Topics covered in this article include:

• Introduction
• Measurement Summary
• Instructions
• Available Tests
• Image Acquisition Suggestions
• Detailed Discussion of Measurements
• Example File

Introduction
 

The radiation light/field coincidence (aka Rad/Light) routine automatically analyzes the phantom to provide accurate measurements for field size, shape and geometric coincidence of radiation and optical fields. The routine automatically detects the nominal field size and recognizes radiation fields as formed by the MLC or jaws.

Task Group 142 (TG-142) of the American Association of Physicists in Medicine (AAPM) recommends that the radiation light/field coincidence should be checked during monthly Quality Assurance (QA) in the Mechanical subsection of Table II.

Measurement Summary
 

The analysis provides the following results:

• Radiation/Light Field Center Deviation Plot
• Light Field Center to Radiation Field Edge Distances
• Radiation Field Dimensions
• Radiation/Light Field Center Deviation
• Total Rad/Light Misalignment
• Radiation Field Orthogonality

The measurements are completely automated, requiring the user only to drag and drop the image set into the web-based software interface. A detailed report is created.

Instructions

Radiation light/field coincidence images must be DICOM files. If you have added the appropriate tests to your template, you will be able to select the test from the Type drop-down, as shown below.  There are limited options for encoding the analysis into the filename, but this is not the the suggested method. 

Alternatively, limited capabilities to manually identify planar images using DICOM tag values have been added to the image processing system. This is an extension of the existing naming convention system. The PatientID, StudyID, and SeriesDescription DICOM tags are checked and if the text "isoalign" (case insensitive) is found the image set will be processed as an asymmetric field test series. For more details see Manual Identification of RT Planar Images and Individual Catphan Slices through DICOM tags.

When imaging QA tests are added to templates an upload control will appear in the scheduled QA's data entry screen allowing the user to upload images for automated analysis.

To add files to the upload queue simply drag them from a Windows Explorer folder to the drag and drop folder and release them. Alternatively, by clicking on the Add Files button to the lower right of the control a windows file selection dialog will open and files can be selected for upload. Under either method, multiple files may be selected for upload at once.

If the automatically upload checkbox is checked (the default) then file uploading will start immediately as files are added.

If the automatically upload button is turned to off the file upload process must be started manually clicking the Start upload button on the lower right of the control. To clear the upload queue click the Clear button.

Once file series have been uploaded they will be displayed below the upload control. To remove a series from the queue click the Cancel button beside the series. To start processing click the Start Processing button. A description for the image series can be added at this point. Click the Edit button next to the series. Type a description for the series into the text box that appears below Description and either click Save or press the enter key. The description can also be edited after the images have been processed. Descriptions will appear in the report with the analysis of the series. 

While files are being processed users may perform other tasks such as data entry.

Available Tests

Image Acquisition Suggestions

Below are the recommended steps to acquire the radiation/light field coincidence images:

1. Move the couch to 100 cm SSD.
2. Set the jaws or MLC so that the field size is 5, 10, 15, 20, 25, or 30 cm square fields. There does need to be a gap between the field and the edge of the image.
3. Place the phantom on the couch and align the edges of the phantom to the light field while ensuring the central BB is aligned with the crosshairs.
4. Acquire the image.
5. If desired, acquire more images at different field sizes.

Detailed Discussion of Measurements

The Iso-Align phantom has either a central BB or several BBs forming a graticule with arrows at the field edges. In either case, only the central BB is used.

The algorithm assumes the BB and crosshair coincide, and the BB is in the center of the light field. 

Allowed field sizes for the Iso-Align phantom are 5, 10, 15, 20, 25, and 30 cm square fields. The software measures the field dimensions and assumes the nominal field size is the closest from the list above to the measured extent.

The measurement positions on the edges of the radiation field are at 40% of the field size from the center BB (e.g., if the field size is 10x10 cm, the measurement lines are 4 cm to the left, right, top, and bottom relative to the center BB).
 

Measurements

Light Field Center to Radiation Field Edge Distances

The algorithm measures the radiation field edge at eight points, as shown below. The algorithm then calculates the absolute x distance between the x position of the center BB and the x-position of the field edge of the Left-Top, Right-Top, Left-Bottom, and Right-Bottom. The algorithm calculates the absolute y distance between the y position of the center BB and the field edge position of the Top-Left, Right-Top, Bottom-Left, and Bottom-Right. The report includes length and deviation from the expected distance (½ field size). Positive deviations indicate the radiation field edge is outside the light field edge.

Radiation Field Dimensions

The radiation field dimensions table reports the distance between the radiation field edges for the 4 sides.

The deviation compares the lengths to the nominal field size. 

Radiation/Light Field Center Deviation

The radiation field center is the centroid of the radiation field edges. The position of the radiation center is reported in terms of the coordinates set up by the RT Image Position Tag in the DICOM file.

For the Iso-Align phantom, the light field center is assumed to be the position of the center BB. This center is also reported in terms of the coordinates set up by the RT Image Position Tag in the DICOM file.

The difference between the two centers in X and Y is also reported in mm. The Euclidean magnitude of the offset is displayed in mm.

Total Radiation/Lightfield Misalignment

The Rad/Light misalignment provides a single RMS style metric to quantify the overall misalignment of the field.

The value is calculated as follows:

The difference in the deltas (difference from nominal) is calculated:

tlbl = topLeftDelta-btmLeftDelta

trbr= topRightDelta-btmRightDelta

ltrt = leftTopDelta-ightTopDelta

lbrb = leftBtmDelta -rightBtmDelta

The squared vertical and horizontal alignments are calculated

vAlign = ((tlbl+trbr)/4)²

hAlign = ((ltrt+lbrb)/4)²

The total misalignment is calculated.

totaMisalign = sqrt(vAlign+hAlign)

Orthogonality

The orthogonality measures the angle between the top and left sides of the field and the bottom and right sides (the top left corner and the bottom right corner) as a measure of the ‘squareness’ of the field.

Top to Left Side

The slope of the detected radiation field top edge is calculated (rise/run).

The slope of the detected radiation field left edge is calculated (rise/run). When the slope of the left edge is undefined (i.e perfectly vertical) 10⁹⁹ is substituted.

orthogonality_top_left = | tan^-1((slopeLeft-slopeTop)/(1+slopeLeft*slopeTop))|

Bottom to Right Side

The slope of the detected radiation field bottom edge is calculated (rise/run).

The slope of the detected radiation field right edge is calculated (rise/run). When the slope of the right edge is undefined (i.e perfectly vertical) 10⁹⁹ is substituted.

orthogonality_bottom_rght = | tan^-1((slopeRight-slopeBtm)/(1+slopeRight*slopeBtm))|

Below is an example of a completed report:

Example Files

Below are example files for a asymmetric field test series for use in testing: