How it Works
Dosimetry Check is software that uses the existing hardware on linear accelerators
to
reconstruct doses delivered to patients. This is the most complete method of Quality Control
available.
Dosimetry Check works in 3 main steps listed below. (Note: Steps for TomoTherapy are slightly
different)
Step 1
Dosimetry Check uses the Electronic Portal Imaging Device (EPID) to capture an image of the dose
that
is delivered either before treatment or during treatment. Dosimetry Check can also
use
ion chamber arrays or diode arrays for image capture
Step 2
Dosimetry Check then converts the EPID image to an in-air fluence map that is projected onto the
patient to compute the dose
Step 3
Dosimetry Check then compares the reconstructed dose to the dose the planning system intended to
deliver.
Dosimetry Check Dose Algorithm
Dosimetry Check uses both pencil beam and collapsed cone algorithms. The opportunity to use different
algorithms is beneficial when evaluating plans from treatment planning systems. Dosimetry Check has
proven to provide accurate results with the various treatment planning systems available on the market.
In Step 3 above, we show a comparison between Dosimetry Check's EPID based collapsed cone convolution
superposition calculation and the treatment planning system. Using an
independent system to check plans adds redundancy and increases confidence in the delivered plan.
Expectations with a fully commissioned system are for differences to be less than 3 percent in most
cases. Additionally, dose discrepancies can be viewed in 3D right on the patient CT dataset. The primary
purpose of Dosimetry Check is to QA individual patient treatments and not the planning system. The
Dosimetry Check technology has been improved and fully implemented in RadCalc. For more information on
this, please check out the
LAP Academy courses.
Discrepancies in Plan Checks
Discrepancies in plan checks are common when first starting out. Proper clinical commissioning should be
performed.
After commissioning, some common items to check are:
- Visually compare each field image that Dosimetry Check is showing to what the planning system shows
and
verify that you don't have the wrong image associated with a field and that there is not something
obviously wrong. The images should look similar.
- 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 the 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 of fractions.
The above does not apply in RadCalc as the image processing has been
automated.
- 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 worse than the accuracy by which you can measure the radiation
fields. Please be certain to measure the calibration 10x10 cm field at the same time you measured the
treatment fields or that the calibration image being used has been recently measured? The sensitivity
of
the EPID can change over time so it is important to periodically update the calibration image. 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 of 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.
The above does not apply in RadCalc as there are additional tools for delivery
evaluation and the
calibration procedure is different. During commissioning of RadCalc 3D modules
chamber measurements are recommended and this is
used for the EPID dosimetry validation. Additionally, EASY CUBE can be purchased for
another
level of
redundancy checks.
- For IMAT (Varian's Rapid Arc or VMAT in other systems), 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.
In RadCalc, the recommendation is to use integrated imaging (Dosimetry Mode) for all
systems except
for
the Varian C-Series. These will only export the appropriate frames with the required gantry
angle in
cine
mode.
- Are both systems 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. Also, not 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 irradiating a 5x5, 10x10, and 20x20 (or 25x25) field
size. Process the images without a deconvolution kernel and check to see 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.
In RadCalc the above is performed in fewer steps, without leaving RadCalc. Currently,
only the
planning
on a phantom such as the
EASY CUBE would need
to
be performed back in the TPS for the given plan.
- 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?
The result of these measurements should result in some understanding of the reason for the differences
seen.
