You sit anxiously in the waiting room while your cat undergoes surgery to remove a cancerous mass. Finally, the doctor comes out in his scrubs and tells you he was able to excise the malignant tumor and that he believes he got it all. Relieved but not yet out of the woods, you wait for the pathology report. Sure enough, it confirms the surgeons belief that he excised the cancer in its entirety. There are clean margins. Why, then, does the tumor grow back in the exact same spot some months later?