Linear Accelerator
This is a linear accelerator (Linac). It is source of powerful x-rays. The x-rays used for diagnosis have energies around 100,000 eV, the x-rays used for treatment have energies between 4 million and 20 million eV. This is one obvious reason why an x-ray treatment machine is much bigger than the machine used to take a chest x-ray. The name linear accelerator comes from the fact that electrons are produced in the machine and accelerated in a straight line. They then smash into a metal target within the machine and, because the energies involved are so high, produce high energy x-rays. These are then focussed by the machine into a precisely defined beam that is used for treatment.
Modern radiation therapy is a triumph of precision engineering. The gantry of a linear accelerator weighs 3 tons, the couch 1.5 tons. The gantry can be rotated through 360o and positioned with an accuracy of 1/10 of a degree. Working together, the 4.5 tons of couch and gantry, weighing as much as 3 family cars, can be moved so that the centre of rotation can be located within a sphere of 2mm diameter. Scaling down to the size of a dartboard: we are able to place an object, weighing 750Kg so that it will lie within 0.05mm of any desired position around the edge of the dartboard. Every time we treat a patient we take this technological miracle for granted.
We tend to make light of all of this. We don't want to intimidate patients. Facing cancer is daunting enough, people don't want to blinded by science and wizardry. It is, perhaps, worth gently pointing out that day in and day out, and with minimal fuss, we deliver effective treatment of the greatest sophistication and complexity. We try to make the barely possible seem ordinary.
Listen to the Linear Accelerator
On this clip, which last about two minutes, you hear what it sounds like when the radiation beam switches on, treats, and then switches off. All through you hear the room radio playing - which is what this particular patient wanted to listen to during his treatment. First you hear the radio, then the buzzing sound of the linac as it treats (plus radio), then the machine switches off and, finally, you hear a brief laugh as the radiographers come back into the room.
