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Composite Beam Dosimetry

A techniquie for composite beam quality control is where one sums all the beams with all the beams superimposed in the same position. This is typically done with a phantom. Besides analyzing one beam at a time or comparing the plan within the patient, you can also use Dosimetry Check to perform this kind of composite beam quality control. Below is a comparison of the dose measured with the Map Check device from Sun Nuclear Corp. in a phantom at 5 cm depth for five IMRT fields. Buildup was placed above the device so that the measurement is at a depth of 5 cm. The planning system was used to compute the dose for this same plane. The dash lines are from the planning system.
[Images supplied by D. Jay Freedman, Hartford Hospital, Hartford CT.]

Next is a the same comparison using Dosimetry Check for the same five beams. The beams in this case were measured with the Varian aS500 EPID, although Dosimetry Check can also read Map Check output files, so you could also use the Map Check device for the fluence measurement. However, the measurement is not at depth, no additional buildup is used. The measurement is transformed to a fluence map by means of a deconvoluton with the point spread function of the measurement device. Dosimetry Check can then compute the dose for any desired simulation of the plan and compare with the dose computed by the planning system for the same geometry. By measuring the beams individually, one can use the same measured data set to reconstruct the dose to the patient.