How can I get my cat to gain weight?

I have a 13 year old cat who has been surviving Lymphosarcoma for 14 months (YEA!). She's doing great, but the vet wants her to gain more weight. She eats real well and is on meds for her thyroid. She just doesn't gain. Any suggestions would be so helpful.
   
Thank you!
   

Congrats I'm so glad to hear your cat is such a fighter! You may want to run the suggestion by your vet first since there is a medical condition in the mix, but when we get strays or underweight females that have recently given birth, we feed them kitten food to help build up their weight and get them back to better health and ready for adoption. Another suggestion may be to try feeding her canned food depending on how her stomach handles it (I would start slowly to avoid upsetting her stomach too much). There is usually more fat and protein in canned food and may help her put on a little more weight. Good luck with your kitty!

You could try adding alittle oil to your cats food. I took in a stray at one point and while it didn't have cancer, it was rail thin and was having trouble gaining weight. I would cook salmon filets and add the oil that was rendered out to her food. It goes without saying that she loved it and the added calories were just what she needed.