An invivo dosimeter, or IVD for short, is a device used by radiation oncologists to measure radiation inside the body of a cancer patient during treatment. It is comprised of two main parts: detector and electrometer. The detector is usually a radiation-sensitive diode which is sealed in resin and attached by a long coaxial cable to the electrometer. The electrometer is a very sensitive device which measures electrical current. During treatment, the patient is placed on the treatment couch below the particle accelerator gantry. The detector(s) is then either placed on the skin, or inserted into the patient's body. While the patient is being irradiated, the dosimeter measures the radiation dosage. It is a necessary safeguard against overexposure and incorrect beam placement. Because this device is used in/on human bodies in hospitals, worldwide regulations require that it be powered solely by batteries. Most IVD's have rechargable batteries, and are designed to not function unless they are unplugged from the charger.