An article in the British Medical Journal reveals a study in which dogs have proven effective at scenting cancer by-products in urine.
The connection of the ideas that cancerous cells have altered metabolisms, that those altered metabolisms would create chemicals that volatilize, and that they may volatilize in concentrations high enough to be detected by scent receptors that were formed under tens of thousand of years of evolutionary pressure is certainly enough to form a credible hypothesis. The difference between quack medicine and rigorous medicine is that rigor was added through the experiment just conducted, as opposed to going straight to marketing. I would look forward to further experiments.
I would ask, if anyone connected to the study is looking at this, that esophogial cancer be added to the list of likely candidates.