Though canines have a formidable sense of smell, it's still unclear how these dogs managed to come up with such accurate results. Before declaring a breakthrough in prostate cancer detections, scientists will need to replicate the results on a far larger scale, as LiveScience notes:
There is also the potential study design problem that the dogs might have been able to pick up subconscious cues from the researchers about which samples were cancerous and which were not. In research this influence is called the "Clever Hans effect," after a horse named Hans that, in early 1900s Germany, was claimed to be able to solve math problems, read and understand German, and other amazing abilities. Careful skeptical investigation revealed that Hans merely gave the illusion of being able to do these tasks; he was instead reacting to subtle (and unconscious) cues from his trainer.