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If plan check fails

Common things to check first:

• Visually compare each field image that Dosimetry Check is showing to what the planning system shows and verify that
  (a) you don't have the wrong image associated with a field and
  (b) that there is not something obviously wrong, that they look alike.
  
• Did your imaging device rotate with the collimator? Or does it remain fixed while the collimator rotates, as is the case with an EPID? Check that you have the right collimator angle for the situation for each beam. If the imaging device does not rotate with the collimator, then collimator rotation is already accounted for. Either the collimator angle of each beam should be zero or the fluence must be set not to rotate with the collimator angle. Select beam to edit, and under the Move pull down on the Beam toolbar, select Angles to check this.
• Check the number of fractions that you typed in on the plan toolbar. Dosimetry Check multiplies its results by the number there.
• Print dose for isocenter or some other common reference point. Compare the contribution of each beam to that point to what the planning says is the contribution. Is it one of the beams, or all of them that are off?
• Dosimetry Check can be no better or worst than the accuracy by which you can measure the radiation fields. Did you measure the calibration 10x10 cm field at the same time you mesured the treatment fields? If not, maybe you have a shift in gain of the EPID. As mentioned below, you might need to measure the rmu value of a field with an ion chamber to compare to what you are getting with your imaging device.
• Pick the beam that is off the most. Use an ion chamber with build up cap and measure some point within the beam, the central ray usually being the most convenient. Call the reading Fr. Then put the chamber at the center of a 10x10 cm field at the same distance and expose for 100 mu (i.e. in the same plane perpendicular to the central ray as the first reading). Call that reading Cr. Then compute the rmu value for this point:
  rmu = 100 x Fr/Cr
  In Dosimetry Check, use the get rmu tool (near bottom of options on the Evaluate pull down on the Plan toolbar), to get the rmu value for the same point on the same field. Does the measurement agree? If not then the measurement of the field is wrong. If it does, DC is computing the dose for what was measured.
• For IMAT (Varian's Rapid Arc or everyone else's VMAT) you must consider that cine mode might not have the same gain as single integration mode used to measure the calibration field. Further more, cine mode might miss some radiation between images. For this reason you should do an assessment comparing the sum of images taken in cine mode to that taken in single integration mode for the entire arc. This can be done easily with every case if the second single integration of the entire arc is also done.
• Are both sytems computing with inhomogeneity on or off? In Dosimetry Check, from the main tool bar go to Stacked Image Set options, select the Density toolbar, and look at the CT number to density curve that is being used. Then select the Show Density tool and review the density that is being reported for interesting points on the CT scan set. Note also if the density has been changed by an assignment to an ROI. ROI densities can be assigned or unassigned from the Volumes pulldown on the contouring toolbar (off the main toolbar). Select the ROI using the option menu on the contouring toolbar.
• Consider downloading the plan with the dose computed for a single beam, either on the patient or on a phantom, and compare. In ReadDicomCheck, select the same patient but download with a new plan name.
• Consider downloading the plan computed for all the beams for a phantom and compare. Measure and compare a point within the phantom. Again, you can select the same patient, since a patient can have multiple plans, each with a different stacked image set. Your measured beams will be available for all plans for the same patient.
• For a failed measurement with an EPID, consider shooting a 5x5, 10x10, and 20x20 (or 25x25). Process without a deconvolution kernel and check if you get the typical reponse expected for an EPID. Around .93 for the 5x5, 1.0 for the 10x10 since you normalize to this same field, and around 1.08 for the larger field.
• Also for an EPID, measure a series of exposures to a 10x10 for different monitor units. Use one of the fields to normalize all of them. Do you get back the monitor unit for the other fields?

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