How do I find samples/patients in TCGA (the cancer genome atlas) that had radiation therapy?
The process is slightly painful but here is how you do it. 
Go to UCSC Xena, then launch the browser, hit the Visualize tab, pick your cohort of interest, then in the next menu pick phenotype, and then type radiotherapy in the box and pick the radiotherapy related columns you want. Then, once it has loaded the annotation bar, you can hit download and the downloaded spreadsheet will have this information along with the TCGA barcodes. As for clinical data, https://www.synapse.org/#!Synapse:syn7343873/wiki/412112 is a very good pancancer source.