FilmQA™ Pro Comparison Maps

FilmQA™ Pro provides by default the following comparison maps:

• Differential Delta: Difference between dose map value and plan value.
  Delta = |Dose map—Plan|
  Verified position passes if difference is below required Tolerance.
  Measurement unit: Dose <Gy>
• Distance to agreement: Difference between plan location and nearest dose map location where difference of the dose values is below required Tolerance.
  TDA = |Location(Plan) - Location(|Dose map—plan| < Tolerance)|
  Verified position passes if location difference is below required Distance.
  Measurement unit: Length <m>
• Gamma: Composite of Differential Delta and Distance to agreement as described in Low D., Harms W., Mutic S., Purdy J., Med. Phys. 25 (1998) 5, 656-661.
  GAMMA = sqrt(Delta^2/Tolerance^2 + TDA^2/Distance^2)
  Verified position passes if gamma value is below 1.
  Measurement unit: Dimensionless <->
• Gamma (swap): Same as Gamma but dose map and plan are swapped, i.e. dose map is compared to plan.

 

Implementation Details

Dose Map Projection

For all comparisons the dose map is projected to the coordinates of the plan image.

Dose map has (should have) a 2-4 times higher resolution than the dose map. Due to the registration process the dose map is rotated and shifted compared to the plan as shown in the schematic below.

For each plan pixel a corresponding dose map value is calculated during the projection process. The following projections are available:

• Bi-linear bitmap interpolation: Dose map values are correlated linearly both in x and y direction to interpolate the dose value at plan pixel center
  • Fastest interpolation method (default).
• Bi-cubic bitmap interpolation: Dose map values are correlated using cubic functions both in x and y direction to interpolate the dose value at plan pixel center .
• Weighted average bitmap interpolation: Dose map values are averaged across the plan pixel weighted by the area the corresponding dose map pixel shares with the plan pixel

After any change of the dose map registration the projection of the dose map is calculated and displayed at the ‘Projection’ tab as shown in the example below (note difference in pixel size of x and y axis.

 

Distance to Agreement Search

The distance to agreement calculation requires a search for a matching dose values (for specified tolerance). FilmQA Pro starts the search at the comparison pixel and steps through the image clockwise as shown in the schematics below.

If only the passing rate needs to be calculated, the search can be accelerated by stopping as soon as a matching value is found. In case of gamma calculation this is not necessarily the best (smallest) value, i.e. the gamma value is only approximated.

In case of failing (i.e. no matching dose value inside the circle with radius ‘Distance’) the search can be stopped using a maximum search radius greater than the Distance value. The search modes as shown below are available.

Gamma search mode only affects the accuracy of the final gamma value, it does Not affect the passing rate.

• Until maximum radius: Gamma value for all pixel inside the circle with extended radius are calculated (best accuracy of Gamma value)
• Until first value passes: Gamma search is stopped as soon the first value passes (fastest calculation)
• Until first passed value increases: Algorithm keeps searching after first values passes as long as gamma value improves.
• Until percentage of all values passed: Keeps search until a set percentage of the area of the distance circle passes criterion (balanced calculation, good gamma accuracy reasonable speed).

Comparison Map Scaling and Measurement Units

To allow similar presentation of all types of comparison maps FilmQA Pro provides for each comparison map a relative error presentation - those presentation are as follows for the default maps:

• Differential Delta:
  Delta = |(Dose map—Plan)/Plan|
• Distance to agreement:
  TDA = |Location(Plan) - Location(|Dose map—plan| < Tolerance)| Tolerance/Distance
• Gamma:
  GAMMA = sqrt(Delta^2/Tolerance^2 + TDA^2/Distance^2) Tolerance

 

Additional Gamma Map implementation

For educational purposes, FilmQA Pro also provides a Gamma map implementation that does Not use any projection of the dose map to the coordinates of the plan as described above. Such implementation delivers substantially inflated passing rate and passing rates gets even more inflated when the dose map includes additional noise.

• Gamma (no projection, noise dependent): Same as Gamma but dose map is Not projected to plan grid.
  Do Not use this implementation for machine QA!
  See Gamma Map Analysis for examples.