Dogs have a sharp sense of smell, tracking everything from bombs to drugs. But a special group of dogs in San Diego is on the trail of something a lot tougher to track down -- cancer.Diamond the German Shepard is known for her tracking ability. Now, she is on the hunt for what may be the most elusive smell of all.For the past year, Diamond and five other dogs have been training for the biggest of smell tests. They're part of a study by Scripps Clinic to see if dogs can detect breast and prostate cancer in urine samples. It's based on the fact that tumors give off an assortment of chemicals."Cancer gives off odors (and) this is what the dogs are sensing," said Dr. Robert Gordon from Scripps Clinic.This is how each trainig session works: There are six specimens without cancer and one with cancer. The dogs are let loose and monitored to see if they choose the sample that is cancerous.This past week, a Northern California study using labradors and Portugese water dogs claimed the dogs could detect lung cancer in the breath of sufferers with 99 percent accuracy.Doctors said the knowledge could help develop better detection methods, and dogs could act as an initial screeners in remote areas. As for the study in San Diego, the initial results are promising."I would say (90 percent) of the time, she's right on," said Diamond's trainer.