From what I can tell, dogs worked well in the laboratory, but it did not work well in the field. In the lab they usually had 1/5 or so positive samples, but in the field, it was a much lower positive rate depending upon cancer and the dog handler would not be able to give positive reinforcement immediately in the field, also different dogs have different success rates. https://www.livescience.com/61234-how-dogs-smell-cancer.html