Because radiation doesn’t distinguish between cancerous cells and normal cells, there is typically a limit to how much can be used without damaging healthy body tissues. CyberKnife is a system of robotic radiosurgery that delivers radiation so precisely that patients can tolerate a much higher dose with few side effects. While the machine takes continuous Xrays of a patient, a robotic arm delivers beamlets of radiation from 140 angles, all of which converge on the tumor with an accuracy of less than 1 millimeter. Because it is so precise, the veterinarian must know exactly where the tumor is located, says Proulx, who is one of only a handful of veterinarians in the world who are using CyberKnife in pets. “Not all pets and tumors are candidates, but we’ve certainly seen that in dogs with brain tumors we’ve been able to double the survival time.” The procedure costs approximately $12,000.