Is it safe to treat a recurrent esophageal SCC with definitive chemoradiation after a prior endoscopic mucosal resection?  

I would not be concerned about perforation or fistula formation. With an EMR, the resection takes off the mucosa/submucosa and leaves the wall intact. There should be sufficient time from the prior procedure to have full mucosal regrowth and the wall integrity should be well maintained.