A radiation beam analyzer for measuring the distribution and intensity of radiation produced by a Cyberknife®. The analyzer employs a relative small tank of water into which a sensor is placed to maintain a constant SAD (source to axis distance). A first method maintains a fixed position of detector, and raises or lowers the small tank of water. A second method moves the detector up, down or rotationally synchronously in opposite directions with respect to the small tank of water to keep the SAD constant. These methods position the detector relative to the radiation source to simulate the location of a malady within a patients body. An embodiment of the present invention enables measurements of substantially larger fields. This is accomplished by rotating a tank of water 90 degrees from a first position to a second position.
