I’m basically trying to quantity the fat percentage of tumors on CT, any suggestions on how to do this in 3D slicer would be very much appreciated.
This kind of analysis is usually done by segmenting regions that you would like to quantify (tumor and fat), using Segment Editor module, then compute volumes using Segment Statistics module.
You may need to do some experimentation to find a good way of segmenting structures. These should help: