What would be a reasonable healthy data set/control group to compare with a cell line from TCGA? Assume I want to run a simple machine learning of healthy/non healthy classification, how can I use TCGA where it has only data of people with cancer. What do usually people do for example for study of gene expressions?
In the TCGA datasets there is a variable (type) containing the information if the sample was from the tumor or from an adjacent region, which is usually considered as healthy. 

As this samples come from the same patient and the same tissues they are used in the analysis for the comparisons and classifications.