors can increase your appetite.
Good luck.