External beam treatment time
Inside the machine, known as a linear accelerator (or “linac” for short), there is a measurement device between the source of radiation and the patient. This device is called a “Monitor Chamber,” and it is used to monitor the amount of radiation being emitted by the linac. The Monitor Chamber collects electric charges created by radiation as it passes through the device. That electric charge is counted as “Monitor Units.” When a treatment plan is developed, we set a certain number of Monitor Units that we want to be counted with each treatment field. When we “turn on” the beam, the linac begins counting the Monitor Units, and when the number has been reached, the linac knows to turn off the radiation. For safety purposes, we have other safeguards to ensure that the number of monitor units is not exceeded and that the linac turns off the beam at the appropriate time
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