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Make a calibrated picture of each field.

This picture is divided into pixels, each pixel becomes the weighting factor for a pencil beam during dose calculation.

Use an EPID or other device or use film. If using film, consider using a film holder that will attach to your collimator. We would recommend holding the film at 100 cm. Or put the film on the treatment couch. Cover the film with a 1 mm thick copper plate (brass or bronze will also do) for 6x, 2 mm for 10x, 3 mm for 18x. You need the material to shield out contamination electrons. (You can use bolus of dmax thickness, i.e. 1.5 cm for 6 MV, 3.5 for 18MV x-rays, but the scatter within the bolus will act like a low pass filter, rounding off beam edges a little. For best results we recommend you use a copper like plate.)

Expose the film. At 100 cm Kodak EC film in ready pack should enable you to expose up to 650 monitor units. Verification film is limited to less then 200 mu, but our software can scale the measurement up to the intended prescription. We recommend you use verification film for fields with less than 100 mu prescriptions. But this may depend upon the film scanner and software that you are using and the distance to the film. You can certainly use some other type of imaging device which would make this step go faster.

Accumulate all the segments for a beam position onto one image (film). If there are eight beam positions, there will be eight images (or films). A beam position is a beam with distinct isocenter, and gantry and couch angle. If the film or imaging device is mounted in the collimator and rotates with the collimator, then you need an image per collimator angle. If on the couch, you can accumulate collimator angles too. You will also have to correct for film processing. You can expose an extra single calibration film and rescale the prior calibration curve. You can also either shoot a step wedge to generate a calibration curve, or you could include a parallel plate ion chamber in the bolus next to the film (or you could put an ion chamber immediately below the film or use a diode or a mosfit detector). You do not want a phantom under the film or above it.

Your effort, probably the time for one treatment. Go to Step 2

You run a film response curve by exposing a series of 10x10 cm field sizes. This curve gets reused if rescaling for each case, or can be used to calibrate a step wedge that would then be used with each case to generate a response curve.

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