Software Products for the Radiological Sciences
If plan check fails
Common things to check first:
These measures should result in some understanding of the reason
for the differences seen.
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
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
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?
Return to homepage
Math Resolutions, LLC
5975 Gales Lane, Columbia, MD 21045
© copyright 2001 by Math Resolutions, LLC