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How Dosimetry Check Works

Pencil beams are created that cover the treatment field and patient. The area of a beam field is simply divided up into small pixels (in a plane perpendicular to the beam), each pixel being a separate pencil. It is then only necessary to tell each pencil how much radiation it receives to compute the dose to the patient. The distance to the patient surface for each pencil is known from the CT scans and beam position.

The beam film (taken BEFORE or without the patient) gets darker the more radiation a spot gets. Therefore the beam film is a record of how much radiation each pixel gets. It is necessary to convert this information to a unit that will make it possible to compute the dose in centiGray to the patient from each pencil. Each spot on the film is mapped to the monitor units that would produce the same darkness at the center of a 10x10 cm field size (i.e. the field size that you calibrate the accelerator to). We call this resultant number the "relative monitor units" (RMU).

To accomplish this mapping you must run a calibration curve of monitor units versus the value read (after the film is digitized) at the center of a 10x10 cm field size. This curve is used to map the pivel values on the beam film image above to RMU. Here the signal value was the transmitted light.

Once converted to RMU, Dosimetry Check can compute the dose from this beam to the patient in centiGray. Here, zero RMU is black, whiter is larger RMU. The pixels on the RMU image are used as "weights" for the corresponding pencil beams. The fluence map that we have derived here from a calibrated image of the beam completely determines the dose to the patient. This process is similiar to how a planning system computes the dose, except here we are using the measured fluence instead of computing the same from knowledge and models of what kind of things modulate the beam. By starting with measurement instead of a model, we verify the dose and dose distribution that the patient receives.

The dose computed by Dosimetry Check may be compared directly to that computed by the planning system. ADAC IMRT plan (green) versus Dosimetry Check (magenta) with inhomogeneity corrections on, courtsey of Dr. Tianyou Xue.
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